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t
LL SCALE IRRIGATION
A Manual of Low-cost Water Technology
Peter H. Stern
Intermediate Technology Publications Ltd/
International Irrigation Information Center
We are inde
mediate Technology
culture Panels for help in deciding on the
this book and for comments on the text. V’W
Nndson for the information about contour se
in Chapter 4, Mr R.A.
ApPendices 8, C 2nd D, and Mr G.P.C. t-ienry for
case study in Appendix E-l. We are grateful to Mr Faul
Harrison for permission to reproduce the article in Appendix
E-l I. We are also grateful for having been able to draw on
information from the FAO Irrigation and Drainage Paper 24,
‘Crop Water Requirements’, and on the FAO Development
Papers 88 ‘Sprinkler Irrigation’ and 95 ‘Surface Irrigation’.
Where illustrations and drawings have been supplied from
other sources, credits are acknowledged beside each item.
Our cover picture is from a photograph by Douglas Dickins.
Gratefu I thanks are conveyed to Professor H. Shalhevet
and to other referees who scrutinised the text and made
many useful and helpful suggestions.
Published jointly by Intermediate Technology Publications Ltd, 9
King Street, London WC2 8HN, U.K. and the International Irrigation
Information
Center, Volcani Center, P.O.B. 49, Bet Dagan, Israel;
P.O.B. 8500, Ottawa, KlG 3H9.
@ Intermediate Technology Publications
Irrigation Information Center, 1979.
Ltd and International
Published 1979
ISBN 0 903031 64 7
Printed by the Russell Press Ltd, Gamble Street, Nottingham
Telephone: (0602) 74505.
NG7 4ET.
Preface
PART I
THE IRRIGATION
SCENE
Chapter 1
Introduction
What is irrigation?
Water and agrkulture
The geography of irrigation
Chapter 2
The Choice of Technology
The advent of modern irrigation
Changes in attitude
Options for development
19
Chapter 3
Is It Worttwhile Irrigating?
Climate
Soils
Topograph\a
Water
Crops
Labour
Legal aspects
Is it worthwhile?
24
PART II
IRRIGATION
31
Chapter 4
Moisture Conservation Techniques
Run-off interception
Run-off farming
Contour seepage furrows
Mulching
Soil moisture trap
Humid culture
PRACTICE
31
Chapter 5
Swface Irrigation
Basin irrigation
Bxder irrigation
Furrow irrigation
Corrugation irrigation
Wild flooding
Spate irrigation
Trickle irrigation
38
Chapter 6
Subsoil Irrigation
Water table control
Sub-soil pipes
Pitcher irrigation
53
Chapter 7
Overhead Irrigation
Watering can
Hose-pipe
Sprinkler systems
55
PART I II
PLANNING
64
Chapter 8
Climatic Factors
Mediterranean regions
Monsoon regions
Wet tropical regions
The use of rainfall records
64
Chapter 9
Crops and Water
Evaporation processes and consumptive use
Calculating ETo from pan evaporation
Crop water requirements
Net irrigation requirements
Irrigation efficiency
Water quality
69
AND DESIGN
Chapter 10
Physical and Chemical Characteristics of Soil
Soil formation
The composition of soil
Types of agricultural soil
The chemistry of soil
Nutrient deficiencies
Salinity
Acidity and alkalinity
Chapter 11 Soil and Water
Soil moisture
Available water
Plant root zones
Irrigation application
Infiltration rate
Irrigation supply
83
Chapter 12 Drainage
Soil drainage
Surface drainage
Farm drainage
Design of drainage systems
Drainage and salinity
Soil erosion
!
Chapter 13 Source Development
Small catchment storage
Streams and rivers
Wells and boreholes
Chapter 14 Channels and Pipelines
Irrigation channels
Construction of channels
Channel conveyance losses
Pipelines
98
105
Chapter 15 Water Lifting
115
Animabpowered systems
Water power
Wind power
Mechanical pumps
APPENDICES
Appendix A Units, Abbreviations and
Conversion Factors
Appendix B The Measurement of Climatic
Information
I, Rainfall
I I. Air temperature and humidity
I I I. Evaporation
IV. Sunshine
130
131
Appendix C Mapping anti %weys for
Irrigation
137
Appendix
Source
140
D The Measurement of a Water
Appendix E Two Case Studies in
Management
148
ables
1.
2.
3.
4.
5.
6.
7.
8.
9.
IO.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
Irrigated areas of the World in 1952 and 1972
16
Suitable areas for basins on flat land (sq. metres)
Suitable spacing of terrace steps for basins (
Suitable dimensions for border strips
Suggested I:laximum furrow lengths (metres)
Soil infiltration rates and suitable furrow inflows
4
100 metres of furrow length
4
Syphon pipe flows (Iitres/secondI
Length and spacing of corrugations
49
Soil infiltration rates ;nd suitable corrugation inflo
per 100 metres of corrugation length
50
Typical rotating sprinkler characteristics
58
ter intake rates for overhead irrigat’ion
6
ample of specifications and calculations for a s
sprinkler irrigation system
2
Pan coefficients (Kp) for American Class A type
71
evaporation pans
Crop coefficients ( Kc) for various crops and their
72
growing periods
An example calculation for net irrigation water
73
requirements for cotton in Uganda
Field irrigation efficiencies for different methods
74
of irrigation
75
Standards for irrigation water
76
Relative to,lerance of crops to salinity
79
Approximate quantities of mineral matter in soil
84
Typical soii moisture quantities
85
Available water for typical soils at field capacity
86
Typical root-zone depths
86
Moisture extraction pattern in plant root zones
88
Soil infiltration rates (mm/h)
Approximate values of C in the formula Q = CiA
94
for small catchments less than 250 hectares
Drain channel capacities in litres/second for various
95
channel slopes and sections
Suitable channel side slopes for different earth
106
materials
28.
29.
Coefficients of rou hness (n) for
channel
water velocities for
Suggested maxi
earth materials
sof
es
I.
2.
3.
4.
5
6.
7.
8.
9.
IO.
II.
12.
13.
14.
15.
The Aswan High am, Egypt (Photo:
Dickins)
Kenana Sugar Scheme, Northern Su
(Photo: New Civil Engineer)
Three thousand year-old rice terraces at
e,
uglas Dickins)
the Philippines (Photo:
One of the irrigation ca I headworks at t
Nagarjunasagar Dam on the Kristna River, near
Hyderabad, India, for the irrigation of 1X
hectares (Photo: Douglas Dickins)
a. Weir and canal headworks, Krasak River, Central
Java, Indonesia
b: Regulating structure, Mataram Canal, Central
Java, Indonesia
Water supply installation, Urn Isheishat, Western
Sudan
Water supply yard, Urn Isheishat, Western Sudan
Run-off interception with ridges
Run-off interception with terraces made of stones
a. Furrows ploughed across the direction of slope
b. Basins for intercepting run-off
Drainage zf terraces
Contour seepage furrows
Details of seepage furrows and spillway
Suil moisture trap
Transplanting rice, Luzon island, the Philippines
(Photo: Douglas Dickins)
14
14
15
19
20
21
27
27
31
32
32
32
33
34
34
35
39
16. Small gated control structure for basin or
irrigation
17. Irrigation furrows
a2 ( 060: isecx
18. Furrow irrigation at Mision
de la Iglesia Anglicana, Argentina)
49. Watering can
20. Typical sprinkler head (Photo: Wright
21. Simple sprinkler layout (I /lustration:
22. Possible arrangements for a small sprinkler
irrigation system
23. Typical crop coefficient curve
24. Soil moisture quantities
25. Calculation for irrigation quantities with rain%
26. Drainage layout for a 25-hectare far
27. Field drainage arrangements
28. Micro-irrigation system
29. Catchment tank construction
30. Diversion weir
3i. Groin
32. Low dam with washout sect ion
33. Gabion filled with stone, lid open
34. The hydraulic characteristics of channel sections
35. Irrigation canai design chart
3K Concrete pipe design chart
37. Asbestos-cement pipe design chart
3”s. Steel pipe design chart
3d. PVC pipe design chart
40. Irrigation by scoop
41. Beam and Bucket
42. Indian ‘Dall’ (or ‘Auge’)
43. Archimedean screw (Photo: Douglas Dickins)
44. Section of Archimedean screw
45. Chain pump
46. Bucket pump
47. Reciprocating hand pump
48. New No.6 hand pump (Bangladesh)
102
103
105
IO8
112
112
113
?14
116
116
116
117
117
118
118
119
121
reface
This handbook has been written fo
with the development of irrig
and with limited technical a’n
the readers, I hope, will b
because the book is in
district officers, extension an
agents, volunteers and other operators, traine
to Diploma level or the equivalent, working in rur
and usually out of reach of I
phernalia and know-how readil
trained people.
Often, in my experience, small ru;al corn
about irrigation and sometimes embark on irri
without fully appreciating the problems in
extra demands which a change-over from rainrc4tivation will make on their time, labour
-j-he first part of this book is an attemot to look at some of
these problems.
“What is ‘SMG: s!;ale’ and how low is ‘low cost’?“, it may
jutifiably
be asked. !n one country I visited recently the
government described small irrigation projects as those
between 100 and 1,000 hectares. In this book, 100 hectares
has been taken as the extreme upper limit, 2 to 20 hectares
as the range of sizes for most small farm units in the developing
world, and even a one-tenth hectare vegetable plot is a viable
irrigation unit for a family under certain conditions.
On the question of cost, ‘I-- : cost’ is a very relative term,
depending on local economiu circumstances, and no attempt
is made to define it. When a large project is so costly that no
major international
funding agency will finance it, the
conclusion that nothing can be done seems negative and
inadequate if, by introducing more appropriate technology
which makes better use of local resources, the project can be
made feasible. A scale of expenditure which may be well
within the means of a rural community in one country will
be too costly in another. Sprinkler irrigation is too expensive
for small-scale far ’
be ~feasible 117som
The second an4 t
technical aspects
readers will not have
matter has been
some sections, an
of crops, soil and
lines, will be m
mathematical an
will confound others. At t
may be disturbe
risks which hav
the communication
of kna
,further reading
listed at the end
Peter H. Stern
ART I
THE IRRIGATION
SCENE
Chapter 1
What is Errigation?
It may at first sight
activity which is wellin this book we shall be talkin
sense, and because this takes us
commonly understood to be irri
or any other cultivated plants. T
farming, humid culture, and
becai.jse these are important and si
of this book.
Water and Agriculture
Every farmer knows tiiat cultivation r
water and sunshine, or to put it
essential inputs to thegrowth of vegetation ar
water and energy. In the wetter parts of t
rain-fed cultivation is practised, the farmer’s actrvrtres c
of selecting suitable (that is fertile) land, preparing t
for cultivation, and sowing, tending and harvesting his crops.
Natural rainfall provides the water needed. But in many othe
places otherwise favourable for cultivation, natural sainfal
does not provide all the water needed, and irrigation can
make up this deficiency.
In arid and semi-arid regions, there is usually little doubt
about the need for +:rigatio:r. Most of the world’s great
irrigation development are !.o be found in these regions.
Large engineering works on major rivers such as the Nile at
Aswan, Egypt (Figure 1 ), providewater to distributionsystems
serving thousands of people and tens of thousands of hectares
(Fig. 2). Also in these regions there is a very large number
of small irrigation schemes, farmed and operated by traditional
methods which have been passed down from antiquity.
Fig. 1 The Aswan H&h Dam,
Fig.2
Kenana
Engineer)
Sugar Scheme,
Northern
Sudan (Photo:
New Civil
In many other parts of t
sufficient to produce CT
used to make up the f
cultivation season. Here
cut. Potential increases in
against the extra time an
increased productivity.
endanger the fertility of th
The Geography of Irrigation
Irrigation has been
ral thousands
irrigation in India a~
the Nile Delta in Egypt, a
Euphrates in Iraq were u
igure 3 shows rice t
Phiiippines. Today over
irrigated in five continents.
million hectares in 1952, and is stil
figures for irrigated
1952 and 1972,
Fig.3 Three thousand ymr-old rice terraces at Bananue, the Philippines
(Photo: Douglas Dickins)
16
Small Scale Irrigation
These figures, at best, are estimates, and their accuracy
varies with the way the statistics have been assembled.
Sometimes they may be slightly over-estimated because
authorities do not enjoy disclosing figures which show underutilisation o)f investment. However they give a fair idea of
the importance of irrigation in the agricultural economy
of many countries, and demonstrate thit practical experience
in irrigation is widespread. South-East Asia takes the lead
with nearly seventy per cent of the world total.
Table 1 - Irrigated areas of the world in 1952 and 1972
Data from International Commission for irrigation and Drainage
Annual Bulletins. Items marked * are the author’s estimates.
Million Hectares
Region and Country
1952
1972
1. SOUTH-EAST
Burma
Ceylon
China
India
Indonesia
Japan
Korea
Malaysia
Pakistan
Philippines
Taiwan
Thailand
Vietnam
Others
ASIA
0.8
0.2
40.0”
19.7
1.8
2.8
0.4”
0.2”
12.1
0.1
0.6
0.6
0.1
0.1*
79.2
2. NORTH AMERICA
Canada
United States
3. EUROPE
Albania
Bulgaria
Czechoslovakia
Denmark
.France
0.8
0.3
74.0
32.7
3.8
3.4
1 .o
0.3
12.0
1 .o
1.9
1.9
0.2
0.1
132.0
0.6
16.9
--17.5
0
0.2”
0.03”
0.02”
A23
0.2
1.0
0.1
0.1
2.5
Introduction
Region and Country
Germany (Fed. Repub.)
Greece
Hungary
Italy
Netherlands
Poland
Portugal
Rumania
Spain
United Kingdom
Yugoslavia
Others
4. MIDDLE EAST
Afghanistan
Iran
I raq
Israel
Saudi Arabia
Syria
Turkey
Others
5. U.S.S.R.
6. AFRICA
Algeria
Ew Pt
Libya
Malagasy Republic
Morocco
Senegal
Somalia
South Africa
Sudan
Tunisia
Others
17
Million Hectares
1952
1972
0.1*
0.1”
0.1”
2.2
0.05”
0.05*
0.03
0.05”
0.8*
0
0.02*
0.01*
6.1
0.3
0.7
0.4
3.2
0.1
0.1
0.6
0.6
2.3
0.1
0.6
0.1
13.0
0.6”
2.0”
3.3
0.04
0.02”
0.4+
0.05
0.1*
6.5
0.8
3.1
4.0
0.2
0.2
0.5
1.9
0.2
6.0
10.4
0.2
2.5”
0.05”
0.4”
0.01
0.05”
0.05”
0.4
0.5
0”
0”
4.2
0.3
3.0
0.2
0.9
0.2
0.1
0.2
0.6
0.8
0.1
0.2
-m
18
Small Scale Irrigation
Million Hectares
1952
1972
Region and Country
7. CARIBBEAN AND CENTRAL
Cuba
Dominican Republic
Jamaica
Mex ice
Others
8. SOUTH AMERICA
Argentina
Brazil
Chile
Colombia
Ecuador
Guyana
Peru
Venezuela
Others
9. AUSTRALASlA
Australia
New Zealand
WORLD TOTALS
South-East Asia
North America
Europe
Middle East
U.S.S.R.
Africa
Caribbean and Central America
South America
Australasia
AMERICA
0.3”
0.1”
0.04”
1.7
0.1
2.2
0.5
0.2
0.1
4.0
0.1
4.9
1.0
0.05”
0.1”
0
0.05”
0.04’
0.05”
0.1”
0.02
1.4
1.2
0.1
1.3
0.2
0.1
0.2
0.9
0.4
0.1
4.5
0.6
0.03
0.6
1.3
0.1
1.4
79.2
10.9
6.1
6.5
6.0
4.2
2.2
1.4
0.6
117.1
132.0
17.5
13.0
10.9
10.4
6.6
4.9
4.5
1.4
201.2
19
Chapter 2
ethnology
oice
of
e
There are many notable examples of heavy investments in
large irrigation projects which have not turned out as planned
and in which today less than fifty per cent of the irrigation
facilities are actually being used. If iarge-scale planners and
designers, with almost unlimited technical and scientific
resources at their disposal, can make mistakes like these, it is
understandable that it will often not be easy for the smallscale farmer to decide whether or not to introduce irrigation.
But because the output of major irrigation schemes is
sometimes disappointingly low, more attention is now being
given to the small-scale farmer and the scope for improving
product,ivity in small units. The various physical factors,
technical opt ions and social and inst.i t u t ional influences
which need to be considered are discussed in Chapter 3.
Fig.4 One of the irrigation canal headworks at the Nagarjunasagar Dam
on the Kristna River, near Hyderabad, India, for the irrigation Of 1%
million hectares (Photo: Douglas Dickinsl
20
Small Scale Irrigation
The Advent of Modern Irrigation
ivlany of the ancient irrigation systems which originated
sever-PI thousand years ago in the East and Far East have
continued without significant changes in their overall layout
and methods of operation until the present time. Water
supplies are still often unregulated and uncontrolled, and the
distribution and amounts of water available for crops each
year are precariously dependent on rainfall and run-off.
The first applications of modern engineering technology
to irrigation were made in India during the nineteenth
century, followed quickly by large-scale developments in
the southern United States. During the first half of the
twentieth century, traditional irrigation systems in many
parts of the world were modified and improved. The most
significant feature of these works was the development of
the river dam or barrage (Figures 1 and 4) and large canal
headworks which ensured a much more efficient and effective
diversion of river waters, allowing water not required for
irrigation to pass on down the river. Many of the well-known
barrages in Pakistan, India, Egypt and Iraq were built during
the first quarter of this century. At about the same time
scientific engineering was being introduced into t
very extensive rice irrigation systems in Malaysia, Indonesia
and other parts of the Far East (Figure 5).
,
Wr and canal I
,
.
wrks, Kmsak River, Glmtral Java, Indonesia
The Choice of Technology
Fig, 5b Regulating structure,
21
Mataram Canal, Central Java, Indonesia
Usually, as modifications and improvements were applied
to irrigation systems serving largely peasant agricultural
communities, the government responsibility exercised through
irrigation departments and services was confined to head
works and major distribution, and little was done to carry
those improvements down to field level where efficiencies
were often very low. Irrigation efficiency is a measure of the
effective use of the water supplied, and it is expressed as
the ratio of the amount of water needed by the crops to
the amount of water actually supplied. But even with low
efficiencies most of these developments showed positive
economic returns to the governments which were responsible
for them, and at the same time did much to improve the food
situation in rural areas. R.B. Buckley,* in a classic work on
irrigation in India, wrote in 1919; “The beneficial results
which both the security and increase of out,turn (i.e. crops)
confer on the people are incomparably mo9e valuable than
the large revenue derived by the State”.
Where private enterprise was also involved in irrigation
development, rather more attention was devoted to improving
efficiency at all levels including engineering improvements to
minor distribution
systems and field water applications.
The 1920s and 1930s saw theexpansion of the highly efficient
*Irrigation Pocks? Book, E. & F.N. Spon, London, 1920.
22
Small Scale irrigation
sugar cane irrigation in Java, and the Gezira Irrigation Scheme
for cotton, and other crops in the Sudan.
Changes in A ttitude
Until the late 196Os, it had been generally assumed that
the patterns of large-scale irrigation developed during the first
half of the present century were universally beneficial. These
assumptions were justified by results measured in terms of
corporate production and corporate revenue, without looking
too closely into the effective results at family and farm
level. Furthermore, these results were achieved through the
satisfactory functioning of large administrative and managerial
units, which were sometimes government departments
and sometimes commercial enterprises. In both cases they
depended very much on loyal co-operation at all levels, and
although occasionally this had to be enforced, by and large
these orderly systems were accepted by all those working
within them. By the early 1950s major political and social
changes were beginning to stir in many parts of the developing
world, and these changes have had a profound effect on all
institutionalised
activity. Irrigation schemes, which worked
tolerably well in an earlier, more rigid social and economic
system, now ran into all kinds of problems with staff and
labour, maintenance and upkeep, and the effects of these
changes were manifest in falling outputs and deteriorating
performance.
By a curious anomaly the twenty years between 1950 and
1970 which saw so many disappointments in irrigation at
field and farm level also saw an increase of seventy per cent
in the areas of land under irrigation in developing countries,
representing very substantial capital investments in new
works. For many years irrigation planners and development
authorities were not deterred by poor field results in their
energetic promotion of irrigation projects. The problems of
irrigation at the farm level have, however, been receiving
some attention during the past decade. At a seminar sponsored
by the United Nations Food and Agriculture Organization
(FAO) in the Ph’l’I rppines in 1970 these problems were
discussed. Among other “things this seminar concluded that
more attention should be given by governments to smallscale development.
More recently at its world congress in Moscow in 1975
The Choice of Technologbf
23
the International Commission for Irrigation and Qrainage
appointed a committee on “assembling irrigation efficiency
data”, with a view to making recommendations for improving
efficiencies at all levels, including field efficiencies.
Options for Development
There are many instances of large-scale irrigation schemes
being under-utilised, and there are therefore very valid
reasons for the collection of data which wi!I lead to the
better use of these existing facilities. But there is also a great
need for a livelier response to the FAO Seminar recommendation that more attention should be given to smallscale development. The strongest argument in favour of
small-scale irrigation is that it is easier than large-scale
development because the human problems are reduced
to a manageable scale. Certainly the sharing of a common
source of water calls for co-operation between farmers, but
experience seems to show that co-operative activities are
more successful if they are not too large. If thedevelopment
unit, in human terms, is small, then inevitably its resources
for development will also be small. This will mean that
construction work will have to be carried out at minimum
cost, using simple methods, and local materials and labour
wherever possible. This, in turn, will involve choosing the
best available technology for the work in hand. Where
equipment is installed, such as machinery for pumping, it
should be of a kind which can easily be maintained and
repaired by the skills which are locally available.
24
Chapter 3
Is It Worthwhile
Irrigating?
A farmer contemplating irrigation will need to consider a
number of factors. He will have to think about the climate,
the soil, the availability of water, the crops to be grown and
the amount of time and effort which he is prepared to put
into the operation. Even in the simple practice of irrigation
by watering can, most of these factors must be considered,
if only briefly. Is it worth buying a can? Is it worth the effort
of carrying the water? If the undertaking is likely to be more
complex, involving the cost of developing or diverting a source
of water, the purchase of equipment and materials and the
employment of labour, then it will be more important to
look carefully into the factors involved. To do this the farmer
needs the best information he can get.
It is not always easy in remote areas to discover where the
avaiiable information can be found or how to obtain it, but it
is well worth a considerable amount of effort to find it. This
may call for visits to the meteorological service, agricultural
institutions, the water development authority (if it exists)
and other organisations. If there is an agricultural extension
service - and there are very few developing countries without
this service - contact should be made with the nearest local
officer who will usualiv be only too pleased to help a farmer
interested in developing his resources.
Climate
The climate is a very important factor in any sort of
cultivation. In places where there are fertile soils and conditions are favourable for irrigation but there is no rainia!!,
the need for irrigation is obvious. In other places where there
is some rainfall, but it is insufficient in quantity or badly
distributed in time, crops might be very much improved
with irrigation. If rice is to be grown in paddies, then irrigation
is a necessity. Whatever the circumstances it is important to
obtain all possible information about the climate, the most
useful data being rainfali,, ternperature, evaporation, humidity
and the daily amounts of sunshine. Appendix B explains how
Is It Worthwhile
irrigating?
25
these quantities are measured and recorded. As climatic
conditions vary with the time oi year, and irrigation may
involve growing crops during months when there is no
traditional cultivation, climatic conditions throughout the
year need to be carefully studied. Because there are also
variations from one year to another, records should be
studied for as long a period as possible. Chapter 8 describes
how this information is used in different climatic regions.
Soils
Most farmers understand soils, know where the better soils
are to be found and how to use them for rain-fed cultivation.
The introduction
of irrigation can sometimes produce unexpected results if the farmer does not know a few scientific
facts about his soils. A summary of the more important
physical and chemical features of soil is given in Chapter 10.
The extra water of irrigation will bring about beneficial
changes in most soils, increasing the amount of organic
material (arising from extra cultivation), and, by keeping the
soil wet, facilitating growth and the movement of nutrients
from the soil into plant systems. But, the more water in the
soil, the less air, and as air is also needed for the processes
which maintain a soil’s fertility, too much water can be
harmful. Furthermore, some soils contain harmful soluble
materials which irrigation water may bring up to the surface
and subsequently precipitate (or release), seriously affecting
the crops.
Every country has a department or section of soil studies,
usually part of, or associated with its Ministry of Agriculture,
and, largely through the help and encouragement of the Food
and Agriculture Organisation of the United Nations, a large
proportion of the agricultural land in the world has now been
covered by soil survey maps. In many countries these maps
show not only the distribution and types of soils, but also the
land classif ied in accordance with its agricultural capabilities.
These are all useful to the farmer and although he may not
always be able to interpret them himself, there may be
specialists in the agricultural service who can advise him on
the basis of the mapped information, and he should seek
this advice.
Topography
By topography
we mean the form and shape of the land.
26
Small Scale Irrigation
Almost any land, however steep, can be irrigated if it can be
cultivated, depending upon the method of irrigation and the
skill and resources of the farmer. But because it is important
to control the water supply so that it will not be wasted, it is
easier to irrigate by surface methods if the land is not steeply
sloping. If basin irrigation is planned, each basin must be as
level as possible. One of the advantages of overhead irrigation
is that proper control of the water is not so dependent on the
slope and shape of the land. But if water is to be supplied
in pipes under pressure, it is necessary to know the height
or ‘head’ for pumping. If water is to flow by gravity from
one point to another, we must be certain that the starting
point is higher than the delivery point.
All such information
abcut heights and topography is
obtained from topographical surveys and mapping. Existing
maps, usually to a scale of 1 in 50,000 will give a general
idea of the topography of an area, but this scale LX be too
s,nall for irrigation planning. For a very sma!l it :-:Tetion
development it may be possible to judge height:: h’y eye,
but this will be rare, and some topographical survlr.. work
is usually essential. Appendix C gives some guidance on
survey and mapping for irrigation.
Water
One of the commonest mistakes which people make when
thinking about water for irrigation is to under-estimate the
quantity which will be needed. In rural situations a good
water supply for domestic and animal use (Figtirtis 6 and 7)
may be a very inadequate supply for irrigation. For example
if all the water consumed in a month by a rural community
of 1000 people with 250 cattle and 500 sheep and goats were
used for irrigation, this would provide two irrigations a
month to an area of about a quarter of a hectare.
The amount of water needed for irrigation depends not
only on climatic conditions and the total area to be irrigated,
but also on the crops to be grown. Tree crops and other
perennial crops, which grow all the year round, need water
throughout the year. Seasonal crops, which are cultivated
from seed until they are harvested, need water only during
the cultivation season. If irrigation water for seasonal crops
is to be taken from a source which is liable to dry up during
the dry season, the timing of the irrigation season will be an
Is It Worthwhile
Fig.6 Water supply installation,
Irrigating?
Urn Ishaishat, Western Sudan
Fig.7 Water supply yard, Urn Isheishat, Western Sudan
important consideration. The water re ’
are discussed more fully in Chapter
It is therefore important to know how mcpc
needed for irrigation in terms of the crops t
the area of land to be irrigated, and how mu
meet this need. As the capacity of many
springs or streams will vary very much with t
year and from one year to another, great care
28
Small Scale Irrigation
in assessing the ‘safe’ i ,aever-likely-to-fail) supply.
quest ion the Gove{; tn ient water development or water
resources organisatl-In should be consulted. They s
know if there :gre xry records of the flow of a spring or
stream and where lilese records ma*; be found. If no records
are available, th::n the farmer himM should start measuring
the source which !le intends to use ?L’ays of
suggested in A;ICYT!4x Il.
A!! natural 5:;’ fxe wa%,rs and g::xn::waters contain some
dissolved salr:.;, In some places wan& may contain so
salt that it is lrr~:,uitable for irrigation. it is therefor
make sure rhz+. the water proposed for irrigatio
too saline ?GX~$ The degree of salinity of wat
determine,; !.J? laboratory tests, and if there is
about the quairty of the water to be used, professional advice
should b? ;(aught.
Whe, :i ‘rrigation is tised to exten
season, This often means cultivatin
of the year when the climatic
different from the rain-fed cropping
to be considered in the light of possible new c
Whes’. which can be grown ilnder
or CL,qxus will not thrive under ir
because it is too hot. Fdddy rice,
monsoon conditions, cannot be gr
irrigstion in thecool dry winter in Pa
If d farmer intends to grow vegetables under irrigation for
sale, he must be sure that his produce will se I. Some years
ago 5 large dam was constructed in one count y in SE. Asia
to enable the farmers to grow vegetables under irrigation
during the dry season in addition to their traditional r
ric,: during the monsoon. Littie was done, either
don-x agency which provided the dam or b
to encourage the farmers to avail thems
facility apart from a pilot project of a few hectares, on which
some farmers were persuaded to grow irrigated cucumbers.
At the end of the first irrigation season the farmers had
severai tons of cucumbers which they could not sell. The
only possible market for this produce would have been in a
city 500 km away, and the cost of the transport could not
IT It Worthwhile
Irrigating?
29
possibly have been recovered from sales.
Labour
Irrigation requires more labour than rain-fed cultivation.
In addition to the activities associated with dry land farming,
the irrigation water supply has to be managed and controlled.
On large projects much, and sometimes all of the water
control may be mechanised, but this will not be the case with
most small farms where the farmer may have to manage his
water source, look after his supply line, and distribute the
water on his land.
With rain cultivation it is possible to leave the farm from
time to time to participate in other non-farming activities,
as no great harm comes to the crops. Under irrigation, water
must be applied when it is due, and under harsh climatic
conditions a day or two of de&y in watering may result in
serious crop losses. The farmer is ?herefore much more tied
to his land when it is irrigated. Irrrsated rice is particularly
labour-intensive, and thededicatedconmitment
to cultivating
successful paddy rice may not come easily to those not
traditionally accustomed to this typa of cultivation.
Legal Aspects
A number of countries now have legislation which governs
the use of limited water resources, and it may be necessary
for a farmer to obtain a licence to use a source of water.
In countries where such legislation exists, the farmer should
consult the recponsible authority, which could be the Water
Development or Water Resources Department, about his
requirements. It may be that his needs are so small that they
fall below the minimum for which licences are required, but
it is as well to have the legal position clearly understood
before embarking on a development.
There may be other !egal aspects to be considered, such as
common rights to a source of water, or access to land for the
conveyance of water from the source to the field. Where
several farmers share in the development of a source, this will
call for some co-operatrve agreement over the management
and use of the supply. Examples of how this is done in some
countries are given in Appendix E.
ts it Worth while?
If having looked at all these rather daunting questions in a
30
Small Scale Irrigation
general way, the farmer decides to irrigate, he will then need
to consider each one in more detail. .His approach to the
problems of planning for irrigation will depend on the
geographical and climatic region in which he is situated, on
his financial and technical resources and on the farming
system that he intends to follow. Part I I of this book describes
the various methods of irrigation which can be practised
and Part I I I looks at the problems of planning and design.
31
PART II
IRRIGATION
PRACTICE
Chapter 4
oisture
Conservation
Techniques
Run-off is the term given to that part of water from rain+all
(and other forms of precipitation) which flows away from
the land on which it has fallen. Where soils are impermeable
and shallow and where land slopes are significant (say more
than 5%), run-off occurs quickly, and only a little moisture
remains in the soil. Under semi-arid conditions and high
temperatures, direct evaporation from the soil quickly
removes any moisture held in the surface layers and it may
be impossible to grow crops. Various measures can be taken
to reduce these soil moisture losses and to arrest run-off
from the land so that crops can be grown.
Run-off ln terception
Where surface water runs down a slope too quickly this
can be retarded by creating level or nearly level ridges on the
land to check the movement of the water, thereby enabling
more water to be absorbed by the soil (Figure 8). Intercepting
the run-off also helps to reduce the erosion of precious soil
on steep slopes.
Fig.8 Run-off interception
with ridges
The ridges may be made in various ways according to local
soil conditions. If there are stones and gravel on the ground
surface they can be coliected to form the ridges. This method
was practised by the Nabateans in the Negev Desert two
thousand years ago and some of their constructions are still
used for growing crops today. In the gently-sloping plain of
the Gezira, Sudan, where stone and gravel are rarely found,
32
Smal I Scale Irrigation
ridges for intercepting run-off are made of the local
In steep, rocky terrain the ridges may take the
terraces made of stones (Figure 9). Where land is
in furrows, the furrows will intercept run-off if
across the direction of the slope (Figure 1Oa).
Fig.9 Run-off interception
clay soil.
form of
ploughed
ploughed
with terraces made of stones
Fig. IOa Furrows pr’oughed across the
direction of slope
Fig. IOb Basins for intercepting
run-off
Where land is gently sloping or almost flat, run-off can be
arrested by the construction of basins (Figure lob). Most
systems of embankments or terracing will have to cater for
heavy storms or prolonged rainfall, and provision must be
made for surplus water to move down the slopes into streams
and drainage Iines. I n a few cases, with rock terraces and
permeable soils, percolation through the terraces may be
adequate, but usually it will be necessary to direct surplus
water along the terraces or ridges into some prepared waterway such as a natural drainage line (Figure 11 j.
Run-off Farming
Run-off farming is the name given to the practice of
concentrating surface run-off for cultivation in desert and
semi-desert regions where there is never sustained stream
flow. It is a technique which was used as long ago as 950 B.C.
Moisture Conservation
Techniques
33
Fig. I I Drainage of terrace
and recent investigations in the Negev Desert have revealed
the remains of extensive ancient agricultural systems based
on this method. Farms up to 3 ha in size in narrow valley
‘lottoms were watered from catchment basins up to 60 ha in
area on adjoining hill slopes. Hun-off from the catchment
basins was collected in channels and directed on to terraced
fields. Modern experimental research has demonstrated that
this system works successfully, and provided the proportions
between catchment area and cultivated area are correct,
cultivation can be supported where the total annl;al rainfall
is less than 100 mm.
Con tour Seepage Furro ws
A system has been developed in Zambia for conserving
run-off in seasonally water-logged areas near to rivers and
streams, known as dambos. A dambo is a grass covered plain
which slopes gently towards the river into which it drains and
which collects run-off from adjacent higher ground. It has
heavy hydromorphic soils which are :,~aierlogged during the
rains and which under natural conditions may remain waterlogged for a considerable part of the following dry season,
In this way dambos provide valuable grazing. As a result of
increasing pressure on agricultural land leading to soil erosion
and rapid run-off from catchment areas above the dambo,
together with cultivation and over-grazing within thedambo,
water passes through the area more rapidly and the land dries
out more quickly. The natural storage in the dambo can be
restored, and even improved, by the construction of contour
seepage furrows. These are small ditches following contour
34
Small Scale I rrigaticrn
levels across the dambo (Figure 12). These ditches, 15 cm
deep, 45 cm wide at the bottom are best dug by hand, and
the spoil placed on the down-slope side of the ditch to form
low embankments. The ditches should be spaced so that
there is a ditch line for every 0.40 m fall down the slope
(i.e. every 20 m on a 2% slope, every 40 m on a lOXislope),
and the spoil banks should be interrupted by a spillway
opening every 25 to 30 m (Figure 13). These openings are
necessary to allow free lnovement of storm water and avoid
damage to the system. The 3.0 m wide spillway opening is
improved by cutting a small drainage channel 5cm deep
down the centre of the spillway to carry small surplus run-off.
Contour
furrows
\
P
1 ines
/-----
Fig. 12 Con tour seepage furrows
:g
depth
0.15m
Fig. 13 Details of seepage furrows and spillway
By increasing the capacity of the dambo for holding water,
the contour seepage furrows will improve the quality of the
natural vegetation, reduce the erosive effects of overgrazing,
Moisture Conservation
Techniques
35
and, if so required, provide sub-soil water for seepage irrigation
well into the dry season. If cultivation is to be undertaken
where contour seepage furrows have been constructed, then
cultivation plots will need to be protected from spill flow
through the spillways by excavating drainage channels down
the slope between the seepage furrows.
Mulching
The evaporation of moisture from soil can be reduced by
mulching, a term describing the placing of material on the
soil which suppresses evaporation and conserves water within
the root zone. Mulching with a blanket of plant residues such
as dead weeds, straw, hay or other waste products is practised
traditionally
in many parts of the world. Gravel in iayers as
rhin as 5 to 10 millimetres can also be used.
Paper and polythene are now widely used as mulches.
Latex (liquid rubber), asphalt and oil have all been used to
establish vegetation in sandy desert situations, but a disadvantage of synthetic mulches is that they tend to be
expensive.
Soil Moisture Trap
An inexpensive development of the mulching principle was
evc\lved on a project in the Western Sudan in the mid-1960s.
A very thin, and therefore cheap, plastic sheet, perforated
with holes 2 mm diameter at 10 cm intervals was laid over
sandy soil and covered with a 5 cm layer of soil. A ridge of
soil was formed to enclose this prepared area (Figure 14).
Fig. 14 Soil Moisture Trap
36
Small Scale Irrigation
Rain falling on the prepared area was absorbed by the soil
surface and excess moisture passed through the holes in the
membrane into the soil below. After the rain had stopped the
blanket of soil above the membrane dried quickly, but
moisture remained in the soil below for much longer. In this
way, with successive showers of rain it was possible to build
up a store of water in the soil, sufficient to bring plants to
where under natural conditions this was not
maturity,
possible. Egg-plants, sown in seed beds and transplanted
during the short wet season in July, vvere cultivated successfully, yielding fruit the following October, while those in an
adjacent control plot without membrane storage failed for
lack of water.
Humid Culture
Where water is scarce, plants can be grown in an enclosed
system in which the water used is recycled. This process is
known as humid culture and it has been demonstrated that
crops and vegetables can be cultivated in this way both in the
United Kingdom, and under more severe conditions in Saudi
Arabia.
For this system the cultivation is enclosed in chambers,
usually consisting of polythene sheeting supported on metal
hoops to form semi-circular tunnel sections, which may be
from 80 cm to 3 m in height. To prevent moisture escaping
laterally through the soil, the edges of the polythene are
buried to 15 or 20 cm, or attached to timber or concrete
foundations.
Air inlets are provided for ventilation, and
the movement af air causes some moisture loss. Water is
introduced initially by irrigating and it is found that plants
will grow for many weeks before additional water is needed
to make up for losses.
Further Roading
M. Evenari, L. Shanan and N.H. Tadmor, The Negev: The Challenge of
a Desert. Harvard University Press, Cambridge, Massachusetts 02138,
USA, 1971.
M. Evenari, U. Nessler, A. Rogel and 0. Schenk, FieldsandPastumin
Deserts, Eduard Roether, Buchdruckerei
und Verlag, Darmstadt,
W. Germany, 1976.
Doxiadis Associates and Doxiadis lonides Associates, ‘Agronomic
Investigations in the 1964 Season’, Land and Water Use Survey in
Kordofan Province of the Republic of the Sudan, Document DOX-
Moisture Conservation
Techniques
37
SUD-A 35, UNDP/Food and Agriculture Organization, Via deile Terme
di Caracalia, 00100 Rome, Italy, April 1965.
#ore Water for Arid Lands, National Academy of Sciences, Washington
D.C., USA, 1974.
38
Chapter 5
Surface
Irrigation
Surface irrigation systems are those which supply water to
the land at ground surface level. They are also sometimes
known as gravity systems because the water flows under the
action of gravity and without the use of pumps. But because
surface systems often include the use of pumps on main
supplies, the term gravity is not appropriate to all surface
systems. There are seven principal surface irrigation methods
and these are basin, border, furro w, corrugation, wild flooding,
spate and trick/e irrigation.
Basin lrriga tion
The basin method of irrigation is the most widely tised,
and easiest to operate. Most of the rice in the world is grown
in basins, known in the East as paddies. Figure 15 shows
paddy basins in the Philippines. Many other crops such as
Fig. 15 Transpian ting rice, L won Island, the Philippines
Photo: Douglas Dickinsi
cotton, grain, maize, groundnuts and vegeta
basin irrigation, which can also be used in arc
each tree may have its own basin. The met
dividing a field into small units, so that e
!svel surface. It is therefore most suited to
be used on sloping land provided that the soii is
to allow levelling without exposing the sub
(levees, bunds, ridges or dykes) of earth 30 to 50
constructed round the area forming the ba
outlet controls for water (Figure 16). Th
with water to within about 10 cm of the top of
and the water is retained until it infiltrat
the excess is drained off. Basins may be o
square metre to several hectares.
The main disadvantage of basin irrigation is that t
interfere with the movement of animals and ag
equipment used in land preparation and cultivation.
the land is very flat there may be problems in d
excess water from the basins, and this can cause de
also encourages the breeding of mosquitoes. Basin irrigation
cannot be used for crops which are damaged by pro
Fig. 16 Small gated control structure fur basin or border irrigatiun
40
Small Scale Irrigation
standing in 8ater; it ay be used on most soiS ty
from sandy soils to ay, but heavier soils are preferable
because percolation I
The size of basins depends primarily on the rate of water
supply available, the slope of the land and t e texture of the
soil. As one of the purposes of basin irri
a uniform depth of ponded water over
basin should be filled as quickly as possi
of water supply ba
in light sandy soils w
smaller than basins
eavy clays. on flat la
larger than on slopi
nd. Table 2 gives
for various supply flows and soil types fo
Table 2 - Suitable areas for basins (sq metres)
Flow
Litreshec
10
20
50
100
Sand
65
13
32
Soil Type
Sandy Loam
Clay Loam
Clay
700
2,
4
4,i)OO
Water is distributed most evenly
surface is level and variations betwe
levels in a basin should
sloping, basins shou
following the contou
between the steps will
the amount of levelling which can be carried out without
damaging the soil by the removal of too much top-soil. Where
soils are shallow very little land levelling can be done, and
terraces will be narrow. Normally the step or drop between
terrace levels should not be more than 15 cm. Table 3 gives
suggested spacing of contour steps for different land slopes,
the wider spacing applying to soils where land levelling is
permissible and the narrower spacing to soils where it is not.
The size of the basins on each terrace can be determined
from Tables 2 and 3, as an example will illustrate. Suppose
basins are to be formed on land with shallow sandy-loam soil
sloping at 0.5%, and the available water supply is 20 !itres/
second. Assuming that the soil is too shallow to permit land
Ievelling, the terrace spacing, from Table 3, would have
to be 12 metres. A suitable area, from Table 2, would be
Surface Irrigation
41
Table 3 - Suitable spacing of terrace steps for basins (metres)
Land slope %
0.1
0.2
0.5
1 .o
1.5
2.0
3.0
4.0
Spacing
150-60
75-30
30-12
15-6
10-4
7.5-3
5-2
3.75-1.5
400 sq.m., so the length of each basin should be 400/12 =
33.3 m.
The levees or embankments enclosing the basins should be
formed by borrowing soil beside the embankments, and this
can be incorporated with land levelling if this is needed. The
banks should be constructed 0.75 to 1.O m high, which after
settlement would be 0.3 to 0.5 m high, with a base width
1.5 to 1.8 m and top width 15 cm to provide a pathway to
the field.
Water can be supplied to the basins in two ways. It may be
delivered to each basin from outlets on a supply channel, or
it may be allowed to flow successively through a series of
basins from a single channel outlet. For most crops, which do
not need water continuously,
the first method of water
supply should be used. The second, continuous supply
system is suitable for paddy rice cultivated under constant
flooding, or for a single crop in several basins all requiring
the same amount of water at the same time.
The time required to fill a basin can be calculated from the
rate of flow of the supply, the area of the basin and the
depth of water required to fill it. For example, if a basin
800 sq.m. in area is to be filled to a depth of 40 cm with a
20 I/s supoly, the time required will be approximately:
0.4 X 800 X 1,000 = 4 4 hours
20 x 3,600
’
This is the approximate time because we have not allowed
for some of the water to be absorbed by the soil during filling.
The amount of water absorbed by the soil will depend on the
42
Small Scale Irrigation
infiltration
rate of the soil (see Chapter 11). For the basin
areas and flow rates in Table 2, the time calculated as in the
example should be increased by lo%, so that 4.4 hours
should be corrected to 4.8 hours.
Border lrrt$a tion
Border irrigation is suitable for large field units of 4
hectares or more. A field is divided by borders (low banks)
into a series of strips which may be from 3 to 30 m wide, and
from 100 to 800 rn long, with an even, moderate slope along
their length. Water is admitted at the ;,r\r, end of a strip and
allowed to flow evenly across the full width of the strip and
at nearly uniform depth. The rate of water application is
adjusted so that the soil receives its correct amount of water
as the sheet of water advances down the strip. When the
water reaches the iower end of the strip, irrigation should be
just about complete.
This system requires a relatively large flow of water, as a
whole strip is covered with water during irrigation. It is best
suited to deep, medium-textured
soils with deep rooting
crops. The land slope down the strip on fairly heavy soils
should be not less than 0.2% to ensure water flow. Lighter
soils, with their greater rates of infiltration, require steeper
slopes, up to about 2%. Cross slopes should be eliminated
wherever possible. This system is more appropriate to large
scale than to small scale irrigation, and can be used for a
variety of crops including grain, Iucerne, pasture and orchards.
Border strips should be as long as possible without impairing water application efficiency. Long strips up to 800 m
can be used on flat land with soils with very low infiltration
rates. On soils with high infiltration rates, the length of strips
may have to be 100 m or less. The length of strips also
depends upon the flow rate of the irrigation supply. Too
small a flow will not reach the end of a strip if it is too long.
Table 4 gives a guide to the widths and lengths of border
strips for different supply flows and soil types.
The levees or banks forming the borders to the strips
should be 20 to 25 cm high after settlement and triangular in
cross-section. Water is supplied to the top of a border from a
which may be
supply channel through outlets or turnouts,
concrete or wooden gate structures (Figure 16), pipes placed
in the channel bank with gate controls or portable syphons
Surface Irrigation
43
Table 4 - Suitable dimensions for border strips
Type
Sands
Soil in filtration Rate,
mm/hr
Flow
litresksec
25 and over
0.2
0.4
0.8
15-30
10-12
5-10
60-90
60-90
75
220-450
100-120
30-70
Loams
7 to 25
0.2
0.4
0.8
15-30
10-12
5-10
250-300
90-180
90
70-140
40-50
12-25
Clays
2.5 to 7
0.2
0.4
15-30
lo-12
350-800
180-300
45-90
3040
I
'
I
SIope
%
Dimensions
Width
Length
m
m
Yaced over the bank.
b-row Irrigation
In the furrow method of irrigation, small channels (furrows)
carry water down or across the slope of the land to wet the
soil, and crops are grown on ridges between the furrows.
The method is best suited to deep, moderately permeable
soils with uniform, relatively flat slopes (preferably not
steeper than 3%), and to crops which are cultivated in rows,
such as vegetables, tomatoes, cotton, maize and potatoes.
Furrow irrigation can be used in fields or plots of any size,
and must be employed where surface irrigation is to be
applied to crops which cannot tolerate standing in water.
In contrast to basin or border irrigation, furrow irrigation
wets only part of the ground surface (between one fifth and
one half). This reduces evaporation losses and the land dries
out more quickly after irrigation. Uniform furrow irrigation
requires considerable skill both in initial land grading and in
controlling the water over local irregularities in land profile.
Furrows are usually V-shaped in cross-section, 25 to 30 cm
wide at the top and 15 to 20 cm deep. Wider U-shaped
furrows with a greater wetted area are sometimes used on
soils which take water slowly. The spacing of furrows will
depend on the crops to be grown, the space needed between
rows for tillage and weeding and the lateral movement of
water through the soil. The beds or ridges between the
furrows may be flat or slightly rounded. Many crops are
cultivated in rows 0.75 to 1.OOm apart, with one row on
44
Small Scale Irrigation
each ridge (Figure 17a). Vegetables are often planted with
two rows 40 cm apart on each ridge (Figure 17b). Because
water moves downwards through permeable soils more
quickly than through impermeable soils, furrows should be
closer together in more permeable soils
0.75
to i.oa rtj
t
lb)
Fig. 17 lrriga tion fun-0 vvs
For uniform irrigation, furrows should have a constant
slope along their length. If this is not possible because of
the topography, a slightly increasing slope is preferable to a
decreasing slope. If the furrow slope is too steep, the velocity
of the water will produce erosion, resulting in the formation
of gullies and loss of soil. To avoid erosion the slope should
not be more than 2% for most soils. Light sandy soils erode
more easily than heavy clays. Where the natural land slope is
more than 2%, the furrows should run at an angle to the line
of greatest slope, and on steeply sloping land they may run
almost along contours. The minimum practical slope for
furrows is O.l%, although flatter slopes are feasible.
The length of a furrow will depend on its slope and the
texture of the soil. If the furrow slope is too flat and the soil
is permeable, water may never reach the end of the furrow
even if the water supply fills it at entry. If the slope is too
Surface lrrlgation
45
steep the water will collect at the far end, and land at the
head of the furrow will be shr?rPof water. For slopes up to
about 1% in medium soils, furrows may be 300 to 400 metres
long, but actual lengths have to be determined largely by trial
and error in each situation. Table 5 gives suggested maximum
furrow lengths for different soils, slopes, and water applications. The application depth is the total depth of water
applied to a field in one irrigation (see Chapter 1 1).
Table 5 - Suggested maximum furrow lengths (metres)
Furrow
Slope %
0.05
0.1
0.2
0.3
0.5
1 .o
1.5
2.0
C/a ys
Application,
mm.
Loams
Application,
mm.
200
300
100
200
75
125
400
450
510
570
540
450
400
320
400
500
620
800
750
600
500
400
270
340
370
400
370
300
280
250
400
470
530
600
530
470
400
340
90
120
190
220
190
150
120
90
190
220
300
400
300
250
220
190
Sands
Application,
mm.
Water is admitted to the head of esch furrow, either one at
a time or in groups, and ,the rate of flow is adjusted so that
the furrow flows full but not overflowing, and so that soon
after the water has reached the end of the furrow the required
amount hasseeped into thesoil on each side of the furrow and
beneath it, to satisfy the irrigation requirement (Figure 18).
The rate of flow into a furrow depends primarily on the
intake rate of the soil and the length of the furrow, and will
be determined largely by experience in the field. Furrows in
light sandy soil will accept water more rapidly than furrows,
in clay soil. Table 6 gives infiltration rates for various soil
textures and suitable furrow flow rates per 100 metre length
of furrow, for furrows 1 metre apart.
For furrow spacing other than 1 metre, the inflow figures
in Table 6 should be altered proportionately,
the inflows
being lower for closer spacing and higher for wider spacing.
The figures obtained from the table should therefore be
multiplied by the actual spacing. Thus if 0.6 I/s is the appropriate rate of flow for 1 metre spacing, the flow for 80 cm
spacing should be 0.6 x 0.8 = 0.5 I/s.
-
46
Small Scale Irrigatic>n
Fig, 18 Furrow irrigation
Anglicana, Argentina)
at Mision ia Paz (Photo: Mision de la Igfesia
100 metres of furrow length. Furrow spacing 1 metre.
Infiltration Rate
mm/h our
Fwrro w Inflow
l/see/l 00 m length
l- 5
5-10
10-20
0 03-0.15
0.15-0.3
0.3 -0.5
0.5 - 0.8
0.8 2.7
30-100
The time that water is allowed to flow int
depends on the water application required. In
is often a matter for the farmer’s judgement, but
of application (D) and the rate of inflow into th
are known, the time of flow can be calculated fro
formula.
where D is in millimetres
S = spacing of furrows in metres
L = length of furrow in metres
Q is in Iitreslsecond.
If, for example, thesoil is a sandy-loam, and an appropriate
furrow inflow rate (Table 6) is 0.7 I/s per 100
to a furrow 285 m long should be 0.7 x 2.85 = 2 I
required application is 150 mm and the f
apart, the time of watering for each furrow wi
T ,150 x 0.8 x 285
3,600 x 2
If the available supply is 20 I/s, then this wi I water 2W2 =
10 furrows at the same time. The field area watered by a
furrow is the length of the furrow mul
ed by the spacing,
or 285 x 0.8 = 228 sq.m. The area whi
can be watered at
the’ same time is therefore 10 x 228
2,280 sqm, and
watering this area will take 4.75 hours. If the tota
cultivation in the field is 285 m x 100 m, or 28,500 sq.m,
the time nEeded to irrigate the whole field with a 20 I/s
supply will be:
28*500 x 4.75 = 59.4 hours
2,280
This is very nearly 60 hours, and if irrigation is carried out
for 12 hours a day, it would take 5 days to water this field.
For annual crops such as grains and vegetables, furrows
are constructed as part of the ploughing operations each year.
48
Small Scale Irrigation
The field channels supplying the furrows may or may not be
re-formed during ploughing, depending on the irrigation
layout. On gentle uniformly sloping land with long furrow
runs, the field channels are likely to be fairly permanent and
undisturbed by ploughing and other activities. Where; there
are short furrow runs and therefore many field channels, it
may be necessary to reconstruct the channels annually.
Outlets from the field channels to the furrows may be
made in various ways. The most common practice is to cut
openings in the field channel bank, at the same time blocking
the channel with soil or a timber or metal check just downstream of the lowest furrow of a group to be irrigated. When
the watering of one group of furrows is completed, the
channel bank is repaired and the block in the channel moved
to a new position upstream or downstream for the next
group of furrows.
Although widely used, the practice of breaking field
channel banks is not efficient because, unless the operators
are very skilled it is not easy to ensure equal and uniform
flow into the ,furrows. A popular and more efficient system
is to use syphon pipes which carry the water over the channel
bank. These may be made of aluminium, rigid plastic or
synthetic rubber. To start a syphon flowing the whole pipe is
immersed in the water in the field channel. Keeping one end
under water the other end is blocked by placing a hand over
it and the blocked end lifted over the channel bank and
lowered until it is below the level of the water in the channel.
When the hand is thzn taken away, the water will flow. For
flexibility
of operation it is better to use more than one
syphon pipe for each furrow, so that adjustments to the flow
can be made by adding or removing a pipe. The flow in a pipe
can be reduced by raising the lower end to reduce the head.
The flow through a pipe depends upon its diameter, length
and the head across it, which in the case of syphons is the
difference between the water levels in the field channel
and in the furrow when it is flowing. For short syphon pipes,
the lengths of the pipes can be assumed constant, and flows
taken as varying with the diameter and head. Table 7 gives
syphon flows for different heads and pipe diameter.
Corrugation irrigation
A variation of the furrow method is corrugation irrigation,
Surface Irrigation
49
Table 7 - Syphon pipe flows (littes per second)
Diameter of syphon. mm
5
0.05
0.19
0.42
0.75
1.17
10
20
30
40
50
Head, cm
15
10
0.07
0.26
0.53
1.06
1.65
0.08
0.32
0.73
1.29
2.02
20
0.09
0.73
0.84
1.49
2.33
in which close furrows about 10 cm deep are used without
raised beds. The corrugations are spaced 40 to 75 cm apart,
and during irrigation the whole soil surface is wetted. This
method is used for close-growing crops which are not cultivated in rows, such as grain, pasture or lucerne.
As with normal furrow irrigation, the length of the corrugations depends on the slope of the land and the texture
of the soil. This method can be used on steeper slopes than
furrows and the corrugations are usually run down the
greatest slope. Cross fall should be avoided if possible,
and if unavoidable it should never exceed the slope of the
corrugations. Corrugation irrigation is best suited to medium
textured soils (silt loams or clay loams), in which water can
move easily laterally through the soil. Tabie 8 gives suggested
maximum length and spacing for corrugations for different
slopes and for deep and shallow soils.
-
-..
Table 8 - Length and spacing of corrugations
Slope
%
Clays
Length Spacing
m
m
Loams
Sands
Length Spacing Length Spacing
m
m
m
m
Deep
Soils
2
4
6
8
10
180
120
90
85
75
0.75
0.65
0.55
0.55
0.50
130
90
75
60
50
0.75
0.75
G.65
Q.55
0.50
70
45
40
30
-
0.60
0.55
0.50
0.45
-
Shallcw
Soils
2
4
6
8
10
120
85
70
60
55
0.60
0.55
0.55
0.50
0.45
90
60
50
45
40
0.60
0.55
0.50
0.45
0.45
45
30
-
0.45
0.45
-
50
Small Scale Irrigation
The rate at which water is admitted to t
depends on the infiltration characteristics of
length of the corrugations. Table 9 gives infiltration
different soil textures and suitable inflow rates
length of corrugations spaced at 0.65 metres.
rates for
etre
Table 9 - Seil infiltration rdes and suitable corrugation inflows
per 100 inetresof corrugation length. Corrugation spacing 0.65 m.
h filrration Rate
mmhour
Corrugation In f/o w
l/m/lW
m length
the same way as those for furrows (pa
is complete, the pipeline IS mov
the field, and so on until the
watered.
As with all irrigation equipment, the use of gated pipes
makes the operation more costly than using earth channels.
Not only must the cost of the pipelines be considered, but
also the cost of raising the water up to one metre to provide
the necessary pressure in the pipeline. Professional advice
should be sought in designing a gated pipe system.
Surface Irrigation
51
Wild F Jooding
Wild flooding, a term which originated in the USA, is a
system used primarily for steep land with low-income crops
where the uniformity of water distribution
is not an important factor. Water is delivered at several points from a
supply channel running along the upper edge of a sloping
plot or field, and is allowed to move freely down t
It requires skill in the selection of points for releasi
but otherwise is a simple system involving a minimum amount
of land preparation and does not require much I
operation. This method can be used for perenni
crops which protect the soil from erosion. It is dangerous to
use it on light erodible soils.
Spate lrriga tion
The use of short flood spates from upland and mountainous
areas to irrigate land in plains and lowlands where rainfall is
insufficient for cultivation is known as spate ir
practised traditionally in many parts of the Mi
notably in the south and east of the Arabian peninsular, in
the Sudan on the borders of Ethiopia, and in Ethiopia.
It is, in effect, another form of the run-off ,farming described
in Chapter 4. An annual rainfall which may vary between
ents during a
50 and 300 mm falling on steep rocky catc
ods in ‘wadis’
short rainy season produces short torrent
(water-courses which are normally dry) which can be diverted
on to cultivable land with deep soil capable of absorbing
large quantities of water in a relatively short time. Because
the soils are deep their moisture capacity is great enough to
support a crop on one or two good inundations.
The method is inevitably hazardous because the flood
flows are quite uncontrolled and traditional earth and brushwood diversions are easily carried away. Water-courses
emerging from hills into plains have a habit of changing
their courses every few years, and this can seriously affect
diversion arrangements. The introduction of more permanent
masonry or concrete diversion and headworks in recent
years has helped to stabilise some of these systems. For
irrigation on a small scale, and where the right conditions
are found, there is much to be said for spate irrigation.
Trickle irrigation
The application
of water to the soil at a very low rate
52
Small Scale Irrigation
(2 to 10 litres per hour) through small outlets (tricklers or
emitters) is known as trickle, drip or dribble irrigation.
Water is supplied to the tricklers through polythene pipes
(12 to 16 mm diameter), laid along rows of crops so that the
trickier discharges on to the soil in the immediate vicinity of
the crop stems and roots. The water is supplied under low
pressure (1 to 3 atmospheres), and the supply-line may
include equipment for injecting fertilizer into the water
supply. The small orifices (openings) of the tricklers are
liable to clogging and therefore the water supply is filtered.
This system is expensive because of the equipment needed,
and it is therefore limited to high-income market-garden
crops. It is very economical in water use and highly suitable
for light, controlled irrigation applications.
Further Reading
Booher, Surface higation,
Food and Agriculture Organization of
the United Nations, Via delle Terme di Caracalla, 00100 r?;Drne, Italy,
1974.
Ivan E. Houk, /rrigation Engineering, Vol. I, John Wiley and Son, New
York, USA, 1951, Ch.16.
Josef D. Zimmerman, Irrigation, John Wiley and Son, New York,
USA, 1966, pp.1 07-l 43.
Bruce Withers and Starrley Vipond, /rrigaFion: Design and practice,
B.T. Batsford Ltd, 4 Fitzhardinge Street, London WlH OAH, 1974,
pp .35-46.
FAO Irrigation and Drainage Paper 14, 7iickle lrrigation, Food and
Agriculture
Organization of the United Nations, Via delle Terme di
Caracalla, 00100, Rome, Italy, 1973.
L.J.
53
Chapter 6
’
I rrigatio
In many parts’ of the world conditions are favourable for the
cultivation of crops by the application of water below ground
surface. This method of sub-surface irrigation (also known as
sub-irrigation) is practised widely on a small scale wherever
there is low lying alluvial land adjacent to a river or stream
and where the land and river bed are sufficiently permeable
for a water table to be maintained in the ground at a suitable
depth for plant growth. Where rivers and streams are uncontrolled, adjacent lands may be inundated at times of high
flood, and if the flood period is followed by a long dry
season, the water table in theadjacent landswill fall gradually,
corresponding with the seasonal regression of the water
source. The fall in river levels is usually much .Tore rapid
than the fall in groundwater level, which allows time for
crops to be established and for their roots to pursue the
receding water table.
In some countries in the Middle East and North Africa, in
places such as along wadi (dry stream) beds or in coastal areas
where a water table is not too deep, but out of reach of plant
roots, a method is practised of digging holes over the water
table and planting dates or vegetables in them.
Water Table Control
Measures to control river basins by means of dams or
barrages, or to drain and reclaim low-lying and flooded land
will affect the behaviour of water tables in these areas. A
reservoir may create a permanent water table in peripheral
alluvial land, fluctuating with the level of the reservoir itself,
where previously the water table was seasonal or transient.
Drainage and reclamation works may help to lower a natural
water table during a wet season and, by controlling the
drainage outflow in an ensuing dry season, these works
may help to maintain groundwater at a level suitable for
cultivation. This practice is common in many large alluvial
plains.
By keeping water below ground level, it is used efficiently
54
Small Scale Irrigation
Normal irrigation losses t
water evaporation are practically eliminated and
colation losses are controlled by the rate of drainage from the
area. There is, however, a serious danger with this ethod.
If either the water or the soils contain harmful salts, these
salts may be brought to the surface of the ground by capillary
and economically.
action, and then it will be impossible to remove th
by heavy applications
of water at the surface a
lowering of the controlled water table so that t e salts will be
leached out .
Sub-soil Pipes
Attempts have been made to irrigate through perforated
pipes buried in the soil. To be effective, as many pipes need
to be laid in a field as there are furrows in furrow irrigation,
and this makes the system extremely costly rn equipme
If the pipes are to be out of danger of ploughing, they sho
be at least 40 ems below ground level, so heavy installation
costs are added to the high equipment costs.
Pitcher lrrigation
A new technique, using ordinary baked
pitchers has been developed experimentally a
Pitchers of about 30 cm diameter were burie
diameter, 60 cm deep) filled with manured
the pitchers being just above ground lever. Press
were sown around the pitchers, Jvhich we
good quality irrigation water, adding water
Gourds, pumpkins and melons were cultivated succesfully in
this way.
A similar method has been practised experimentally
in
Iran using the Kuzeh, a clay jar with a narrow neck a
15 cm high and 8 cm in diameter. Several jars were buri
the ground and connected to a hose-pipe water supply by
short lengths of plastic tubing, Plants were
wetted soil round the jar, using both pure and
Further Reading
R.C. Monclal, ‘Pitcher Farming’. Appropriate
Technology
Vol. 1,
intermediate Technology Publications Ltd, 9 King Street, London
WC2E SHN, England, 1974, p.7 (reprinted from World Crops, March/
April 1974).
‘Kuzeh Pot Irrigation’, lrrinews, No.1 1, Volcani Center, P.O. Box 49,
Beit Dagan, Israel, 1978.
Chapter 7
rrigatio
Watering Can
The simplest piece of overhead irrigation e
watering can (Figure 19), much used for small-scale
in temperate regions, but w ich is not so co
developing countries in tr ical or sub-t
Since all the water has to be carried by han
can method is limited to small plots wit
source of water.
Fig. 19 Watering can
The size of plot which can be irrigated depends large
upon its distance from the source, and the time that it takes
to fill the can at the source. One man with
an easily accessible canal, river or shallow
a plot of about 500 sq. metres. If he has to fete
from a point 100 metres away, he could cnly m
half this area. All other overhead systems of irrigation require
water under pressure. This pressure may be derived from an
elevated source such as a tank or reservoir, or it may be
produced by a pump.
Hose pipe
Wherever there is a piped water distribution system, it is
possible to connect a hose pipe to a tap or outlet, and,
56
Small Scale Crrigation
provided there is sufficient pressure in the water as it emerges
from the hose pipe, this can be used with a nozzle to throw
water over a plot of land. For example, if water is available
at a standpipe or tap at a pressure head of 20 m, then one
man, using a 12.5 mm diameter hose pipe 200 m long and
working continuously 9% hours a day, could irrigate a plot
of about 800 sq. metres. If his pipe were only 100 m long,
then he could irrigatea plot of about 1200 sq. metres, because,
with less friction, the shorter pipe will deliver more water in a
working day.
A major disadvantage of this system is likely to be the cost
of the water supply. An 800 sq. metre plot might require
160,000 litres in a six-month irrigation season. This quantity
would be equivalent to six months of domestic water for
45 to 20 people in a rural area. Where piped water for domestic
use is in short supply, it is very unlikely that it would be
permissible to use it for irrigation. But there are places where
piped water is available for irrigation and where the hose pipe
can be used with advantage.
Sprinkler Systems
Of the various systems of irrigation the sprinkler method is
the nearest to natural rainfall. Water, distributed under
pressure through pipes, is discharged as a jet or spray into the
air over the land to be irrigated. Because of its application
efficiency,
its adaptability
to different types of terrain
without the need for land levelling and because of its ease of
operation, sprinkler irrigation is used extensively all over the
world. However, it involves high initial costs for pumping
machinery, piping and portable field equipment, and because
supplying water under pressure uses a lot of energy, it is
costly to operate. For a scheme drawing water from an
existing river or canal, initial costscould be between US$l2OO
and $2000 per hectare. If the existing water source were at
about ground level, annual costs for pumping energy and for
the operation and maintenance of the system, excluding the
farmer’s time and labour could be between US$lOO and $200
per hectare.
Sprinkler irrigation is best suited to medium and large
farms of IO hectares and above. It is also feasible to use
sprinkler systems on small farms down to about half a
hectare, but small units are relatively more expensive in
Overhead Irrigation
57
initial cost (over US$2000 per hectare) and relatively less
efficient in water use. Sprinkler irrigation is particularly
adapted to light applications and therefore to coarse textured
soils (sands and loamy sands) which have low moistureholding capacities, because uniformity of distribution is not
so dependent on the depth of application or on soil intake
rates. Distribution is, however, liable to be distorted by the
wind, so that areas with high prevailing winds are less suitable
than areas with moderate winds.
Sprinkler irrigation pipework may be permanently buried
below ground, partly permanent and partly portable, or fully
portable in which all the pipework is laid on the ground
and moved as required. Fully portable systems are most
appropriate to small scale development.
For small scale intensive cultivation, rlozzle lines (pipes
with nozzles at regular intervals) may be u3ed. These apply
small quantities of water at low rates and are therefore
suitable for nurseries and seed beds. Installation cost is high,
and because the nozzlesarevery small, they are easily blocked
and need frequent cleaning.
The most common type of sprinkler used with portable
systems is the rotating head sprinkler, consisting of a head,
with one or two nozzles, which is rotated slowly by the
action of the water passing through it, and which waters a
roughly circular pieceof land round the sprinkler (Figure 20).
Fig.20
Typical sprinkler head (Photo: Wright Rain)
58
Small Scale Irrigation
Rotating sprinklers operate under a wide range of pressures
and discharges. For every operating pressure there is an
optimum nozzle diameter to give the best dispersion of the
water. Sprinklers are classified broadly into three groups
according to their operating pressures, and these are given
together with other characteristics in Table 10.
Table 10 - Typical rotating sprinkler characteristics
Characteristics
Pressure
MediumPressure
HighPressure
Operating pressure, atmospheres
Nozzle diameter, mm
Discharge, I/s
Diameter of coveragt!, m
Sprinkler spacing, m
l-2
1.5-6
0.06-l
6-35
9-18
2-5
6-20
0.25-l 0
25-80
18-54
5-l 0
20-40
1O-50
80-l 40
54-l 00
LOW-
b
High pressure systems involve high energy costs for pumping, and medium and low pressure systems are therefore more
appropriate to the small scale operator. Low pressure systems
are used for irrigating orchards and tree crops below the leaf
canopy, for soils with high infiltration rates and for covering
small areas. Medium pressure systems cover larger areas and
are generally used for field crops. High pressure systems
are used for high standing crops such as sugar cane. Giant
sprinklers, consisting of a single high pressure nozzle operating
at up to 10 atmospheres and covering a circle over 100 m in
diameter, can be used for irrigating sugar cane and tree crops
above the leaf canopy.
For very small plots, up to about 1000 sq. metres, a single
small rotating sprinkler will be adequate. For larger areas,
sprinklers are mounted, usually on 25 mm diameter standpipes, 1 to 2 m high, at equal inkrvals along the pipe which
supplies them, so that the pipe-line with its sprinklers waters
a strip of land. When this strip of land has received its quota
of water, the pipe and sprinklers are then moved laterally to
water a second strip of land adjacent to the first. In this way,
a field unit can be watered by one or two sprinkler lines
(Figure 21). The sprinkler line (or lateral) is made up of
sections and all the equipment is provided with special joints
and couplings so that it can be dismantled, moved and reassembled quickly. The lateral pipe may be supplied directly
from a portable pump for a small scheme, or from a hydrant
Overhead Irrigation
59
on a buried pressure pipe system on a large scheme. The
portable lateral line is well-suited to all crops which are
cultivated in rows.
Fig.2 1 Simple sprinkler layout (Illustration:
Wright Rain)
The circular area wetted by one rotating sprinkler depends
upon its discharge and operating pressure. The precipitation
rate over this area is not uniform, hut by arranging the
wetted patterns of sprinklers to overlap \ F igure 2 1) reasonably
uniform applications are achieved. The overlap is usually 50%
or more of the diameter of the wetted area. Lateral pipe lines
are made up of standard pipe length: between 5 and 12 m,
and the spacing of sprinklers along a lateral and the distances
between lateral positions are in multiples of pipe lengths.
Owing to friction losses along a lateral pipe, the discharge
from each sprinkler on a lateral will fall progressively along
the line. The number of sprinklers on a lateral, and therefore
the length of the lateral, will be determined by the permissible
drop in discharge between the first and last sprinklers. 7 his
drop should not be greater than 10% of the discharye of the
first sprinkler. The length of the laterals may also be decided
in relation to the shape and size of the fields.
A sprinkler irrigation system is designed to provide a
calculated depth of water at a fixed rate of application. The
application rate is determined from the intake characteristics
of the soil. Water intake rates for overhead irrigation are
given in Table 11. These rates are slightly lower than those
for surface irrigation (Table 9) as there should be no ponding
of water at the soil surface.
60
Small Scale Irrigation
Table 11 - Water intake rates for overhead irrigation
Soil type
Clay
Clay-loam
Silt-loam
Sandy loam
Sand
In take rate mmhour
1-5
6-8
7-l 0
8-l 2
1O-25
The amount of water which reaches the soi
less than the discharge of the sprinklers becaus
evaporation and wind in the air, by evaporatio
foliage and wet soil, and by deep percolation in
ratio of the effective water application to the tota
from the sprinklers, is known as the application efficiency,
and this is usually from 70% to 80%.
The required depth of application is talc
soil characteristics, as described in C apter 11) a
frequency of irrigation from the crop water requirements as
described in Chapter 9.
There is a wide choice of sprinkler equipment availa
there are many possible combinations ef sprinkler spacing,
nozzle discharge, and operating pressure to give the requ
watering performance. It mey therefore be necessary to
professional advice in choosing the appropriate e
and designing a sprinkler layout. The following example
illustrates how a simple scheme might be designed.
A farmer proposes to use sprinkler irrigation on a piece of
land 160 m x 120 m adjacent to a river from which he can
pump water (Figure 22). For the crops which he intends to
grow, a medium pressure system will be suitab e, and from a
study of the climatic and soil conditions he has found that he
will need a maximurn application depth of 60 mm every 10
days. The maximum intake rate of the soil is 8.5 mm/h, and
with an application efficiency of 80%, the gross application
would be 75 mm, at a maximum rate of 10.6 mm/h.
Because there are many different type.; of sprinkler irrigation equipment available, each with a range of sprinklers
with different specifications and characteristics, there may be
several systems and layouts which will adequately meet the
farmer’s requirements. In this example we consider two
Overhead I rriganiow
--"
--
---
16Qm
--
---+
I
i
la)
River
Fig.22 Possible arrangements for B smell sprinkler i~i&ation system
62
Small Scale Irrigation
Table 12 - Example of specifications and calculations for a small
sprinkler irrigation system
Details
Units
Sprinkler specifications
Operating pressure
Nozzle diameter
Discharge per sprinkler (q)
Spacirig (S)
Application rate (r-1
arm
mm
I/s
m
mm/h
Field details
Number of sprinklers on
lateral n _ 120
Possible arrangements
(a)
(69
2
4
3
4.5
.37
i2 x 12
9.5
I-IO
13
10
I30
18
13
h
h
h
h
7.2
0.3
7.9
15.8
7.9
8.7
ET
17.2
9
6.5
3.25
3.70
S
Number of lateral positions
Application time, to apply
75 mm gross, t - 75/r
Add time for moving equipment
Total time for one position
Time for 2 positions per day
Number of days required for
complete irrigation,2
2
Pump supply required
Q = q.n.
days
I/s
possibilities, the details of which are given ir! Table 12
Of the two possibiiities ia) mdy cost a little more in
equipment, needing 13 sprinklers and risers instead of 10.
But in other respects it appears to be the better arrangement.
The lower operating pressure and lower discharge mean less
energy for pumping and therefore lower operating costs.
Regarding the time programme, (a) fits a lo-day cycle
better than (b). On the other hand a farmer may prefer a
slightly faster cycle (6% days instead of 9 days) if it gives
him more time for other activities. If he is prepared to
irrigation for 24 hours a day, scheme {a) \rJouici i&t: G &vs
and scheme (b) just over 4% days.
Overhead irrigation
63
It will be clear that many factors need to be considered in
selecting equipment .and planning a sprinkler irrigation
system, and it may therefore be prudent to seek professional
advice on these matters.
Further Redng
Arthur F. Pillsbury, Sprinkler irrigation, Food ancl Agriculture Organizattion of the United Nations, Via delle Terme di Caracalla, 00100
Rome, Italy, 1968.
Josef 0. Zimmerman, Irrigation, John Wiley and Son, New York, USA,
1966, Ch.9.
Bruce Withers and Stanley Vipond, Irrigation:
Design and Practice,
B.T. Batsford Ltd, 4 Fitzhardingc Street, London WlH OAH, England,
1974, pp.47-54.
64
PART III
PLANNING
AND DESIGN
Chapter 8
Climatic
Factors
The importance of information about the climate when
planning for irrigation has been mentioned in Chapter 3, and
Appendix B explains hew the most important climatic
factors, rainfall, temperature, evaporation, humidity and
sunshine are measured.* In this chapter we describe how this
information can be used.
Most of the countries where irrigation is needed fall Into
one of three principal climatic regions: mediterranean,
munsoon and wet tropical. The climatic characteristics of
each of these regions are different, and farmers will view their
irrigation possibilities in different ways accordingly.
Mediterranean Regions
The Mediterranean Regions take their name from the
conditions common to the countries bordering the Mediterranean Sea, and include not only Mediterranean countries
but also other parts of the Middle East, the Black Sea area,
the west coast of the United States, the coastal belt of Chile,
the southern tip of South Africa and parts of South and
Western Australia. For the purpose of this section semiarid regions which have seasonal conditions similar to the
Mediterranean regions (but drier) have also been included.
Examples of these are central Arabia, Iraq and parts of the
western United States. In all these regions most of the rain
falls in the cool winter months and the summers are dry and
warm or hot. Where winter rainfall is usually adequate, rainfed crops are grown in the winter; irrigation may be needed
either to supplement a shortage of rainfall for the winter
crops, or to extend cropping into the summer months. When
irrigation is needed in the winter it is clearly important to
know something about the patterns of rainfall in order to
plan irrigation requirements. If irrigation in the summer
months is contemplated, information will be needed about
*Wind is also an important climatic factor, but its measurement
sive enough in many countries to be of general practical value.
is not yet exten-
Climatic
Factors
65
significant rainfall at the beginning and end of the summer
and about the length of the summer period.
If the farmer is planning to irrigate during the summer
months he will almost certainly know from experience which
these morrths are. But rainfall records will be useful in
planning his cropping seasons, and also in assessing the
capacity of his sourceof water for irrigation if he is proposing
to use a small local source such as a spring or stream. If the
farmer has no previous experience of growing crops under
irrigation in the summer, then a study of records of temperature and humidity during the summer months would
help him to choose the most suitable varieties for the hot
season.
Monsoon Regions
The Monsoon Regions lie in the tropics and sub-tropics,
and they are characterised by a dry, and sometimes very
dry winter and usually a well-defined wet season in the
summer. These regions include the densely populated countries of the Indian sub-continent and southeast Asia, much
of central and tropical South America and of northern
Australia. The natural time for cultivation is with the summer
rain, and irriga&n may be needed either to supplement rain
when it is deficient, or to enable crops to be grown in the dry
winter months,
Information about climatic conditionswill assist the farmer
just as for the Mediterranean regions, except that the seasonal
conditions are reversed. Temperature and humidity will be
particularly
significant if consideration is being given to
irrigating in winter crops or vegetables which are traditionaliy
grown in the humid summer months.
The Monsoon Regions include many of the main rice
growing areas of the world in South-East Asia where irrigation
has been practised for hundreds of years. As paddy rice is
grown under flood irrigation, rainfall tends to supplement
the irrigation; rainfall records will indicate the proportion of
total water requirements which may be expected from natural
rainfall, and will thus contribute to the economical use of
irrigation water. If the quantity of rain falling in a heavy
storm is measured, then the irrigation supply should be
reduced by an equivalent amount. In practice this may not
be easy to achieve in an open channel supply because of the
66
Small Scale Irrigation
time lag betwee reducing the supp
reduction being f t at the field.
the
Wet Tropical Regions
Although the Wet Tropical Regions, which lie on or near
the equator and at low altitudes, have
occurs for most of t
year. As temperatur
quickly exhausted
need ,for irrigation.
Congo Basin, and eq
South America.
With no clearly defined winte
suitable crops can be
regions, provided that
out the growing season, and provid
sunshine for maturin
annual rainfall patter
are two identifiable
dry periods. Under such
rainfall and other
cropping patterns can
climate.
Sometimes sunshine f
factor which determines
planting dates for a particular crop.
the best sunshine period occurs after the second rains and
that the crop is planted during the first rains. If the total
rainfall in the two seasons is insufficient to mee’t t
requirements, irrigation will be needed
rainfall, particularly at the critical time
during the dry interval between th
The Use of Rain faN Records
Because rainfall varies from year to year in tota
and in its timing and pattern each year, it is necessary to
measure rainfall as accurately as possible for a number of
years. The length of the record needed is discussed in Chapter
15. A minimum of five years is usually considered essential,
but even records for one complete year are better than
nothing. In places were the average rainfall in the wet season
is less than 500 mm, the amounts of rain each year are very
Climatic
Factors
67
variable. Short term records may therefore be unreliable.
For example, suppose records of annual rainfall for five years
gave the following results:
Year
mm
1
440
2
580
3
290
4
310
5
380
A vet-age
400
To the farmer planning to irrigate the low rainfall years are
important, in this case years 3 and 4. If he had only measured
rainfall in years 1 and 2, he would have little idea of what
was going to happen in year 3. Hence 5 years’ records are
better than 2 or 3. But because climate shows trends, with
successions of wet years and successions of dry years, records
for longer than 5 years should be obtained if available.
In this example, the farmer may have found that during
the five years of records he got his best crops in years 1 and
5, and that in the wettest year, 2, results were poor because
the wet season lasted for too long, and this damaged the
ripening crop so that he had a bad harvest. These observations
would tell him that the amount of water needed for a good
crop would be equivalent to the rainfall in years 1 and 5,
and the average amout; ? for these two years was 410 mm. If
410 mm is taken as the seasonal crop water requirement,
then in year 3, the driest year, witn 299 mm of rainfall, the
difference between 410 and 290, which is 120 mm, will give
the amount of extra water to be provided by irrigation in
year 3. In year 4,410-310 = 100 mm of irrigation would have
been needed.
While it is important to know the total amounts of irrigation water required in a season, it is even more important
to know the maximum amount of water to be provided in
one watering. At the height of the irrigation season this
maximum amount of water will have to be provided in a
given time, and this determines the capacity of the system,
3r the rate of flow which the system must be designed to
deliver.
Rainfall is measured daily, and daily amounts are added
up to give monthly and annual figures. A study of daily and
monthly records will show the length and frequency of dry
periods when irrigation will be needed, and this information,
in conjunction with temperature and evaporation figures
will enable the maximum irrigation requirements to be
68
Small Scale Irrigation
determined, on the basis of crop water requirements,
,2xplained in the next chapter.
as
69
Chapter 9
Crops and Water
Plants need soil, sunlight, air and water to enable them to live
and grow. Water is an essential component of all plant tissue
and fulfils three primary functions. It keeps plants erect by
filling the cells which make up plant tissue; it acts as a
cooling agent in evaporating from the leaves, preventing
overheating under hot conditions; and it carries nutrients in
solution from the soil into the plants through their roots.
The growth of plant material is produced from the combination, with the aid of sunlight, of a gas in the air, (carbon
dioxide), with water and nutrients from the soil.
Evaporation Processes and Consumptive Use
Evaporation is the process by which water, in the form of
water vapour, enters theatmosphere from open water surfaces
such as the sea, lakes, ponds, rivers, or from wet land surfaces.
Transpiration is the evaporation which takes place at the
surfaces of plant leaves, described above Ftapotranspiration
is the total movement of water vapour into the air from land
which supports plant life. It includes transpiration from the
plants, evaporation from damp soil and evaporation from
any open water that may be present in furrows or depressions
following irrigation or heavy rainfall.
The amount of water used in evapotranspiration is the
quantity which is important for irrigation planning, because
in the absence of rainfall, irrigation has to provide this water.
Evapotranspiration
varies with climatic conditions in the
same way as open water evaporation. When the climate is hot
and dry, the rate of evapotranspiration is high; when it is cool
or humid the rate is low. When there is a wind it is higher
than when the air is still. Evapotranspiration,
like rainfall
and evaporation is expressed in terms of depth of water
(millimetres), and the rate of evapotranspiration in millimetres
per hour. In regions where there are marked seasonal changes
in climate there will be corresponding changes in the rateof
evapotranspiration;
where there is little seasonal climatic
change, the rate will be much the same throughout the year.
70
Small Scale ! rrigation
Under natural conditions without irrigation, the actual
evapotranspiration
which takes place from land supportin
vegetation at the end of a hot dry season will be very
because the soil is quite dry and there is therefore no available
moisture. The term potential evapotranspiration OSused to
describe the evapotranspiration
which could oc
these conditions, if water were freely availab
which is what irrigation would provide.
Consumptive use is a term which originated in t
States and describes the quantities of water
supporting vegetation and crops. I: is the:-ef
quantity as evapotranspiration,
and is also
depths of water.
It is not easy to measure evapotranspiration accurately,
but it can be estimated from measured climatic data.* For
this purpose a quantity known as the reference crop evapotranspiration
is used, defined as “the rate of evapt:transpiration from an extensive surface of 8 to 15 cm tall green
grass cover of uniform height, actively growing, completely
shading the ground and not short of water”. It is given the
symbol ETo and can be calculated in a number of ways, one
of which will now be described.
Calculating ETu from Pan Evaporation
The measurement of evaporation by pans is described in
Appendix E3. Pan evaporation (Epan) can be converted to
ETo by multiplying it by a pan coefficient, Kp. The value of
this coefficient differs for different types of pan, and for the
same type of pan it varies with the relative humidity of the
air and the speed of the wind at the time of measurement,
and with the siting of the pan in relation to crops and vegetation. Table 13 may be used as a guide to selecting the
appropriate value for Kp with American Class A type evaporation pans.
Crop Water Requirements
Under the same climatic conditions,
different crops
require different amounts of water, and the quantities of
water used by a particular crop vary with its stage of growth.
Initially during seeding, sprouting and surly growth, a crop
uses water at a relatively slow rate. As growth develops this
*It can also be estimated by measuring changes in soil water content.
Table 13 - Pan Coefficients (Kpl for American Class A type
evaporation pans
Cropped Area
Less than 40% Over
40%
7U% 70%
Relative
Humidity
Wind:
Light
Moderate
Strong
Very Strong
0.65
0.60
0.55
0.50
0.75
0.70
0.60
0.55
Dry-Fall0 w Area
Less than 40%- Over
40%
0.85
0.75
0.70
0.60
rate will increase, reaching a maximum in most cr
approach of flowering and then declining towards
The actual amount of water used by a crop, or crop
evapotranspiration (ETcr) is related to rzf
transpiration (ETo) by a crop coefficient
Kc.ETo. The coefficient Kc may vary fro
during initial gro
to over 1.O at mi
falling below 1.0. or irrigation system
mum crop water requirements are the s
needed. Figure 23 shows a typical crop coefficie
Stages of growth are shown as percentages of
Jrowing period, and this curve indicat
with growth for most crops. There are,
Kc
Litial;
I
LL”P
,de"elopmentj
-Growing
mid-season
i
I
late
Period-
Fig. 23 Typical crop caefficien t cume
I
I
72
Small Scale Irrigation
between crops, and Table 14 gives crop coefficients for a
seiection of crops at mid season and at final growth, under
both humid and arid conditions, and the ranges of their
growing periods. Generally growing periods are shorter in
warm climates with long hours of sunshine, and longer in
cool climates.
Table 14 - Crop Coefficients (Kc) for various crops and their
growing periods
Crop
BarIey,Wheat
Green Beans
Maize
Millet
Sorghum
Cotton
Tomatoes
Cabbage
Cauliflower
Relatives Uumidit y Rela rive Humidity
more than 70%
less than 20%
(humid)
#idFinal
Fin81
M/d. . (arid’
Season Growth
Season Growth
Gro wing
Period
Days
1.1
0.95
1.1
1.05
1.05
1.1
1.1
0.25
0.85
0.55
0.3
0.5
0.65
0.6
1.2
1.0
1.2
1.15
1.15
1.2
1.2
0.2
0.9
0.6
0.25
0.55
0.65
0.65
120-165
7590
80-110
105-140
120-130
180-195
135180
1.o
0.85
1.1
0.95
8085
Net lrriga tion Requirements
A farmer planning irrigation must consider all aspects
of crop water requirements including the growing period
variations, his cultivation programme for each crop, and any
possible contribution from rainfall. This can be illustrated by
an example taken from a proposed development in Uganda.
One of the crops to be grown was cotton, and a variety which
Srequired water for 5% months. Two cropping patterns were
possible which would allow for picking during the relatively
dry weather between October and Mlorch: an early crop from
May to October or a later crop from Au&St to January. Net
. irrigation requirements (In) for the cotton were calculated
from climatic data, and the results xe shown in Table 15.
The table shows the effective rainfall (Rd) and reference crop
evapotranspiration, (ETo) in monthly amounts for a year.
As would be expected, evaporation (and consequently
evapotranspiration) are high when the rainfall is low. Effective
rainfall was takeyas the ‘2094dry monthly rainfall, which
Crops and Water
73
means that amount of of rainfall in a particular month which
is not exceeded for 20% of the total number of years of
records. Thus the ‘20% dry’ rainfall for March at 66 mm
means that in say 30 years of rainfall records for March, the
amounts for March for 20% of the years (i.e. 6 years) were
66 mm or less.
I-Table 15 -
An example calculation for net irrigation
requirements for cotton in Uganda
water
All water quantities are in millimetres.
Rd = effective rainfall, ETo = reference crop evapotranspiration,
Kc = Crop coefficient, ETcr = Crop evapotranspiration,
In = rwt
irrigation requirement.
Month
March
April
May
June
July
August
Sept.
Oct.
Nov.
Dec.
Jan.
Feb.
Totals
Rd
Eto
66
196
170
94
122
137
104
91
61
38
18
38
171
159
161
153
158
158
156
161
159
164
171
157
1,135
1,928
Early Crop
Kc ETcr In
56
0.35
99
0.65
1.05 161
1.20 190
1.05 164
0.65 105
0
5
39
53
60
14
775
171
Late Crop
Kc ETcr In
0.35
0.65
1.05
1.20
1.05
0.65
55
101
169
191
172
111
0
0
78
130
134
93
799
435
Several interesting facts emerge from this table. Because
rates of evapotranspiration are much the same throughout
the year in Uganda, the total amounts for ETcr and the peak
monthly values are similar for the two crops. But the irrigation
requirements are very different, due to the rainfall pattern,
so that the second crop would need two and a Mf times as
much irrigation water as that for the first crop. Also the
maximum irrigation requirement for thesecond crop (134 mm
in Dscember) is more than twice as much as that for the first
crop (60 mm in September), so that the second crop would
need a correspondingly greater irrigation system capacity.
74
Small Scale Irrigation
Irrigation Efficiency
The amount of water (If) delivered to the field in which
crops are growing is greater than the net crop irrigation
requirement (In) owing to the field application losses. With
surface irrigation these losses arise from deep percolation in
free draining soils, and from overspill and wastage of water.
With overhead irrigation the field losses occur primarily
through the direct evaporation of water in the air before
reaching the crop and from the effects of wind distorting the
spray pattern. The ratio In/If is known as the field irrigation
efficiency, which may be expressed either as a factor or
as a percentage. Table 16 gives field irrigation efficiencies
appropriate to different types of irrigation.
Table 16 - Field irrigation
efficiencies for different
irrigation
lrriga tion Method
Surface
Basin (except rice), border, furrow,
corrugation
Flooded rice
Overhead Sprinkler
methods of
Efficiency
0.4-0.6
0.3
0.6~Xl.8
In the example given in Table 15 the net irrigation requirement (In) for the early crop in the month of maximum
demand (September) is 60 mm. If the field efficiency for
surface irrigation here is taken as 0.5 then the field irrigation
requirement for the month would be 60/0.5 = 120 mm. If
sprinkler irrigation is used with an application efficiency of
0.7 the sprinkler application required for the month wouM
be 60/0.7 = 86 mm.
Where irrigation water is supplied to a field by open
channel, losses occur in the channel from evapcration and, in
the case of earth channels, from seepage through the sides
and bed of the channel. Suitable factors for these conveyance
losses to give overall irrigation efficiencies are given in Chapter
14.
Water Quality
In general physical impurities such as sediment and silt in
suspension in irrigation water are not harmful to agriculture
and may help to maintain the fertility of the soil. Chemical
Crops and Water
75
impurities dissolved in the water can be harmful when present
above certain fairly well-defined limits.
:.. All natural waters contain some impurities. Rainfall picks
up traces of gasses such as carbon dioxide from the air.
Surface run-off dissolves small quantities of materials on the
land surface and groundwater sometimes contains quite high
concentrations of salts. The principal salt constituents that
may be present in irrigation water are chlorides, sulphates
and the element sodium. Particular salts are not necessarily
harmful, but the total concentration of salts above a particular
limit may be harmful. Common salt (sodium chloride) is one
of the most widespread impurities in water. It accounts for
four-fifths of the dissolved salts in sea water.
The quantity of salt in a sample of water can be determined
by boiling the sample dry and weighing the solid residue
which is left, and this gives the Tutal Dissolved Solids (TDS),
expressed in parts per million (ppm) by weight equivalent to
milligrams per litre (mg/l). Salinity is also indicated by the
electrical conductivity of a solution, expressed in micromhos
per cm, conductivity increasing with salinity. Because even
very small traces of boron can be harmful to crops, the boron
concentration in parts per million is also used as an indicator
of the quality of irrigation water. Standards for irrigation
water in terms of these various quantities are given in Table 17.
Table 17 - Standards for irrigation
water
TDS = total dissolved solids
Qmlit y
of water
Good
crops
suited
All crops
50-500
TBS
w/l
O-600
Conductivity
micromhow’cm
Boron
pm
o-o.5
Moderate
Injurious to
sensitive crops
500-2200
600-2000
0.5-2
Poor to
unsuitable
Harmful to
most crops
over 2200
over 2000
over 2
The actual concentration of salts which is harmful in
any particular ‘situation depends very much on the chemical
characteristics of the soil and on the type of crops being
grown. Under favourable conditions where soil hasa low clay
content it may be possible to cultivate under irrigation with
76
Small Scale Irrigation
highly saline water up to 5,000 mg/l. The different
of salt tolerance of various crops are shown in Table 18.
Table 18 - Relative tolerance of crops to salinity
High tolerance
Medium tolerance
Barley
Cotton
Date palm
Grasses
Rape
Spinach
Alfalfa
Cantaloup
Figs
Grapes
Maize
Oats
01 ives
Peppers
Potatoes
Rice
Rye
Sorghum, WhecJt, Vegetables
Low
tolerance
Citrus
Clovers
Field beans
Green Beans
Soft fruits
Further Reading
FAO Irrigation and Drainage Paper 24, Crop Water Requirements, Food
and Agricuiture Organisation of the United Nations, Via delle Terrne di
Caracalla, 00100 Rome, Italy , 1977.
77
Chapter 10
Physical and Chemical
Characteristics
of Soil
Soil has aptly been defined as the material in w
grow. lt is, therefore, any earth material which c
water and plant food in a state in which plants
A fertile soil is a soil in which plants grow wet
is maintained by replenishing the soil with the requir
for plant growth.
Soil Forma tiun
Soil material is formed from the breaking-down of rock
into small particles by a process known as weathering.
Weathering occurs under the influence of rain,
wind, temperature change and the chemical acti
acid water containing carbon dioxide fr
weathering rock, from which soils are for
the parent material. The chemical and physical properties of
its parent material play an important part in determi~in
characteristics of a soil.
If the weathering rock is on a slope, the particles pr
will be washed down by rainwater until they reac
ground or some obstacle which prevents further movement,
when they will settle and accumulate to form the basic
ingredients of soil. In exposed places wind also plays its part
in the transport of weathered particles. Soil is also formed in
some cases direct!y over its parent rock without lateral
particle movement.
In time seeds, carried by the wind, in bird droppings or by
surface water, will arrive and germinate. The development of
plant life and the recycling of plant nutrients through the
decomposition of vegetable material completes the formation
of a fertile soil.
The Compusition of Soil
A fertile soil contains two distinct solid components:
mineral and organic matter. The organic matter usually
amounts to one hundredth to one-tenth by weight of the
mineral materials, except in the case of peat, in which the
78
Small Scale Irrigation
solid matter is nearly all organic The process of deco
sition of vegetable matter at the soil surface
material known as humus, which gives top-sail
dark colour.
As every farmer knows, the appearance and co
of most soils varies with depth below the ground
often distinct layers or horizons can be identified.
of the agricultural activity takes place in t e top-soil or upper
layers, the composition of the sub-soil or lower
an important part in determing the drainage chara
a soil. A sandy or gravelly sub-soil provides good
heavy clay sub-soil, poor drainage,
Under arid and semi-arid conditions soils wit
lower horizons, which obstruct the penetration of
sometimes of plant roots also, are quite comma
impervious layers, often known as hard-pan, a
shallow top-soil, they present problems for t
wet weather or under heavy irrigation they
waterlogged, and in hot dry weather wit
their moisture quickly and become parch
Types of Agricultural Soil
Agricultural soils are described by t
and the texture, Cuhvial soils are
washed down by rainwater but not t
and rivers. Soils formed from material
by streams and rivers are known asalluvial soils. Vertisols are
soils which are formed over their parent rocks without lateral
particle movement The colours of SOiiS vary from almost
black (Black Cotton), through shades of red (Red Mediterranean) and brown (Brown Forest) to light yellow.
The texture of soils depends upon the relative proportions
of different-sized particles in their make-up. The proportions
of different particle sizes can be determined by mechanical
analysis, and these mineral components can be classif ied
according to particle size as follows:Name
Gravel
Coarse sand
Fine sand
Silt
Clay
Size Limits
(Partr’cle diametwsl
Above 2 mm
2.0-0.2 mm
0.2-0.02 mm
0.02-0.002 mm
Less than 0.002 mm
Physical and Chemical Characteristics
of Soil
79
The textural descriptions follow the domina
sizes in their make-up, so that soils are describ
loam, silt or clay, or combinations of these (sandy loam, s
clay/loam etc). The term loam indicates a well-graded c
ponent in which no one particle size dominat
ferences between these types are not rigid but t
may be used as a general guide:Sandy soils -- 60% or more of sands
Loams - Some sand and not more than 30% c
Clay soils -- Over 30% clay and less than 50%
The terms ‘light’ and ‘heavy’ refer to the amount of power
required to draw a cultivating implement through the soi
this context, sandy soils tend to be light, loams
clays heavy, although clay soils have a lower b
(weight of a unit volume, excluding contained
sandy soils.
The Chemistry of Sod
The particlesof weathered rock which make up the mineral
component of soil consist of about fifteen principal e
usually combined with oxygen as oxides and often combined
with each other. These major mineral constituents are listed
in Table 19.
Table 19 - Approximate
Element
Silicon
Aluminium
Iron
Potassium
‘Sodium
Titanium
Calcium
Magnesium
Barium, Phosphorus,
Manganese, Sulphur,
Chlorine, Fluorine,
Chromium and others
quantities of mineral matter in soil
Symbol
Si
Al
Fe
K
Na
Ti
Ca
Mg
Ba, P,
Mn, S,
Cl, FI,
Cr
Percentage
by weight
71.3
13.7
6.9
3.0
1.1
1.1
1 .o
0.7
0.9
loo.0
80
Small Scale Irrigation
The principal elements in the organic component of soi
are carbon (C), hydrogen (H) and oxygen (01 combi
carbohydrates, and nitrogen (N) as protein. The quantities of
these organic materials vary extensively with the type of soil.
Oxygen, carbon and hydrogen are obtained from air and
water. Nitrogen isabsorbed from the air by plants and r
in the soil by the biological decomposition of plant re
In addit ion to oxygen, carbon, hydrogen and
plants need potassium, phosphorus, calcium, magnesium,
sulphur and iron. The amounts of these nutrients which are
used are very small, and most soils contain adequate supplies
for plant growth for many years. These nutrients
active ingredients on which plants can feed when they are
dissolved in water to form the sail sdutiun or when they are
retained by adsorption on the surfaces of particles of colloidal
clay or of humus. Under natural conditions these active
ingredients take part in an almost completely closed biological cycle, passing from the soil into plants through their
roots, down to the ground when the plants drop their
or die, and then back to the soil through organic decomposition. Agriculture breaks this cycle by removing crops, so
that nutrients are lost from the soil, and compensating
measures have to be taken to replace the losses, by applying
manures and chemical fertilizers. The three most important
nutrients which may need to be replenished in soils are
potassium, phosphorus and nitrogen, but deficiencies in
other nutrients, sometimes used in minute quantities, may
also need to be mzde good.
Nutrient U eficiencies
In irrigated areas, particularly in arid or semi-arid regions,
soils may be deficient in organic matter leading to a shortage
of nitrogen, and in phosphorus and potassium. These deficiencies injure plant growth, cause poor colour and affect the
quality of crop products. Shortages of iron, magnesium and
manganese may cause leaves to turn yellow. It is not always
easy to -diagnose nutrient deficiency correctly, and if in
doubt a farmer should seek professional advice.
Salinity
”
Soils having a high concentration of soluble soils are
known as saline. They occur most commonly in low-lying
areas in arid and semi-arid regions where groundwater, often
Physical and Chemical Characteristics
of Soil
07
quite heavily charged with salts in solution, lies n
surface and is subject to intense evaporation leaving
in the surface layers of the soil. Salinity can aiso be produced
by irrigating with water containing dissolved salts ano maintaining a high water table so that the salts are deposited in
the surface soil layers.
Salinity reduces the productivity of crops in various ways,
by affecting the soil structure, and making it difficult ,for
plants to absorb water and nutrients. Some plants are more
salt tolerant than others. Salt concentrations in the surface
layers can be reduced by leaching, or washing the salts down
to levels below the plant root zone.
The most common soluble salts are those of sodium,
potassium, magnesium and calcium. Excesses of the chlorides
and sulphates of these elements form a white crust at the soil
surface, often known as white alkali.
Sodium carbonate, sometimes associated with potassium
carbonate, produces black alkali soils forming sodium hydroxide which dissolves organic matter causing a dark-coloured
crust at the soil surface. These salts are harmful to plant
roots, damage the structure of the soil, and may reduce the
availability of plant nutrients.
A number of secondary elements needer’ for plant growth
such as manganese, lead, zinc, copper, chromium, iron,
fluorine, and others can be harmful if present in excess.
Some of these, for example, lead, boron, copper or fluorine,
while not necessarily harmful to plants may be toxic in food
consumed by human beings or animals.
Acidity and Alkalinity
The acidity and alkalinity of substances are measured by a
quantity known as the pH value which refers to the concentration of hydrogen ions (electrically charged particles) in
solution. The pH value of water, which is neutral, is 7.
Values above 7 are alkaline and below are acid. Alkaline soils
are those with pH values greater than 8.5. Acid soils, which
are produced from large quantities of organic matter and
occur in marshes and swamps, have pH values of 5 or less.
Further Reading
Ivan E. Houk, lrt-igation Engineering,
York, USA, 1951, Chs.2 and 3
Vol.1, John Wiley and Son, New
82
Small Scale Irrlyation
Orson W. Ivaelsen, irrigation Principles and Practices, John Wiley and
Son, New York, USA, 1958, Ch.11.
83
-Chapter 11
oil an
ater
Soil Moisture
The structure of a soil consists of a framework of solid
material enclosing a complex system of pores and channels
which provide space within the soil for air and water. When
all these spaces are filled with water, the soil is saturated.
A soil can only remain in a saturated condition if it is below
water-table level and cannot drain freely. It may be temporarily
saturated above a water-table during and immediately after
irrigation or heavy rainfall. The maximum amount of water
or moisture which a soil can hold at saturation depends upon
the volume of its pore spaces and is known as its saturation
capacity.
Although regarded as old-fashioned terminology by soil
scientists, the moisture in soil can be divided into three
classes: gravity, capillary and hygroscopic. Gravity water can
only remain in soil which is above a water-table for a short
time, because it drains out under gravity. Capillary water
occurs as thin films on the soil particles or as droplets or thin
threads within the pore structure. Capillary water is the
principal source of water for plant growth, and the amount
of water retained by soil after gravity water has drained out
is known as the field capacity. Hygroscopic water consists
of a thin film held on the soil particles so firmly that it is
not available for plant growth.
When plants growing in soil are short of water they start to
wilt or droop. If water is supplied at this stage, the wilting
point, they will recover. If, however, they continue without
water they will reach a point beyond which they do not
recover with additional water. This is known as thepermanent
wilting puint; this term is also used to define the level of
moisture content in the soil when this state is reached. This
level includes all the hygroscopic water and some capillary
water. The difference between the moisture content of a soil,
and its moisture content at the permanent wilting point is
known as the available water.
These various soil-moisture quantities, are shown diagram-
84
Small Scale Irrigation
-
A
T
Saturation
Available
-?I
wat .er /
/ Percolation
after
/ irrigation
TOTA
son
MOISTU
Field
Capacit
jStorage
for plant
growth
I
Permanent
wilting
point
Fig.24 Soil moisture quantities
matically in Figure 24. These quantities are often described
as constants, but this is misleading, because they are only
constant for a given soil, and vary with the texture and
composition of the soil. When soil is irrigated its water
content will be raised initially to the saturation level, but if
the soil is free draining the “gravity” component of the water
will drain away. Gravity water will drain from the root zone
in less than a day in coarse sandy soil, or in three or four
days in heavy clay soil. This component therefore is not
normally counted as being available to crops unless drainage
is prevented by underlying hard-pan or by a high water-table.
Table 20 gives typical figures for soil moisture quantities
for different types of soil, expressed as percentages by weight
of dry soil.
Soil type
Fine sand
Sandy loam
Silt loam
Clay loam
Clay
Table 20 - Typical soil moisture quantities
Percentages by weight of dry soil
Permanent
Saturation Field Capacity Wilthg Point
15.20%
20-40%
30-50%
40-60%
40-70%
3-6%
6-l 4%
12-l 8%
15-30%
2545%
l-3%
3-8%
6-10%
7-16%
12-20%
Available
Water
2-3%
3-6%
6-8%
8-14%
13-20%
Soil and Water
A vailable Water
The available water is the water which is accessible to
vegetation. As rainfall and irrigation quantities are usually
expressed in terms of depth of water it is convenient to
express available water in similar terms. To convert the
percentages of Table 20 into depths of water it is necessary
to know the dry density of the soil. If we call x t
density of a particular soil in g/cm3, and f is the percentage
by weight of available water, then a cubic metre of the soil
will contain 10 fx kg of water. The volume of this quantity
of water will be 10 fx litres. This volume in a metre cube of
soil is equivalent to a depth of 10 fx mm. Table 21 gives the
results of this conversion of available moisture at field
capacity for typical soils.
Table 21 - Availablla water for typical soils at field capacity
Soil type
Finesand
Sandy loam
Silt loam
C!ay loam
Clay
Dry
Density
g/cm3
1.60-1.76
1.28-1.68
1.10-1.50
1.10-1.50
1.44-1.54
Available Water
mm/metre
96
23
3-6
6-8
8-14
13-20
30-50
40-100
60-120
go-210
190-300
The total available water in a particular soil will be the
available water per metre depth multiplied by the depth
of the soil. Thus a soil 1.5 m deep with 80 mm available
water per metre depth will contain 80 x 1.5 = 120 mm total
available water. The same soil 0.5 m deep would contain only
40 mm of available water.
Plant Rout Zones
Different plants have different rooting depths, and young
plants have much shallower root systems than mature plants.
Table 22 gives typical rooting depths for various crops in
fertile soil under unrestricted conditions. The figures in the
table should be taken only as guides because root patterns
depend much on local soil conditions and water availability.
Shallow soils must contain shallow root systems. Excessive
irrigation, maintaining a high water-table will also tend to
produce shallow roots, whereas drought conditions with
86
Small Scale Irrigation
water available only at considerable depth wi!l encourage
leep roots.
Table 22 - Typiral
root-zone depths
Depths in metres at full growth
Shall0 w
Beans
Broccoli
Cabbage
Cauliflower
G rass
pasture
Lettuce
Onions
Potatoes
Rice
Spinach
0.5-O-7
0.4-0.6
0.4-0.5
0.3-0.6
0.4-0.6
0.3-0.5
0.3-0.5
0.4-0.6
0.5-0.7
0.3-0.5
Medium
Barley
Carrots
Clover
1.0-l .5
0.5-l .o
0.6-0.9
Eggplant
Grains
(small 1
Peas
Peppers
Sweet
potatoes
Tomatoes
Water
melons
0.9-I .2
0.9 -1.5
0.6-l .O
0.5-l .o
Deep
Alfalfa
Cotton
Deciduous
orchards
Maize
Sorghum
Sugar
cane
1.0 -2.0
1.0-l .7
1 .O-2 .O
1 .o-2.0
1 .O-2.0
1 .o-2.0
1.0-l .5
0.7-l .5
1.0-l .5
source of data: FAO irrigaticn and Drainage Paper No.24, Crop
Vater Requirements, Table 39.
Generally most of the water used by plants is taken from
the upper half of the root zone, and because of this only
about half of the available water is actually used. This is
illustrated in Table 23.
Table 23 - Moisture extraction
Roe t zone depth
pattern in plant root zones
Percentage of available water usx?d
First quarter
80%
Second quarter
60%
Third quarter
40%
Fourth quarter
20%
Av. 50%
Irrigation Application
As plants cannot readily use more than half the available
moisture in the soil, irrigation is needed when this half is used
up. The amount of water to be applied to a particular crop in
Soil and Water
Fig.25
87
Calculation for irrigation
one irrigation application is therefore half the available
moisture in t.he root zone of the crop when the soil is at field
capacity. The interval between applications is determined by
the rate at which the plants use the available moisture, as
described in Chapter 9. If rain also adds water to the soil, it
must 5e allowed for when estimating irrigation requirements.
The calculation of irrigation requirements will be illustrated
by an example. In a particular month of 30 days it has
been estimated that a crop will need 240 mm of water, or
an average of 8 mm per day. If the soil is clay loam with
120 mm/m available water at field capacity and the effective
root depth is 1.35 m, the total available water in the root
zone will be 1.35 x 120 = 162 mm. Half this quantity, say
80 mm will be used by the plants, and at a rate of 8 mm
per day, the irrigation interval should be 80/8 = 10 days.
If 40 mm of rain falls during one of the lo-day periods,
the deficit to be made up at the end of this period will be
only 40 mm (80 mm less 40 mm). An irrigation application
of 40 mm can be applied now, or, alternatively irrigation
could be postponed for 5 days on account of the rainfall,
and a full application of 80 mm applied after 15 days interval.
A day by day calculation for readily available soil moisture
and irrigation requirements for a month with rainfall on the
lines of the above example is given in Figure 25. it has been
assumed that the first irrigation in the month is due on the
second day and that at the end of the first day the residual
readily available water in the soil is 25 mm, requiring an
additional 55 mm to bring it to field capacity of 80 mm. it
has also been assumed that any water in excess of field
e
a0
Small Scale trrigation
quantities with fain fall
Table 24 - Soil infiltration
rates [mm/h)
Infiltration
rate
l-5
Clay-loam
5-10
Silt-loam
1O-20
Sandy loam
20-30
capacity drains away too quickly to be used by the crops.
These calculations give net irrigation requirements, and gross
field requirements can be calculated after allowing for losses
(as described on page 74). if the field application efficiency is
O.& the field irrigation application would be 133 mm.
Infiltration Rate
Water enters soil under the action of gravity, and this
process is known as infiltration. The rate of entry is greatest
when the soil is dry at the start of watering, decreasing as
the top-soil becomes saturated to a nearly constant rate,
which is known as the infiltration rate for irrigation. The
infiltration
rate is expressed in miliimetres depth of water
per hour, and Table 24 gives typical infiltration rates for
different soils.
if the application rate is higher than the infiltration rate,
water will be wasted; if it is lower, evaporation losses may be
unnecessarily high. The infiltration
rate will give the time
required to water a piece of land. A soil with an infiltration
Soil and Water
89
rate of 20 mm/h will absorb an application of 80 mm in
80/20 = 4 hours. The methods for calculating rates of flow
into furrows and corrugations are described in Chapter 5.
Soil intake rates for sprinkler irrigation, shown in Table 11,
Chapter 7, are slightly lower than infiltration rates, to avoid
ponding water at the soil surface.
Irrigation Supply
In the foregoing sections it has been assumed that the
farmer has available figures for the characteristics of his
soil which he can use for calculating his irrigation requirements. In practice, especially for small scale developments,
these figures are unlikely to be available, and much will
depend on the farmer’s judgement and trial and error in the
field in determining his irrigation quantities. But some
knowledge of the methods of calculating these quantities is
extremely useful, because by making appropriate assumpticns
from the tables in this chapter quantities can be caiculat;sd
which will serve as a check against conclusions reached in the
field by trial and error.
While it is convenient to think of irrigation applications in
terms of depths of water it is difficult to measure these
depths without scientific equipment, and the farmer in the
field needs to relate his irrigation requirements to quantities
which he can measure and with which he is familiar. The
most easily measurable quantity is time, if it is not required
to great accuracy, and many irrigation systems are operated
on the basis of time. A farmer may know from experience
that it takes him half a day to irrigate a particular field.
He may qualify this by adding that it holds only when his
source of supply is running properly; in other words, when
his supply channel is flowing full. if his supply is reduced
the same field may take a whole day to irrigate. In the
example of Figure 25 an irrigation application rate of
6.5 mm/h has been assumed so that a full application of
80 mm takes 12 hours. With a constant source of supply,
smaller applications take proportionately
less time. if the
field application efficiency is 0.6 the gross rate of application
will be 6.5/0.6 = 10.8 mm/h. If the area of the irrigated field
is 2 hectares, the field water supply needed will be:2 ’ l”#oo ’ ‘Om8 = 60 litres per second
3,600
90
Small Scale Irrigation
In this way a farmer can check that his source of water is
sufficient to water the area of the field.
Further Reading
Ivan E. Houk, lrrigatian Engineering, Vol.l, John Wiley and Sons, New
York, USA, 1951, Ch.4.
G.V. Jacks, Soil, Thomas Nelson and Sons Ltd, London, England
1963, Ch.lV.
91
Chapter 12
rainage
The term drainage is used here to describe a
whereby surplus ws’:er is removed from agricu
includes both the intssrnal drainage of soils, and the collection
and disposal of surface run-off.
Soil Drainage
As has been described in Chapter 11, it is the capacity of
soils for holding water which enables plants to grow by
drawing water and nutrients in solution in the water from the
soil: through their root systems. Since air is a so necessary
to these processes, soils should not be permanently saturated
with water. A good soil therefore has good inter
characteristics, which means that water must be
fairly easily through the soil so that excess water can be
removed when required.
Heavy clay soils are poor draining; light sandy soils ar
draining. if soils drain too freely, they will be waste
irrigation water, and this can be a problem if water supplies
are limited. The best soiis are those of medium texture,
composed of a mixture of large and small particle sizes,
and deep enough (say over 40 ems) to have sufficient water
holding capacity to sustain plant life for a week or ten days.
A type of soil which occurs extensively in tropical and subtropical regions consists of a shallow, reasonably permeable
upper layer, which may be only 10 cm deep, lying over a
as hard pan).
very dense impermeable clay (often kn
cause the upper
This soil has serious drainage problems.
layer, which may have good internal drainage characteristics,
is shallow, it is quickly saturated by rainfall or irrigation.
As surplus water cannot drain easily through the underlying
impermeable clay, the top soil may remain waterlogged for
many days or weeks.
Wherever possibie I~,nd chosen for agricultural development
should have natural/y well-drained soils. Freedom of choice
is not always possible, and a farmer may often have to use
poorly-drained soils. With some poorly-drained soils measures
$2
Small Scale Irrigation
can bp taken to impr
such as ploughing al
surface, and heavy sub-soil can be broken up and mixed witn
lighter top-soil by digging or ripping. Soils which drain fre
but too slowly, such as; some heavy alluvia! soils. can
improved by sub-soil drains, which m
brushwood, specially
pipe, laid 80 cm or
sub-so!1 drains are laid at 2
expensive. Low-lying land
may need deep boundary
soil drainage water.
Surface Drainage
In addition to t
the soil, consideration should be gi
surface run-off arisi
excessive irrigation.
than 588 mm in a se
land, but heavy st
removed. I f the tan
collect in the furrows a
ditch is needed to
basin flooding, o
Where cultivated
produce their ow
to provide a protective cabch drain
cultivation
to prevent the fun-o
sweeping over the cultivation.
Farm Drainage
Field drainage wJi
is removed to a gsint
farm or to neighbowring
that a drainage syrte
natural drainage outlets,
d isposal areas. F igur
25 hectare farm unit
field drainage.
or
into
Design of Drhage Systems
Surface drainage systems are designed to evacuate surplus
water at a rate which is calculated or- estimated from data
Drainage
SQOm
--
t
directlion
93
_
of
Unit
Fig.26 Drainage layout for a 25hcatw
farm unit
about local conditions. It is usually based on the maximu
surface run-off which may be expected after
and for a small catchment area, this can be c
the formula:
Q=CiA
where
Q is in litres per second
C is a coefficient of fun-off
i = rainfall intensity in mm/h
A = area of catchment in ha.
Fig.27 Field drainage armngements
94
Smal I Scale Irrigation
The rainfall intensity of a storm is the total amount of
rainfell divided by the duration in hours. The coefficient
C varies with the topography, vegetation cover, and soil
characteristics, and it is not easy to select the correct value
for a particular situation. Table 25 may be used as a guide for
the choice of C, for small catchments under 250 hectares,
but b&cause it can vlery so much with local conditions, the
figures in the table must be regarded as very approximate.
Table 25 - Approximate values of C in the formula Q = C i A for
small catchments less than 250 hectares
Nature of Catchment
Poor soil cover, little or no
vegetation, low infiltration
Land with fair to good soils,
50% vegetation or cultivation.
medium infiltration
Deep soils, forest or dense
vegetation, high infiltration
Slope of catchmegt
5-10%
O-5%
10-3UX
1.8
1.9
2.2
1 .2
1.4
1.7
0.8
1 .o
1.2
Figures for storm rainfall intensities are not always easy to
obtain. In the troptcs and sub-tropics average storm rainfall
intensities between 20 and 50 mm/h are quite common. For
short periods of half an hour or less, intensities are much
greater and may be 200 mm/h or more. With a rainfall
intensity of 30 mm/h on a 10 ha catchment with C = 1.4,
the estimated run-off from Q = C i A would be 420 I/s.
In some situations, where flat agricultural land can accept
temporary flooding, it may only be necessary to remove
surplus water gradually, say over a period of 48 hours. If,
for example, a typical heavy storm on a 10 ha catchment
amounted to a total of 150 mm in 5 hours (i.e. average
intensity 30 mm/h), and the soil were already saturated at
the start of the storm, the volume of water to be removed
would be 15,000 m 3. To remove this in 48 hours would
require an evacuation rate of 86.6 Vsec.
Drainage channels are usually trapezoidal in cross section,
with side slopes varying between 3: 1 (horizontal: vertical) for
sandy soils and 1%: 1 for stiff clays. The full supply level in
field drains should be kept below the root zone of crops,
which may mean that the water level is 1 or 2 metres below
~
Table 26 - Drain Channel capacities in litredsesnd
for various channel slopes and s+tions.
Channels are trapezoidal with sideslopes l'/:l (horizontal:verticaI)
and roughnesscoefficient0.035
8
D
Channel slope
1/10,000
l/5,000
l/2,000
l/l ,000
1lSOO
l/333
l/250
l/200
l/100
0.01%
0.02%
0.05%
0.1%
0.2%
0.3%
0.4%
0.5%
1 .O%
+Velocitiesgreater
0.10
0.15
2.5
3.6
5.6
7.8
11
13.5
15.5
17
24
Bed width (6) and water depth (D) in metres
0.40
0.15
0.20
0.25
0.30
0.65
0.25
0.30
0.40
0.50
9.3
13
21
30
42
51
59
66
94
16
23
36
51
71
a7
100
113
160
33
47
74
107
148
183
212
237
330
59
a4
133
190
266
330
380
422
595'
121
172
271
390
540
670
765
855
1210+
0.50
0.85
0.60
1.00
243
345
545
770
1090
1330
1540
1720*
2430"
378
518
845
1200
1690
2065
2385"
2670*
3775"
than 1 m/s
8
96
Small Scale Irrigation
adjacent ground level, and the excavated drain section wi
very much larger than the required water cross-section.
Drainage channels are designed according to hydraulic
theory in the same way as irrigation supply channe
Chapter 14), which means that for a given land slope, flow,
and soil material, there may be several possible so
Table 26 gives flows in Iitres/sec for a range of channe
and sections with side slopes 1%: 1 and with an assumed
roughness coefficient of 0.035.
A high roughness coefficient at 0.035 (see Table 28) has
been used in the table because drains tend to be obstructed
with weed and rubbish. To avoid risk of erosion, the velor.ity
of flow should always be calculated by dividing the discharge
by the water cross-sectional area. Safe velocities for different
earth materials range from 0.5 m/s for fine sands, to 1 .l m/s
for stiff clays and 1.5 m/s for cobbles and shingle. In general
it is advisable to ensure that velocities are less than 1 m/s.
Drainage and Satin it y
The importance of water table control in helping to overcome the dangers of salinity in some arid parts of the world
has already been mentioned in Chapter 6. Where irrigation
is practised in these areas, effective land drainage Is essential
to keep the water moving downwards through the soil, to
prevent the accumulation of harmful salts at the soil surface.
There are many examples in India, Pakistan and the Middle
East of large areas of previously fertile irrigated land now
covered with a white crust of salts, abandoned and incapable
of supporting crops.
Soil Emsion
Soil erosion occurs through too rapid surface drainage of
erodible soils, resulting from the removal of protective
vegetation. Surface run-off after heavy rainfall picks up and
carries away soil particles, and in this way valuable fertile
land is destroyed.
The problem is more widespread in tropical countries,
where intense or high rainfall is experienced and where
cultivation and alluvial panning activities may be carried
out without advice or against the law in remote areas. For
example, the clearing of primary jungle in mountainous parts
of Malaya and the cultivation of vegetables, maize, rice,
rubber trees, and other crops without soil protection has
resulted in erosion taking the top soil and carrying it to the
flat land in the valleys. This has the double disadvantage of
denuding the higher land of valuable top soil and a
depositing it in a haphazard fashion on the lower Ian
creating serious problems of silting and consequent fl
in the river systems.
Further Reading
Bruce Withers and Stanley Vipond,
irrigation:
Design and Practice,
B.T. Batsford Ltd, 4 Fithardinge
Street, London WlH QA
1974, pp.1 56-l 88.
98
Chapter
13
ouree Develo
Water for irrigation can be obtained from a variety of sources.
It can be diverted from springs, streams or rivers, lifted from
rivers and lakes, or drawn from wells and boreholes. All fresh
water, whether it is on the surface of the land or underground, originated from rainfall. Surface run-off from rainfall
collects in rivulets and streams, eventually combining into a
single main stream or river. An area served in this way by a
single drainage outflow is known as a drainage basin or
catchment area.
A dam built across a river will collect run-off from the
river’s catchment area above the dam, and this provides a
reservoir which can be used for irrigation. This principle of
collecting water is used in many large irrigation schemes.
There is a finite limit to the number of river systems in the
world which can be treated in this way, and there are very
large areas of semi-arid land supporting subsistence cultivation
where there are no convenient river systems which could be
developed.
This is not to say that there are no drainage systems in
these semi-arid areas. There are, but because of a number of
reasons associated with the physical characteristics of the
area, only a very small part of the precipitation on the land
surface ever appears as outflow. The general drainage slope
of the terrain may be too flat so that water moves over it
slowly and is all evaporated on the way. The land surface
may consist of shallow porous soils overlying impermeable
material which take up most of the rainfall and return it to
the air by evaporation. The basin may consist of predominantly
porous rock formations which absorb the rainfall to feed
underground reservoirs outside the basin. Under any or all of
these conditions, the total surface run-off in a year will consist
only of surplus run-off following exceptionally heavy rainfall,
and this yield may be too low to justify the construction of a
dam and reservoir. Run-off from small catchments in semiarid areas, intercepted and collected before it is lost, can
provide a valuable source of water. Run-off farming, described
Source Development
99
in Chapter 4, is one example of this; small catchment storage
is another.
Small
Catchmen
t storage
There are many parts of the world where subsistence
cultivation is carried out with an average annual rainfall of
500 mm, which means that the actual rainfall in dry years
will be around 300 mm. Because of the low and unreliable
rainfall and poor soils, about 10 hectares of land are needed
to produce the staple crop (millet or sorghum usually) that a
rural farmer needs to support himself and his family. This
low rainfall is insufficient to grow tomatoes, egg-plant,
peppers and other vegetables to supplement the staple food
and add interest and nutrition to the diet.
What could a farmer with 10 hectares of land and without
much scientific knowledge do under these conditions to
improve this situation? He could set aside say 1000 square
metres of his land (one per cent of the total) for catchment
irrigation. Of this 1000 square metres, 700 square metres
would be prepared as a catchment apron, from which run-off
would be fed into a catchment tank, and 300 square metres
would be used as a vegetable garden, irrigated by watering
can from the tank.
Fig.28 Micro-irrigation
system
In a dry year, with 300 mm of rain, the catchment apron
would receive 210 cubic metres of water. Some of this water
would be absorbed by the soil of the apron itself, some
would return to the air by direct evaporation and some, say
15G cubic metres, could be collected. If, allowing for losses,
100 cubic metres of water from the tank could be used on
the vegetable garden, the garden would then receive 301, mm
100
Small Scale Irrigation
of direct rainfall plus 330 mm from the tank, aking a total
of 630 mm. Figure 28 illustrates this principle.
The ef feet iveness of this micro-irrr’gatiun system wi I Bdepend
upon the capacity of the catchment apron for delivering runoff and on the efficiency of the tank for storage. A very
suitable natural surface for a catchment apron is
and there are situations where this can be used.
surface is to be used, it should consist of wellheavy soil, and the slope chosen so that water will run off
more quickly than it is absorbed, but not so steep that
erosion develops. In any case, the run-off should be arrested
byasilt trap before it isadmitted to the storage tank. Artificial
waterproofing
of a catchment apron can be done, using
concrete, brickwork, masonry, bitumen or polythene.
The storage tank should be water-tight. If excavated in
heavy alluvial soil, it may be sufficiently water-tight without
lining. But in most soils, lining will be necessary. This can be
done with brickwork, concrete masonry and cement plaster,
or with membrane materials such as synthetic rubber, PVC
and polythene. Concrete or masonry lining may be expensive,
and call for skills which may not be readily available to the
farmer. Of the synthetic materials polythene is the least
expensive. Figure 29 shows one type of catchment tank
construction, with a pillar-supported roof.
Fig.29
Catchment tank construction
Source Development
1
Streams and Rivers
In upland areas where streams flow for at least six
in the year, water can be diverted for irrigation. In some cases
a direct diversion can be made by excavating a channel
leading from the stream just above a natural barri
usually it will be necessary to build a diversion s
across the stream to pond up the water so that it ca
into an irrigation channel.
A diversion structure may be either (a) a weir, intend
and therefore strong enough to be overtopped when
stream is in flood, or (b) a groin constructed across only a
part of the stream’s bed, or (c) a low dam wit
section designed to be destroyed when the strea
Examples of these diversions are shown in Figu
Weirs and small dams can be constructed w
brushwood, or earth, or combinations of these.
be constructed with sawn timber, masonry,
concrete.
It is very easy to under-estimate the stren
resist water pressure and the energy of flowing wa
small diversion structures are constructed temp
knowledge that they will be destroyed by floo
times part of the structure may be perma
temporary, as with the dam and wash-out section. A simple
form of construction which can be permanent is with
of gabions, which are baskets made of heavy duty
wire or wire mesh, placed in position in the stream bed and
then filled with boulders (Figure 33). A full gabion 2 x 1 x 1
metres weighs about 1% tons.
In planning to use water from a stream for irrigation we
must know how much water is needed, and if the stream is
capable of providing this water for the whole of the irrigation
season. We have discussed a method for calculating the water
required for irrigation in Chapter 11. Streams in tropical and
sub-tropical regions have a tendency to flow violently and
liberally during the rains, and for the flow to drop rapidly
and progressively as soon as the rains are over. Their flow
patterns may also vary considerably from one year to the
next, and it is therefore important to have records of the
flow of the stream for as long a period as possible. Streams
fed from springs usually, have less variable flows than streams
which are not.
Small Scale Irrigation
Fig.3U Diversion weir
Fig. 3 1 Groin
Fig.32
Low dam with washout section ,
Most countries have a Water Development or Water
Resources Department or Organisation which carries out
systematic measurements of the principal rivers and streams,
and collects and collates all hydrological data for the country.
Fig.33
Gabion filled with stone, lid open
This department or organisation s
information about a strea
information
is available
hydrologist may be able
comparison with an adjacen
records. In any case, as soon as a
a stream for which there are no re
to start records.
Gauging a stream at a point w
involves two activities. One, whi
special technical skill is the regular mea
levels by reading the height of t
staff gauge, which is a post or
stream and graduated in metres and cen?imetres.
The second activity is the actual measurement of the flow,
which requires technical knowledge and equipment, if
be determined accurately. This is done by one of the
described in Appendix D, and is best carried ou
officials and organisation equipped for this sort
this is not possible, then an approximate metho
used, but again, not without technical advice and assistance.
The most important information needed about the stream
flow is its steady discharge or rate of flow. If the irrigation
requirement at the point of diversion (which will include
conveyance losses as described at the end of this chapter) is
15 litres per second and the water is needed at this rate every
day for 5 months, the stream must be capable of producing
this for all of this period. This can only be determined from
104
Small Scale Irrigation
daily records for as long a period as possible. The use of a
rating curve or a measuring weir is explained in
The construction of weirs, dams or barra
streams and rivers is usually undertaken by
or large private enterprises, and is not vv~rk
done by the single farmer or small village unit without
assistance. Under certain conditions it is possible to divert
water into irrigation off-takes without a structure across the
river. This can be done where the river channel is on a very
flat slope in an alluvial plain, with water-Ieve
ground level at the river bank, and if t e land is sloping away
s 2 c\dt in t
from the river. Under these conditi
bank will allow water to flow on to the :B$acent land. An
interesting example of this method ass%3 by the effect of
ocean tides is to be found in tht: swamp estuaries of the Great
and Little Scarcies Rivers in Sierra Leone, West Africa. Fresh
water in these rivers, elevated by the risin
flow through cuts in t e river banks into r
A far more usual tuation is to find
several metres above
diversion structure on the river s
further upstream to raise the wat
land, then the use of some water-lifting device wi
for. Methods of iiftirrg water are described
Wells and Boreholes
The capacity of a well or borehole for supp ying irrigation
water is usually not very great. There are exceptions and
some large irrigation schemes depend on borehole water.
For small-scale operations borehole supplies tend to be too
costly, but shallow wells are feasible sources, and are used
in many countries. Here we use ‘well’ to mean a wide hole
one metre or more in diameter, excavated by hand, and
‘borehole’ to mean a narrow hole drilled by machine.
The yield of a well or borehole is the maximum rate at
which water can be extracted, and this often varies with the
time of year. It is important to know the lowest yield during
the period when irrigation water will be needed. Some
methods for measuring yield are described in Appendix 0.
Chapter 14
lrriga tion Channels
The simplest method of conveving water fro
land to be irrigated is by allowing it to flow un
channs!s or canals made of earth. While the size and slope of
a channel can be determined by trial ;3nd error, it is heipful
to knJw something abocr the theory of flow. For very smal!
channels (less that 1 I/s capacity) the cross-sectrow ca
V- or U- shaped; the most usual channel cross-section is
trapezoidal. These ttIrce channel sections are shown
in
rigixe
Type
34.
of
Channel
-
R
Fig.34
The k ydrautic characteristics
R=D
of channel sections
In straight-sided channels it is convenient to define the
slope of the sides, and this is usually expressed as the hori-
106
Small Scale Irrigation
zontal to vertical ratio of the slope. Th
2:l would be set out by measuring 2 metr
and 1 metre vertically. In an earth channel,
should be as steep as possible without bein
water. Slopes can be steeper in stiff clays
Table 27 gives suitable side slopes for diffe
Table 27 - Suitable channel side slopes for different earth
materials
Slope
Material
Sal-Id
Sandy lox;:
Clay loam
Clays
Gravel
Rock
Hofizontal/Vertica~
3.1
2%:1 to2 1
2:l to 155.1
2:l to 1 1
l?41 to I.1
1:l to %81
If k is used to denote the side slope, t
cross-section of a triangular or trapezoi
expressed in terms of k and the water depth
The wetted perimeter IPI of a water
length of the boundary between the water an
the channel, and expressions for P are
34. The hydraulic mean radius (I?) of a channel is obtained
by dividing the sectional area (A) by the wetted perimeter
(thus R = A/P).
The rate of flow (Q) in a channel in cubic metres per
second, the velocity of flow (V) in metres per second and
the water sectional area (A) in square metres, are 4ated in
the equation Q = AV. lf the flow is required in litres per
second, then Q = 1000 AV. Thus for a given section area, the
flow Q varies with the velocity V. The velocity of flow
depends on (a) the slope of the channel, so that the steeper
the slope thegreater thevelocity, and (b) the friction between
the water and the bed and sides of the channel which depends
on the material used to form the channel. The friction factor
is defined by a coefficient of roughness, n, which varies in the
range 0.01 to 0.05. Table 28 gives values of n for some typical
conditions.
Channels and Pipelines
Table 28 - Coefficients
of roughness (n) for different
channel
Type of channel
Earth, straight
Earth, winding
107
types of
n
and uniform
and sluggish
0.016-0.025
0.025-0.040
Rough stoney
beds, weads on earth banks lChannels with weeds and bushes
0.025-0.035
Concrete
0.012-0.018
The velocity
of flow is related to the coefficient of roughness, the hydraulic mean radius and the slope in Manning’s
formula: vi+
R’3
5%
In desigrng a supply channel we usually know the flow
requirec’
and wherever possible the slope of the channel
should beapproximately the slope of the land on the proposed
channel alignment. Knowing the material in which the
channel will be dug, we can make a suitable choice of roughness coefficient (Table XJ. If we decide on a trapezoidal
section, we can select an appropriate side slope (Table 271,
and we then have to choose dimensions which will give us a
velocity V which will satisfy the supply required (Q = AV).
If the velocity is too grea’t the material in the bed and sides
of the channel will beeroded by the water, and safe maximum
velocities for different materials are suggested in Table 29.
1)
Table 29 - Suggested maximum water velocities for different
earth materials
Material
Sand
Sandy Loam
Alluvial silt
Loam
Clay
Gravel
,
Velocity m/s
0.5
0.6
0.8
0.9
1.2
1.2
will be clear from the foregoing that the design of a
channel can be quite a laborious mathematical process, and
wherever possible assistance should be sought from technical
advisers or text books and charts. Figure 35 is an example
of a design chart, for a range of channel sizes, slopes and
It
108
Small Scale ‘rrigatio,~
Slope,
Fig.35 Irrigation
canal design chart
PC?
cat
r\t
Channels and PipeOmes
809
water velocities. This chart has been based on a
roughness coefficient, n, of 0.025 for trapezoidal channels
with side slopes 29. Suppose, for example, we require a
channel in a clay-loam soil to carry 60 I/s on land sloping
at 0.1% (1 in 1,000). The vertical 60 I/s line intersect
horizontal 0.1% slope line at a point between two t
channel lines of the following dimensions:Bed width, m
Water depth, m
(1)
0.10
0.25
(21
.l
.3
Either of these sections will be approxi
ely correct, but
to be on the safe side we should chaos
will actually carry 80 l/s. It will therefore carry
reduced water depth.
In selecting a design, we should also check t lat the veDocity
of flow is less than the maximum safe ve
erosion. Velocity lines are shown on Figure 3
purpose. Channels which i-ire lined with c
or stone can take higher velocities, up to a
this will allow smaller cross-sections for a
lining is costly and is not usually feasible e
distances.
It should be noted that the lines on Fi
channels with a roughness coefficient of
channel design lines can be used for other rou
efficients, bu-i the flow figures will then be altere
tionately. A channel with bed width 0.15 m and water depth
0.30 m wi II carry 100 I/s at a slope of 0.15% according to
Figure 35, with a roughness coefficient, n, od 0.025. If n is
0.035, the capacity of tht: channel would be:
,oo x 0.025
0.035
= 71.4 I/s
If n is 0.015, the capacity of the channel woul
100 x o’o25
0.015
= 167 I/s
Construction of Channels
Before a channel can be designed a survey must be carried
110
Small Scale Irrigation
out to determine the alignment or route of the c
the fall in land level along its lengih. A field charm
delivers water through an outlet or control an
level in the channel should be 15 to 20 cm higher than the
highest point in the field which is served by that o
height is known as the command height in the fiel
If there are several outlets, each one she Id be checked for
command height.
It will be clear that if a channel is to deliver water on
relatively flat land, it must be built up above ground level,
and earth must be borrowed from the edge of the field or
elsewhere to form the channel. Where the channel is serving
as a conveyor with no outlets it is usually designed and
constructed so that the water levei is approximately at
ground level, and the material excavated to form the channel
is placed on either side to give the necessary bank height
above water level, which should be half the full supply water
depth in the channel. This height is known as
The water level at the head of a channel
level at its tail plus the drop along its length. If the channel is
carrying water from a stream with a weir, the water
the head must be no higher than the minimum leve
stream at the weir. If the channel is taking water
pumped supply, the water level at the ead of the channel
will govern the pump lift needed. It may happen that the
designed drop along the channel is too littie for the lie of the
land, and if this is the case, drop structures or artificial
waterfalls will be constructed in the channel, in concrete,
masonry or brickwork, so that the earth sections of the
channel can still follow the best design slope.
To excavate a trapezoidal channel, the bed width and total
width at ground level are set out on the ground with pegs and
string. If the earth is firm, the bed section is dug first to the
full depth required, placing the earth on both sides, at least
15 cm outside the lines of the edges of the channel. The sides
are then excavated and formed to the required side slopes.
It is important to place the earth outside the final edges of
the channel to prevent it rolling back into the excavation
before it has been consolidated. Small channels are best
excavated by hand, but hand work can be assisted by ploughing with a ridger type plough to break a hard compacted
surface.
Channels and Pipelines
I ‘i 1
Channel Conveyance Losses
Some of the water flowing in an earth cha
on its way by evaporation from the water surfac
transpiration
through vegetation growing on
banks, and by percolation or seepage through
sides of the channel. The evaporation from the
channel is relatively small and usually less than
channel flow. The evapotranspiration
an
can be quite considerable. They are very diffi
because they depend so much on local conditrons.
be determined by measuring the difference
into and out of a channel, and measurements show
losses on large irrigaticn schemes to be of the order of
70% of the water supplied. On single well-maintained
in impervious material they may be much I
50% to 70% losses may be expected in e
highly permeable materials channels need to
materials can be used for lining, such
masonry, brickwork, asphalt. Channels can a
buried membranes of butyl rubber, PVC an
sheeting, and sprayed bitumen. All linin
be expensive.
Pipelines
While long pipelines are expensive and
usually feasible for irrigation supplies, it may
carry an open-channel supply through a pipe for short
distances, such as acrossa road or drainage g Ily. The carrying
capacity of a pipe depends upon its section area, the roughness of its insidesurfaceand the tread or difference of pressure
of the water between entering and leaving the pipe. Head is
lost in the pipe through friction, which varies wit
material from which the pipe is made. Common materials
for irrigation pipework are concrete, asbestos-cement, steel
and poly-vinyl-chloride
(PVC). The charts in Figures 36 to
39 show the relationship between head losses in metres
per 100 metre length of pipe, and pipe flows in litres per
second for a range of pipe sizes for these four different
materials, when the pipes are Flowing full.
As an example in the use of these charts, suppose we need
to carry a 20 I/s supply in a pope 40 m long. If we use an
asbestos-cement pipe 20 cm dia, the 20 cm line on Figure 37
142
%mall Scale Irrigation
Fig.36 Concrete pipe design chart for various pipe sizes measured by
inwnal diameter
86
16 '
lo.-
i
I
i
Fig.37 Asbestos-cement
pipe
measured by in ternal diameter
design
chart
for
various pipe
sizes
Channels
Ibad
Fig.38 Steel pipe
in ternal diameter
design chart
and
Pipelines
113
I848 , m/lo8a
for various pipe sizes measured by
cuts the 20 I/s line at 0.2 m/l00 m head loss. The total head
lost thrp lgh the pipe will be 0.2 x 0.40 = 0.08 m. This
means that if the pipe is used to bridge a g;g in a channel
system, the water level in the channel at the
of the pipe will be 8 cm below that at the u
a I5 cm dia. asbestos-cement pipe were
“drop” wouid be a lot greater, in fact 0.84
(Figure 37). If a PVC pipe were used, with less friction, the
head lost would be, from Figure 39, 0.068 m for a 20 cm
pipe and 0.27 m for a 15 cm pipe. The choice of pipe wi II
depend on local site conditions and the availability and cost
of piping. Where the land is very flat and head losses need to
be kept to a minimum, a larger diameter pipe should be used.
114
Small Scale Irrigation
.-
c -...-
+-
l
.
t3
bS
-
2
Fig.39 PVC pipe design chart for various pipe size measured by in ternal
diameter
If there is adequate fall along the line of the canal, then a
smaller diameter pipe can be used.
Further Reading
There are many standard text books on the theory of open channel and
pipe hydraulics, and a technical library should therefore be consulted.
Chapter 15
While the earliest irrigation schemes in history probz~bly
depended only on the gravity flow of water, it was not long
before man became aware of the need to lift water for
irrigation, and many devices were developed for this purpose
in different parts of the world using man or animai power.
The adverrt of mechanical power and modern machinery
revolutionised the technology of water lifting, enabling water
to be raised to heights and in quantities enormously greater
than had hitherto been possible. Cheap oil fuel and hydroelectric power where available contributed to the popularity
of power driven pumping, and pump schemes for irrigation
were developed wherever they were found to be financially
and economically viable.
The success of the mechanical pumping system has,
however, been marred by the very high incidence of failure
where it has been applied inappropriately
to agricultural
communities which cannot manage installations which are
too dependent on imported equipment and spare parts and
on skilled labour which is in very short supply. Because of
this, and because of the increasing costs of oil fuels and other
sources of power, research and development resources are
now being developed on quite a large scaie, to re-discovering
the use of man, animal and wind power for water lifting, and
to improving traditional technology with the aid of modern
knowledge and materials.
In this chapter we summarise briefly :t-e different types of
water-lifting devices available for irrigation, without entering
into great detail. Useful references to these subjects will be
found at the end of this chapter.
Manual Water Lifting
The simplest form of manual device is the water container
which is iiiied and carried to like pianis iu Lt: irriydied. In the
Central Valleys of Oaxaca, Mexico, where the water table is
1 to 10 m below ground surface farmers open wells in the
fields at intervals of 20 to 40 m. A 10 to 14 litre clay or
116
Small Scale Irrigation
metal vessel is used to draw water from thzse
been estimated that a farmer can water up to
metres i.n an &hour working day in this way. The watering
can (Figure 20) is dnother vessel used in this way.
be raised by scooping, and a device for doing thi
in Figure 40.
.
I
Yb
1
.
.
.
Fig.40 lrriga tion by scoop
Man-po wred System
A system commonly seen throughout t
East uses the principle of the lever to raise a
well or irrigation channel (Figure 41), know
name as the ‘shadouf’. The Indian ‘dall’ or ‘auge’ (
uses the lever in a similar way.
Fig.4 1 The beam and bucket
Fig.42
The Indian ‘Dali’
The Archimedian Screw, said to have been invented by
Archimedes about 200 B.C. is shown in Figures 43 and 44. It
is still used in Egypt and India, for low lifts of between
0.25 m and 7.30 m. Limited to working between fairly
constant operating levt:ls, it is one of the more efficient
water lifting machines in terms of output for energy input.
Fig.43 Arcchimedean screw Phero:
Doughs Dickens)
Fig.44
Section of Archimedean
screw
Hand operated chain and bucket pu
Figures 45 and 46. They both consist of
suspended and rotated vertically. In t
or washers are ;jttached to the chain, w
vertical tube in its upward movement; in the latter case
118
Small Scale Irrigation
buckets are attached to the chain. The chain pump was used
ore
extensively in mines in Europe in the 16th Cent
recently it has been develofzd in China.
Fig.45
Chain pump
Fig-45 Bucket pump
The reciprocating pump is the type of
widely used, primarily for community water sup
for irrigation. The origins of the reciprocating pump are
believed to date from about 275 B.C. A
o&n reciprocating
pump was used as a ship’s pump in
early Greek and
Roman navies, and archeological rem
pumps from late Roman times have
Wooden reciprocating pumps were in corn
throughout the 16th and 17th centuries and in the 19th and
early 20th centuries metal hand pumps were manufactured
on a large scale in the United States and Europe.
The main parts of a reciprocating hand pump are shown
in Figure 47. The handle, connected to the pump rod, moves
the plunger up and down inside the pump cylinder. As the
plunger rises, water is drawn through a non-return valve at
the bottom of the cylinder, into the cylinder. On the downward stroke, the bottom valve closes and the water passes
Water Lifting
119
n
Drop
pipe
Cap
ICJ
-P
Cyl in&x
Hand pump nomenclature
Fig.47
Recipmcating
hand pump
Cylinder
120
Small Scale Irrigation
’
through a second non-return
valve in the plunger, to the
upper side of the plunger. On the next upward stroke, this
water is forced up the drop pipe (or rising pipe), w
second intake of water is drawn in through the bottom of
the cylinder. Water is thus raised to the pump head and
discharged through the spout.
The pump cylinder may be placed below the pu
or within
it, depending on the depth of the water surface
below ground level. Where this depth is less than about 6 m,
the cylinder can be in the pump stand, connected to a rising
pipe and .foot valve in the water. Where the depth to the
water surface is greater than 6 m, the cylinder must be placed
below the pump stand, either in the water or
6 m above it, and connected to the pa~mp sta
pipe.
If it is required to raise water to a paint ab g/e the pump
stand, a version of this pump known as the %I and force
pump is used, in which ,the spout is replaced by a de
pipe. For this version a water seal is
the pump stand, ano a linkage arrang
which keeps the pump rod moving vertically
movement.
The output of a hand pump is direct
energy capacity of its operator. While a
generate quite considerable
energy in s
average output for prolonged work is 0.1 to
and 0.08 horsepower is usually taken as t
for a hand pump. Energy is lost in working against friction in
the pump and the ratio of available power to input power is
known as the efficiency
of the pump, often expressed as a
percentage. A usual efficiency for handpumps is 60%, so that
the available power is about 0.05 horsepo
The power used in lifting water isgive
Horsepower
=
(Flow in l/s) x (
76.1
-
Thus for a hand pump with 0.05 available or-se power,
Flow x Lift = 3.8, approximately.
For a ift of 1 m, deliveuy
would be 3.8 I/s, for a lift of 20 m, deliver would be 0.2 i/s.
Despite its relatively simple mechanism, the hand-operated
reciprocating
pump has had a poor record of performance in
a number of developing countries. Banglades is one country
which has successfully adopted the reciprocating hand pump
for small-sca!e irrigation.
With assistance from UNICEF, a
hand pump programme for shallow groundwater,
known as
MOSTI (Manually Operated Shallow Tubewell for lrsigatC3n)
was evolved, which included a simple borehole which could
be driven by manual labour and a hanc’ pump for irrigating an
average of 9.2 ha of rice or 0.25 ha of dry
MOSTI system was introduced in the late 19
there were 60,000 units in use. The cost o
in 1976 was US$70. The pump finally ado
programme, known as the ‘New No.6’ (Figure 4
between the USAID ‘Batelle’ pump and aw earlier Oocally
manufactured pump.
Fig.48
New No.6 hand pump Isirnglad@shl
122
Small Scale Irrigation
Fig.49 Cross-section of a
semi-r0 tary pump
Fig.50 Cross-section of a
helical rotor pump
Fig.51
Cross-ction
of a diaphmgm pump
Water
Lifting
123
Other types of hand pump which
are availab
semi-rotary
and helical rotor pumps (Figures 49,
the diaphragm pump (Figure 51).
Animal-powered
S ys terns
Animal power has been used fcr water lifting in parts of
the world for many c\ uries. One of the
methods is the Persian
eel or Saqia, WI
rise a writer w
forms, but they all
buckets, operated by an animal harnessed to
rotates in a horizontal
circle to provide the
(Figures 52, 53). Another use of animal power w
ated in India (sometimes called the Mot) is illustrate
igures 54 and 55. It is usual y assumed t
working on a circul;r
exert a iorc
of its weight at a speed of 0.7 metres per second. For an
ox weighing 400 kg this is e uivalent to an out
horse-power.
Fig.52 Persian
Dickins)
whet
I“sagia’I
at Dendera,
Egypt
(Photo:
Douglas
124
Small Scale Irrigation
Fig.53
Details of Persian wheel IIllustration:
Fig.54
Indian ‘mot’at
Isi@ of Djerba, Tni.ia
Jamas Goodman)
(Photo: Douglas Dickins)
Water POwef
The energy of flowing water is used for raising water, as in
the water wheel (Figure 56) and the hydraulic ram or hydram
(Figures 57, 58). Water wheels are common in countries in the
Far East, and can be used where there is a fairly constant flow
in a river- or large canal. The hydraulic ram is a self-acting
pump in which a stream of water falling through a small height
Water Lifting
Fig. 55 &tails
325
cf h-idian m&
raises a portion of the water to a greater height. The rate of
delivery will depend upon the supply available, the working
head and the heigh 1 to be lifted. If a supply of 20 i/s were
used with a fall of 5 metres, a delivery of 1 I/s could be
raised to a height of 60 metres.
Fig.56 Water wheel on the Nile near Juba, Sudan
126
Fig.57
Small
Scale
lrriyatirjri
/ TDG low-cost Hydraulic
Ram
Water Lifting
127
Wind Power
The use of wind power for irrigation is practised in many
countries. Commercial wind-pumps as developed in Europe,
the United States and Australia, have a reputation for long
service and reliable performance when adequately maintained.
They are, however, expensive for the small-scale farmer,
and tend to suffer from the usual problems of mechanical
equipment which is dependent upon imported materials
and components. Research is now being undertaken for
the production of a low cost windmill, more suited to the
developing country situation (Figure 59).
A very wide range of mechanical pumps is available to the
irrigation farmer, and there is no doubt that under many
conditions a mechanically-powered
pump is the answer
to ihe problem of lifting water. To describe .Lheseis beyond
the scope of this book, and the reader should consult the
appropriate technical works on this subject
Further Reladiag
Volunteers
in Technical
Assistance,
Water Lifting
and Transport’,
Using Water Resources, VITA, 3706 Rhode island Avenue, Mt. Rainer,
Maryland, 20822 USA, 1977.
t
-:
s,
Fig.59
Low-cost
windmill
Water lifting
129
F. Eugene McJunkin,
Hand Pumps, Technical Paper No.10, Inteanational
Fleference Centre for Community
Water Supply, P.0. Box
140, 2260 AC Leidschendam, The Netherlands, 1977.
S.B. Watt, A Manual on the Hydraulic Ram for Pumping Water, Intermediate Technology
Publications
Ltd, 9 King Street, London WC2E
8HN, U.K., 1977.
John Boyd, Tools for Agriculture, Intermediate Technology Publications
Ltd, 9 King Street, London WC2E 8HN, i976, pp.93-101.
Orson W. Israelsen, irrigation Principles and Practices, John ‘4ViOeyand
Sons, New York, USA, 1958, Ch.5.
N.B. Webber, Fluid Mechanics for Engineers, Chapman and Hall Lt
11 New Fetter Lane, London EC4P 4E E, England, Ch.10, pp.252~263.
P.L. Fraenkel, The Power Guide: A Catalogue of Small Scale Power
Equipment, Intermediate
Technology
Publications
Ltd, 9 King Street,
London WC2E 8HN, U.K., 1979.
130
ix
t
To convert from the unit column
convelsion factor.
multiply
rs
by the apprq.Jriate
ENGLISH
METRIC
Length
mrllrmctre
eent4morrc
matre ! 1 1.000 mml
t- 1 DO0 m)
square centrmetre
kctuare metre
mm
Cm
m
”
km
0.0394 rn
0.394 I”
3261 II
1@34vd
0621 mite
cm2
Unit
Abbr.
Inch
I”
rn
foot (= ;2 n-8)
yard f- 3 ftf
mile b 5,290 It)
I= 1.760 ydl
in 2
ft2
vdz
hectare t- 10.000 m2)
sctuare krlometre f- 100 ha1
$
ha
km2
0 155
10 76
1.196
247ac
0 366
volumb
cubic CO”tim@tra
litro Im 1,000 cm31
cm3
I
0.061 m3
0.220 tmfxvral
I
0 264 us Qallo”
I
0 0353 rt3
sware
Fo;ror
25
4mm
Conwmion
Canvbrrion
Factor
ktlometre
vi
m~lc
gallon
square inch
square fool
square yard
acre I= 43.560 64
same mile I= 640~~
cubrc Inch
rmporial gallon
I- 1 20 us !pllon)
‘JS gallon IQ 0 838
rmparial gallon1
cubic toot
254cm
U36a8m
0.914 m
16Q3km
1:
2
;:2
vd2
ix
rn 3
6 45 cm2
0.0929 m2
0.836 m2
04tTiha
259Rm2
16.4 or13
4 55 I
3791
113
fd
28.32 I
6 23 imperial
gellOll
cubrc metre I- 1.000 i)
113
rt3
vd3
7.48 US QattO”
0.0263 m3
0.765 m3
1.233.5 m3
r”3
m3
35 31 (13
1 309 ,d3
Waight
gramme
krlogramme t= 1 .OOO gl
tonne i= 1,000 kg1
9
kQ
t
0 0353 02
22051b
0 994 ton
ounce
pound I= 16 01)
ton t= 2.240 Ibl
a?
lb
28.35 g
0.454 kQ
1 016 tonne
Velocity
metie per serortd
ml5
3 281 ftls
foot per second
ftls
0.3048
0.0353 f&
13.21 imperial
gallon per minute
15.95 us gallon
per mrnute
35.31 113/s
cubic foot per second
t- 2 acre-feet
per day approx.1
fr3ls
26.32 l/s
d/s
0.0283
gtcr”3
0 0361 lb/in3
lb/in3
27.69 g/cm3
kQh3
0.0624
pound per cubic
inch
pound per cubic
foot
lb/it3
16.02 kg/m3
kg/cm2
kg/cm2
kg/cm2
atm
14.22 lb,‘.“2
0.966 atm
10 m water
14.7 lb/in2
pound per
square inch (psi)
(= 2.31 ft water)
lb/i”2
lb/in2
0.0703kgtcm2
kW
-
1341 hp
0.986 hp
horwpower
(= 550 ft Ib/secl
hp
hp
0.746kW
1.014 metric
horsepower
Rstr of flow
line par second
cubtc metre per
secor-d (= 1,000 l/S1
Density
gramme per cubic
centtmetre
k ilogramme per
cubic metre
Pressure
k ilogramme per
square centimetre
atmosphere
I= 1.033 kg/cm2;
POWer
kilowatt
metric horsepower
lb/It3
cublc yard
acre.foot
t- 43.560 ft3l
m/s
m3ls
0.969 atm
131
ix
io
I. RAINFALL
Precipitation
and the Hydrological Cycle
Water falls on the earth’s surface in the fDrm of snow, hail, rain, or
as ~re~~~~tat~~~~
drizzle and condenses upon it as dew.
uch of the preoriginates from water vapour in the atmosphere.
cipitation on IandareJsmovesover
the land surface as runoff, collecting
in streams and rivers which flow into the oceans or other water masses
such as lakes and inland seas. All these water masses are continually
giving up water to the atmosphere by evaporation. Water also evaporates
from wet land surfaces, and enters the atmosphere through the evapotranspiration of vegetation. This movement of water from the atmosohere to the earth’s surface and back to the atmosphere is known as the
hydrological cycle, which is illustrated in Fii9lure 81, and it is this cycle
which enables life as we know it to exist.
ater
table
dry
season
I
3.
3.
4.
1. Interception
5.
6.
2.
Evaporation
Fig. B 1 Hydrological cycle under humid
Key
Key
Transpiration
Percolation
Capillarity
Lateral
conditions movement
The amounts of precipitation
which fall in the course of a year vary
extensively from one place to another. Some places (deserts) have very
little precipitation
or none at all. Others, notably in the tropics, have a
great deal. Because of these variations, and the effects which they have
on agricultural
practices and crop production,
information
about
precipitation
can be very useful to the farmer and cultivator.
Rain fall Records
Most inhabited areas of tne wsrOd have tried to build ups
records of rainfall, although the extent of cover, the duration of the
records and the expertise available for collecting and anatysing these
records, vary considerably. It is unusual to find that a proposed irrigation
2rea has been adequately surveyed and covered by systematic rain
gauge records for a period long enough to assess reM9e mini
maximum and mean ~rec~~it~tion. This applies ~~~t~c~~a~~y to tr
and semi-arid areas, where variations from year to year can
great.
Therefore as long a period as possible should be gi
gation of a project before the design stage is reach
practice the time factor is too short, due to the l~rge~cy of a
which funds have been allocated. The question is, how
records are needed for planning irrigation?
The answer
several factors: the urgency of the project, its size, the number of
people and the investment cost likely to be involved if it fails, and the
extent to which information
and experience from a nearby similar
development
can be applied to the project. Records for 20 years are
desirable, 5 years will give some idea of the rainfall characteristics of an
area, and one complete year is very much better than nothing at all.
Data are required on the nature and, more particularly on the amounts
and timing of the precipitation
and on variations in these values over
the study area in which the topography may vary considerably. Rainfall
is usually the most significant form of precipitation
relating to irrigation
projects.
Rainfall Measurement
Rainfall is measured as the depth of water falling on a horizontal
surface over a period of time, which may be a day, week, month or
year. It is normally measured in millimetres
(one inch = 25.4 mm).
Rainfall intensity is the rate at which rain falls a1.d is expressed in units
of depth (mm) per hour or per minute.
Standard Rain Gauges
The normal method of rainfall measurement in many countries is by
a standard rain gauge, made of brass, copper, galvanized iron, glass fibre
or plastic. It consists of a funnel which conducts the rain water to a
container
of 100 mm (4 ir.;hes) capacity housed in an overflow can
that accommodates an additional
75 mm (3 inches) of rain. Measurements of the collected rain are made daily with a glass measuring
cylinder
and recorded. It is therefore necessary to employ a gauge
reader wno is able to record readings every 24 hours.
Less frequent measurements can be made at isolated sites where
larger storage gauges are used, some with an aperture of 203 mm
(8 inches) diameter, and other typeseven larger. One type, the ‘Octapent’
A~~~~~~~
rain gauge, used in Britain, pr A-h-3 Sufficient capacit
to a month between observaticms. Figure B2 shows a
and an Octapent rain gauge.
+
127
+
is
!I--l.
Dimensions in
i-&--+
Meteorological Office
Rain Gauge Mk.2
Octapent Rain Gauge Mk.2A
Fig. 82 Two types of British rain gauges
Rainfall Recorders and Automatic
Rain Gauges
Rain recorders are in use where these can be justified on scientific
grounds or where daily observations are not possible. Some of these
instruments
record rainfall autographically
on a weekly or daily chart
fitted to a cylindrical
drum which rotates by clockwork.
The primary
function
of the recording gauge is to provide information
about the
duration and intensity of rainfall. There are many patterns of recording
gauges in use in different parts of the world.
Network Design of Rain Gauge Sites
The siting network
network is found to
developing the system
series of segments of
of rain gauges is important,
but frequently the
be quite haphazard. Probably the best way of
of rain gauges is to subdivide
the area into a
uniform physiographic
characteristics,
and then
site the gauges at points -I’ Iresent tive of these ~g~eu~ts”
which frequently
arises cc.
e number and type of
::n accurate assess nt of a catchwhich are necessary to e
ment’s rainfall is obtained. The iotl.:-‘.I;ng table gives rec~~~e~~e~
minimum
densities of rain gauges for :ege-;-ok catchrnent areas in
the UK.
Minimum numbers of rain gauges required in reservoired
moorland areas (UK)
Rain gauges
Monthly
Total
sqkm.
Daily
2
1
2
4
2
4
3
6
20
3
7
10
41
4
11
15
From ‘Raingauge networks development and design with special
eteorological
Organireference to the UK’, Bleasdale, World
sation/lnternational
Association
for Scientific
Hydrology Symposium, Quebec, 1965.
II. AIR TEMPERATURE
Thermometer
AND HUMIDITY
Readins
The Celsius (or Centigrade) scale has been
Meteorological
Organisation
(WMOI and is in
and many other parts of the world. Fahrenheit thermometers are also
stili used by some voluntary observers.
On the Celsius scale (‘Cl, the freezing point of water is zero degrees
and the boiling point is 100 degrees, at standard pressure of 1013.25
millibars (mb). On the Fahrenheit
scale (OF), the freezing point of
water is 32 degrees and the boiling point is 212 degrees at standard
pressure. The conversion formula from Fahrenheit to Celsius scale is:
tot
= $ (t°F - 32)
Air Temperature
Air temperature
in the shade is recorded by thermometers housed
in screens (open louvred boxes) about 1% metres above the ground.
The screens provide protection
from precipitation
and the direct rays
of the sun, while allowing the free passage of air. A typical screen is
shown in Figure B3. Maximum and minimum thermometers record, by
indices, the maximum and minimum temperatures experienced since
the instruments were last set. Air temperature varies over a 24-hour day
Fig. B3 Ordinary thennozneter screen on steel stand
from a minimum
around sunrise to a maximum from H to 3 hours
after the sun has reached its zenith, after which there is a steady fall
continuing
through the night to sunrise again. Accordingly
maximum
and minimum observations are best made between 8 and 9 a.m.
Humidity
Absolute
humidity is the quantity of water vapour in the air expressed as mass per unit volume. This quantity varies from near zero
when air is very dry to a maximum, known as saturation, when, at a
particular temperature and pressure, the air can hold no more water
vapour. If saturated air is cooled, precipitation
occurs: if it is warmed it
136
Srnsll Scale Irrigation
starts
to become
increases.
vapour
unsaturated
and its capacity
for absorbing
water
Relative humidity
is the ratio between the actual amount of water
vapour present in a volume of air to the amount which the same volume
of air, at thesame temperature, rfould hold if it were saturated. Relative
humidity
is usually expressed as a percentage, so that ICKY%refers to air
which is fully saturated (at tkoedewpoint).
eters, known as
Relative humidity
is measure with two thermo
wet and dry bulb thermometers,
placed in the screen (Figure
drv bulb measures the actual air temperature. Thewet bulb ther~o~eter
is identical with the dry bulb except that its bulb is covered by a piece
of cloth (muslin)
kept continuously
wet by means of conducting
threads which dip into a small container of water. The wet bulb records
a lower temperature than the dry bulb owing to evaporative cooling,
at all times except when the air is saturated and there is no evaporation.
Then both readings will be the same. The relative humidity is obtained
from the difference
between the two temperatures,
using specially
prepared calculation tables.
III. EVAPORATION
Forms of Evaporation
Evaporation,
transpiration
and evapotranspiration
have been defined
end discussed in C.ha!>ttr 9. The most common method of measuring
evaporation
is by means of an evaporation pan, which is simply a
container with open top and vertical sides, with a means of measuring
the water lost daily by evaporation.
Many different shapes and sizes of
pans are in use; some are raised above ground level, some are suf-k in
the ground. The most commonly
used pan is the American Class A
pan, made from galvanized iron, which is circular, 1.21 m diameter,
25cm deep. The pan is mounted on a wooden open-frame platform
15cm above ground level, and it is important that it should be accurately
horizontal.
The pan is filled to a depth of 20 cm, water-level being measured by
a point or hook gauge. Each day at the same time water is added to the
pan (or extracted after heavy rainfall) to bring the water back to its
correct depth of 20 cm. The pan observations are made in conjunction
with rainfall records. If rain has fallen then allowance for this must be
made in calculating the evaporation
from rainfall and the amount of
water added or extracted.
Evaporation
is recorded in millimetres of
depth in a given time. The readings from a pan are known as pan
evaporation and because the water in a pan tends to be warmer than
open water in a lake, the pan evaporation has to be reduced by a par-1
factor to convert it to open water evaporation. This factor, for the
Appendix
C
137
Class A pan may vary between 0.4 and 0.85 depending upon cli
conditions and geographical location. A figure of:en used is 0.7.
IV. SUNSHINE
Recording
The purpose of sunshine recorders is to enable the hourly or daily
totals of the duration of bright sunshine to be measured accurately.
The standard British instrument
consists of a portion of a spherical
bowl, having a glass sphere placed concentrically
within it. The diameter
of the bowl is such that the sun’s rays are focused as sharply as possible
on a card of sensitised material held inside it. The radiant heat of the
sun, concentrated
by the spherical lens, burns a narrow track in the
specially prepared card, which is printed with blue ink, white lines and
figures being left to mark the hours. Naturally the width and depth of
the burn depend on how brightly the sun is shining. With a clear blue
sky the card will be burned clean through; but towards sunset or just
after sunrise only a faint burn will be seen under the same conditions.
Maps are plans of the land drawn to scale showing physical features
such as rivers, streams, lakes, towns, villages, roads, railways and similar
information.
Most maps also include information
about the heights
(or elevations)
of some places, and many show contour lines (see
below). Special maps are made to show special information
such as
vegetation, geological formations, soil types, land use and so on.
Map Scales
The scale of a map is the ratio between distances on the map and
actual distances on the ground. A map sacle of 1 in 50,000 therefore
means that 1 cm on the map represents 50,000 cm or 500 m on the
ground. Scales of 1 in several million are used for maps of continental
areas and whole countries. When larger scales are used it is convenient
to print a map of a country or district in separate sheets, and a set of
maps to a particular scale is known as a scale series. Thus many countries
have topographical
maps to the l/250,000
series, l/100,000
series,
l/50,000 series. For initial planning of irrigation the l/50,000 series is
useful, but for any detailed planning maps to a larger scale will be
needed.
Heights on Maps
Heights on maps are referred to a datum or fixed base height which
in most cases is mean sea level. By taking mean sea level as zero height,
Small Scale IrripX;on
138
all other heights are shown as positive when above sea level and negative
when below sea level.
Contour lines are lines joining points of equal height. Contour lines
are usually
drawn ai equal vertical intervals which may be any value
from 0.25 m to 50 m, deoending upon the nature of the topography
and the scale of the map. When contour lines are close together, the
land surface is relatively steep; when they are wide apart it is relatively
flat. The slope of the land can be calculated from the spacing of contour
lines. For example, if, on a 1150,000 scale map, the distance
the contour lines representing 240 m and 245 m is 1.6 cm measured
on the map, the actual distance on the ground between these lines is:
tfO
x
50,000
1 in 40, which
= 800 m. The land slope is therefore
5 m in 800 m or
is a 100/4O = 2X% slope.
Contours on maps are very useful in describing the topography.
Because the earth is a continuous
surface, all contours must close on
themselves, though not necessarily on one map sheet. Concentric
contours
increasing in elevation
towards the centre indicate hills.
Valleys normally show V- or U- shaped contours. A contour can never
branch into two contours or cross another contour, and a contour line
is always at right angles to the direction of maximum land slope.
Aerial Photographs
Aerial photographs
are photographs
of the land surface taken
vertically
downwards from an aeroplane flying at a constant height.
They are therefore photographic
maps to a scale which can be calculeted from the height of the aeroplane and the optical dimensions
of the camera.
Aerial photographs
can be very useful for initial field planning
because oi their clarity and detail and because they can be used in areas
previously unmapped, or mapped with insufficient detail. They are also
used extensively
for the preparation of topographical
maps, by means
of photogrammetry,
which is the science of mapping from aerial
photographs.
It depends on accurate ground control, which means the
accurate identification
of points on the ground whose heights are
known or can be measured, and on the skillful use of special plotting
equipment.
Surveying
While existing maps may provide much useful information,
it is
unlikely
that maps to a larger scale than l/25,000
will normally be
available, and in many countries the largest available scale is l/50,000.
A 2 hectare farm, say 200 m by 100 m would be 4 mm by 2 mm on a
l/50,000 map or 8 mm by 4 mm on a l/25,000 map. At these scales it is
unlikely that, if contours are plotted, they will be closer than at 20 or
10 metre intervals. The fat! fr
of a furrow 200 m Uong at a
1% slope is 2 m. It will be ciiear therefore that t ese scales are naot large
enough for farm irrigation planning. and it wi be neceswy to carry
out some field survey.
Field topographical
survey iravotves making a plan of t
mapped, and measuring heights so that high and low points and land
stance will also be
slopes can be identified. Heights an
rce
of water to t
any possible alignments from a
A suitable scale for tile topo
l/500 depending on the size
l-ieight measurements shousd
in height of not more than 0
For overhead irrigation, heig
The topographical
survey c
tachcametry. Methods of sunreying will be mentioned
here, and further informati
books on survey (some references are given at the end of this Appendix).
Grid Survey
A base line is set out on the ground, usuai?y Just outside one
dary of the area to be surveyed. an
at 25 m intervals are set at right angles to th
any other angle measuring device.
at 25 nl intervals along the traverses are fixed on the
wooded pegs. Distances are measur
A surveyor using a survey level
man, then levels
along the traverses, measuring the ground level at each
in terms of distances along each traverse, the positio
such as a stream, road or path, gulley, or boundary. Any important
features on the ground which are not included on a traverse can be
located and levelled by a measured “offset” (at a right angle) from the
traverse. Wherever possible the compass bearing of the base tine is
noted so that the plan can be given a north point.
The result of this survey work is then plotted and the picture of the
area begins to take shape. Any specific detailed survey required, such as
the site of a stream diversion, the alignment of the supply channel and
sites for any other structures which may be needed, will also be carried
out. The channel alignment is plotted as a /ongitudinalprofi/e
showing
the rise and fall of the ground surface along the alignment and the
position of any significant features.
Tacheome try
A tacheometer
is a theodolite equipped with means for measuring
heights and distances optically
by sighting and reading a staff. It is
much quicker than grid surveying by direct measurement and levelling,
and with care and skill can be as accurate.
140
Small Scale Irrigation
Simple Survey Equipment
of instruments such as surveyor’s levels
Nhile surveying with t
and theodolites
is desirable b&ause of speed and accuracy, i. may not
always be feasible to use them, or necessary to survey with so much
detail or accuracy.
Various simple devices can be made using either a
suspended weight, a spirit leve or two connected columns of water to
establish a horizontal
line. The horizontal
line can be
ground by sighting along pegs or vertical rods, and groun
be measured with reference to this horizontal
dart
supply channel planned to follow a contour line can
way. (For further information
on these techniques se
Bench Marks
Bench marks are points of known elevation esta
points for surveys. Permanent bench marks are established by acountry’s
Survey Department and their locations and heights are usually marked
on survey maps. A temporar,!
bench mark is established as a local
reference point for a particular survey. It may be related by ievelling to
a perlnanent bench mark, or if there is no permanent bench mark in the
vicinity, it may be given an arbitrary value (such as 100.00 ml.
References
1, A. Bannister and S. Raymond, Surveying, Pitmen Publishing, Bath UK 1972.
2. John Collett and John Boyd, Eight Simple Surveying Levels, Intermediate
Technology Publications Ltd., 9 King Street, London WC2E 8HN, UK, 1977.
pendix
r urce
easurement of a
Units 0 f Measurement
Water is measured in two ways, by volume and by discharge (or rate
of flow). The international
units of measurement follow the metric
system (based on the centimetre, gramme, second) but American and
British systems (based on the foot, pound, second) are used in many
places. Units and conversion factors are given in Appendix A.
The units of volume commonly
used in irrigation
are the cubic
metre (m3) and the acre-foot, which is the quantity of water required
to cover one acre of land to a depth of one foot. One acre-foot =
1233.47 m3.
The units of discharge are the cubic metre per second (m3/s or
cumec), the litre per second (I/..) and the cubic foot per second (ft3/s
or cusec). One cusec flowing for 24 hours is approximately
2 acre-feet.
The relationship between volume and discharge in a channel or pipe
is expressed by the formula Q = AV where:-
Q = the discharge in m3/s (
A = the cross-sectional are
V = the mean velocity of flow in m/s (or
fth)
Stream-f10 w Measurement
The
current
simplest
rcquircd.
available
will not
in their
anyone
principal
methods of measuring st
meters, weirs and ParshaPI flumes.
and easiest to use,
Current meters are e ensive
outside scientific d
ba described here.
construction,
but once ins
familiar with th
Flea ts
Aong
To use floats a relatively straight reach of a channel 2
with a fairly uniform cros
measurements of width an
average cross-section area.
across the stream at each
strings is measured. A sma
or a piece of wood, is pla
stream of the start of th
the measured distance is noted.
to obtain an average read
velocity of the float, whi
its surface. The surface
the average velocity over the cross-section. This coefficient varies
0.66 for water depths le
over. A figure of 0.7 is often used for smail streams.
Thus, if the measured length is 24 m and the average time to cover
this is 20 seconds the average velocity will be 0.7 x 24/20 = 0.84 m/s.
If the average cross-section is 0.2 m2, the flow (Q = AV) will be 0.2 x
0.84 = 0.168 m3/s or 168 I/s.
To reduce the effects of wind on the floats, a me?hod practised in
India and Malaysia uses a wooden rod or tube 25 to 50 mm in diameter
as a float. The rod or tube is weighted at one end and so floats vertically
with a minimum surface to be affected by the wind. Alternatively
a
bottle, half filled with water, will serve the same purpose.
Weirs
Weirs, among the oldest and most reliable structures for measuring
the flow of water in canals, ditches and streams, are overflow structures
built across open channels. The discharge of the channel concerned can
be determined in relation to the depth of water flowing over the weir
crest. Weirs are mainly of use in measuring comparatively
low discharges
to obtain data, for example, for a proposed irrigation scheme, where
and described 1 ...%; are
sharp-crested, fc!Iycontracted
and free-flowing. A sharp cr’ ,t comprises
a thin plate tapered to an edge not more than 2 mm wi
tracted means that the weir has an approach channel
sides are sufficiently
far from the weir crest to have no
influence on the flow. A free-flowing weir is one where the d
hr,ee common types of weir are
water level is below the crest.
rectangular, the 90” V-notch an Me Cipolletti
trtpezoidal.
and t
weirs are illustrated in Figure
Because of the effect of
,down’ at the crest, t
(HI above the crest is measured upstream of the wei
equal to 4t-l. This water depth is usually recorded one
and more frequently
during flood peaks, and the discharge is calculat
from a formula, or read from a calibration table or chart
Weirs usually
used for flow
measurement
Ret tangular Weir
The rectangular weir consists of a horizontal crest with vertica
The width to height ratio of
fully-contracted
conditions
f the weir shouD
approach channel and the wi
e at Reast
times the head on the crest,
depth from thecrest t
the approach channel should be at least twit
Under these conditions the flow is given by the formulaQ = 1.84 (L - @.LH) t=i’.5
where
Q = flow in m%
L = length of crest in m
H = head on crest in m
TabIF Dl gives flows in I/s for a standard contracted rectangular
weir. The equivalent
discharge formula in English units is Q = 3.33
(L - 0.2H) H’s5 w h ere Q is in ft3/s. L in ft and H in ft.
90’ V-notch Weir
This is an accurate flow measuring device suited for small flows. The
crest of the weir consists of a V-notch, each side being inclined at
45’ from the vertical. For fully contracted conditions
the minimum
distances of the sides of the weir from the channel banks at maximum
water level should be at least twice the vertical head on the weir, and
the minimum height from the bed of the pool to the point of the V
should be at least twice the head on the weir.
The flow over the weir is calculated from the formula Q = 1.368
H2s5, where H is the depth of water above the apex of the triangular
V, in metres, and Q is in m3/s. Table 02 gives flows in I/s for a small
90” V-notch weir. The equivalent discharge formt71a in English units is
Q = 2.49 H2*5, where Q is in ft3/s and H is in ft.
144
Small Scale Irrigation
Table Dl - Flows for standard rectangular weirs (I/S)
Head (HI
cm.
1
1.5
2
3
4
5
6
8
10
12
14
16
18
20
25
30
35
,40
45
50
25
0.5
0.8
1.3
2.3
3.6
4.9
6.4
9.7
13
17
21
50
Cl.9
1.7
2.6
4.7
7.2
10
13
20
28
36
46
55
65
76
Length of crest (L.i cm.
?5
l&I
125
150
7 -4
1.8
2.2
2.7
2.5
3.3
4.2
5.0
3.9
5 1
6.4
7.7
7.f
9.5 12
14
1
14.5 18
22
1
20
25
3
2
29
34
4
i-i
910
17
200
3.6
6.7
1
a9
29
276
208
Table 02 - Flows bra
5
6
7
8
9
10
11
12
13
14
15
16
Cipolletti
17s
Smafi 80’ Y-notch weir (I/S)
0.8
1.1
1.7
2.5
3.3
4.3
5.5
6.8
8.3
10.0
11.9
14.0
Trapezoidal Weir
As its name implies, the sides of this weir are inclined instead of
being vertical as in the rectangular weir. The standard side slope is
1 (horizontal)
to 4 (vertical).
The conditions
for full contraction
mentioned
under the rectangular weir apply also to the trapezoidal
weir. The flow of the standard Cipolletti
weir is calculated from the
formula:
c2 = 1.86LH’.5
where
Q = flow in m3/s
L = length of crest in m
H = head on crest in m
Table 03 gives flows in I/s for a standard contracted Cipoktti
trapezoidal
weir. The equivalent
discharge formula in English units
, L in ft and H in ft.
is Q = 3.75LH1s5 where Q is in f
Table 03 - Flows for standard Cipolletti trapezoidal weirs (I/s!
~~~~~~tal~~4 (vertical
H&xl (HI
cm.
1
1.5
2
3
4
5
zi
IO
12
Length of crest IL9 cm
25
50
0.5
0.9
1.3
2.4
3.7
5.2
6.8
10.5
15
19
24
0.9
1.7
2.6
4.8
7.4
10
14
21
29
39
49
l"s
18
20
25
30
35
40
t"o
75
1.4
2.6
3.9
7.2
11
16
21
32
44
68
73
174
229
100
125
150
1.8
3.4
6.2
9.6
15
21
27
42
59
77
97
119
142
166
232
305
391
2.3
4.2
6.6
12
18.5
26
34
53
73
97
122
149
177
2
2
382
481
2.7
5.1
571
701
822
175
3.2
5.9
212
249
248
291
407
577
7
842
986
E
823
982
l.150
200
3.7
6.8
284
333
465
611
770
941
1.122
1,314
Compound Weir
In some natural streams the range of flow may be such that a V-notch
is desirable for measuring very low flows, whilst a rectangular weir is
more suitable for higher flows. A compound
weir, consisting of a
V-notch in the centre of a rectangular weir will meet this requirement.
The rectangular weir portion may also consist of one or more sections
of different
widths, increasing with height. The total flow of a compound weir may be calculated approximately
by treating each section
independently
and adding the results together.
Parshall Flumes
The Parshail flume, (Figure
021, named after the person who
developed it, is a specially shaped open rectangular channel or flume
comprising three principal sections: a converging section, leading to a
constriction
or throat and thence to a diverging section. The bed of the
146
Small Scale Irrigation
converging section is
and the bed of the
placed at prescribed positions
in the converging
section and n
PLAX
Yaee r surface
Fig. 02 Parshall flume
downstream end of the throat. Under free flowing conditions, meaning,
in this case, when the flow through the flume is free and independent
of water level in the channel downstream of the flume, the flow is
determined
from the first water level gauge (Gl in Figure 02). Under
subrnerged conditions,
when water backs up to submerge the throat,
the second gauge (G2) is also read, and the ratio of the readings Gl :C2
gives the degree of submergence. Where the submergence is greater
than 60 to 70% (depending on the size of the flume) the discharge as
calculated from Gl is corrected by a factor. At 95% submergence the
flume ceases to be effective.
The Parshall flume hat a mu
operate with relatively smaBOhea
with a weir. It is relatively insemitive to the velocity of appm
when properly constructed it is accurate, even when running sub
The velocity
of f!ow is high enough virtually
to elimia:ate sediment
deposit in the structure.
Parshall flumes have b
25 mm to 1.5 m throat
act7 flume size has a cali
Springs
est measures
The yield of a sprin
bucket or drum of known cap
S-litre drum, then the yield is
the spring emerges at one
measurement is easy. But 0
in a number of places, and the flow needs to be co~~~trat~~
where it can be collected and measured.
to a point
Walls and Borehotss
The basic procedure for testin
is to lower the standing :af.ster i~il by bsling or p
_ z*io;er to recover its level.
the time taken for I:-GA
removed from the well by
quantity
required to replac
recovery will give ,3n approximate figure for the yield, sufficiently
accurate for most purposes. The yield of many \nalls and boreholes
varies with rha so?::qp CT -4~ -16m
__. gS-A this should be taken into account.
The QGZitity
of ‘water ~xt~acred during the test can be measured in
containers outside the well, or, in the case of a hand-dug well with a
regular cross-section, by multiplying
the area of the cross section by the
change in water level. If, for example, the water level is lowered 2 m in
a circular well 1.3 m diameter, thevolume of water removed will be:4” x (1.312 x 2 =
3.1416 x 1.69
2
= 2.65 m3
If it takes 4 hours
well is:-
2 65
L
= 0.663 m3/h
4
or
go
= 0.18 I/s
for this water level to recover, the yield of the
148
Small kale
lrrigatian
This well would yield
IA ~ollti~~ously
for 24 hours a day.
If it were intended to use it for irrigation, drawing water f
a day, water could be extracted at a higher rate, depending upon the
storage capacity of the well. Suppose there is no water left when the
level is lowered 2 m at the end of the day, and during the night the
water level rises 2 m ready for watering
the next morning. The to
amount of water which can be extracted in 10 daytime hours will
the yield in 10 hours plus the storage volume, an
10 x 0.663 -I- 2.65 =
3
This is equivalent to a yield of 0.828 m3/h or 0.23 I
The yield of a boreholecan be measured in the
t3 lower the level, and Sming the recovery. If, as is usually the case, the
standing water level is 53 m or more below ground level, it is not easy
to measure this depth without special equipment. The water removed is
meawred as it is pumped out. With a low-yielding
borehoie served by a
hand pump this can be done by filling containers. With higher yields
and delivery
by mechanical pump, the quantity
extracted
is best
measured by a flow-meter or portable channel and weir.
References
1, Development and Resources Corporation, Irrigation Principles and Ractices,
P 81 T Journal No.5, United States Peace Corps. 806 Connecticut Ave.,
Washington, DC 20525, USA, 1969 (Reprint 19761, ~~41-50.
2. Food and Agriculture Organization of the United Nations, Mel/ Hydraulic
Seruceures, Irrigation and Drainage Paper 26/2, Land and Water Development
Division, FAO, Via delle Terme di Carac~!lla, 00100 Rome, Italy, 1975,
Section 7.
ppendix E Two Case Studies in
anagement
I. COMMUNAL IRRIGATION SYSTEMS IN THE PHILIPPINES
Adapted from a paper presented at the Water Management Workshop at
the International Rice Research Institute, the Philippines, in December
1972 en titled “Organisation and Operation of 15 Communal Irrigation
Systems in the Philippines” by P.S. Ongkingco.
About thirty percent of the more than one million hectares of irrigation
land in the Philippines is served by communal system. A communal
irrigation
system includes the characteristics
of being small in size,
usually less than 1,000 ha, basically inexpensive and projects in which
Appendix
E
149
farmer beneficiaries
provide financial and labour inputs during the
system construction.
Also, the systems are operated and maintained
by the farmers on a self-financing
basis. Frequently
some sort of
farmers’ association exists to administer and maintain the scheme.
Maintenance
costs may either be met by a system of payments, for
example one sack (normally 44 kg) of rough rice per hectare per year
for the water used, or the farmers may undertake the weeding and
repair of the canals themselves when necessary. In some cases farmers
co-operate voluntarily
without a formal association.
Communal Problems
Once the communal irrigation system has been established, there are
normally two main problems which affect its satisfactory operation.
The one which is perhaps most likely to produce discord amongst the
farmers is the problem of ensuring a fair distribution
of water to all.
This factor frequently results in dissatisfaction and conflict, particularly
within schemes for which the water supply is inadequate to meet all the
farmers’ needs. The other main problem arises out of the need for
adequate and prompt maintenance
of the canals, which in turn can
affect the first problem, the fair distribution
of water.
In 1972 there were at least 16 communal systems in the province of
Laguna, which lies about 70 km to the south of Manila around the
southern end of lake Laguna de Bay. Most of these communal systems
received some initial assistance, notably from the National Irrigation
Administration
(NIA) or the Presidential Assistance on Community
Development (PACD), both of which are Philippine government organisations. Their sizes range from 14 to 1300 hectares, for a total of 4570
hectares through the province. The assistance given was either grants
for basic materials such as cement and steel, or technical assistance for
design and the supervision of construction,
or both.
In many of the schemes the distribution
of water and weeding and
repairing of canals is made the responsibility
of one person, often called
a ditch tender or water master, who may be paid, for example, a
certain weight of rice per year by each of the farmers served. In some
cases, such as the Prinza Irrigation
Dam System, Calavan, the ditch
tender may not be able to do all the work alone and so may have to
hire labour on his own account. At the Prinza scheme his twelve years
of service as ditch tender affords him sufficient experience to be able
to advise the farmers on such questions as the times to spray against
pests, or apply fertilizer. The farmers whom he serves also come to him
regarding their problems in water delivery and schedule and his decisions
are respected because he was appointed by the mayor.
Other systems, particularly
the smaller ones, have no formal farmers’
organisation,
or ditch tender. The work of cleaning and repairing
irrigation canals is done by the farmers but under the direction of an
official. In the case of the 60 hectare Pangil system, this official is the
150
Small Scale Irrigation
vice-mayor,
a farmer-land-owner
in the system, who also leads the
campaign to kill rats which attack the rice plants.
The position of water master to the 40-farmer Dalitiwan system at
Majayjay is unpaid and was inherited from his father and grandfather
before him. Originally
it was “given” to his family because they were
the owners of the largest rice farm (3 hectares). When repairs or maintenance of the canals are needed, the water master tells the farmers
where and at what time to report to do the work.
Some systems apparently
have no water master or official permanently in charge of the distribution
of water and of maintenance of
the canals. When the water supply is insufficient,
it is divided by the
farmers amongst their farms, in some systems on a rotation basis.
However, many of the farmers on such systems, for example the
Cortadilla and Mayputat systems at Santa Maria, whose land is furthest
away from the main canal, say that the water distribution
is unfair.
Within the municipality
of Magdalena there are two systems which
together irrigate about 170 hectares. After one unusually dry season,
the town mayor, anticipating conflict between farmers over the abstraction of water from the system, assigned a policeman to help distribute
the limited supply with occasional supervision by himself. The farmers
appreciated
this action even when they were allocated water only
2 days a week.
Technical Problems
In order to try to ensure that farmers’ rice yields are not depressed
due to lack of water, it is important to make certain (if possible) that
the supply to the communal irrigation system is adequate to fulfil the
needs of all the farms. If this can be achieved, then the chances of
conflict between members over the distribution
of water will also be
reduced.
If the total annual quantity
of water available from the surplus is
insufficient,
there may be little that the farmers’ association or its
members may be able to do, but there are situations where action can
be taken to improve the position.
The Niugan system at Cabuyao serves about 500 hectares during the
rainy season, but only about lo-20 hectares during the dry season due
to low water supply. Some enterprising
farmers have installed tube
wells to assure an adequate water supply in both seasons of the year.
Some even provide water to nearby farms, for a fee.
Technical and financial assistance to the farmers’ association of the
Santo Angel system enabled them to construct
a concrete dam to
replace the earlier brushwood
dam, which was unable to provide a
satisfactory water supply. Now all the fields of the 120 hectare system
can be well irrigated since leaks in the original dam have been eliminated,
assuring the farmers of a twice-a-year harvest of 3.2 to 3.5 t/ha. In
addition, the farrners are now relieved of repairing the dam every time
Appendix
E
151
there is a flood. However, since the completion
of the new dam the
farmers’ association has become inactive, the members pay no fee, and
there are no reserves to pay for repairs and maintenance.
Conclusions
To sum up, some of the main problems which those communal
systems have met, and for which solutions are needed:
a. equitably sharing the water supply,
b. sharing the task of canal maintenance,
c. overcoming technical problems in the water supply.
Sharing water and tasks of maintenance are achieved most satisfactorily where this is under the control of a respected member of the
local community,
who may or may not be paid for his services. In the
Philippines, a local government officer such as mayor or vice-mayor,
even if he is also an irrigating farmer, carries enough weight to be
acceptable as an arbitrator. This would not apply in many countries.
II. FAIR DISTRIBUTION
Reprinted from The New Scientist
IN BALI
17th November 7977.
The Balinese evolved what is probably the most socially sophisticated
system of village irrigation anywhere in the world. Every owner of land
in a particular ecological unit -watered
by the same stream or canal belongs to a common organisation,
the sebak, which maintains the
system and controls water use. It meets once every Balinese month
of 35 days, and has its own system of law called awigawig. Every
landowner has to provide free labour one day a month for repair and
maintenance
- the more land he owns, the more labour he must
provide or pay for.
The sebak decides democratically
on planting times -simultaneous
planting is preferred to keep pests and diseases down, but if water is
insufficient
for everyone’s land to be irrigated at the same time, the
sebak works out a complicated
planting and cropping rota to stagger
the demand for water. Though landholding is far from equal, water is
distributed
with scrupulous fairness. Its supply is regulated into each
parcel of land by a length of coconut tree trunk spanning the inlet.
For every one tenah of land (0.35 hectares) he owns, each sebak
member has a right to one tektek of water. A tektek is a gap four
fingers wide cut into the coconut trunk. Anyone who attempts to cheat
the system, and take more water than he is entitled to, can be tried by
the sebak meeting and fined heavily.
The technology
of the system is as primitive as its organisation is
advanced. The weirs are just piles of stones in the rivers. The coconut
Small Scale Irrigation
152
trunk inlet regulates only the width of water, not the height. In the
heavy rains of November and December flash floods often sweep awL,r
weirs and break down the bundhs between fields. Farmers usually delay
planting for as much as two months, until water supply is more even.
That delay costs an extra crop of rice in most areas.
In Bali the small-scale irrigation effort has gone into upgrading the
village systems - building permanent, solid weirs and primary canals
and providing control gates that can regulate the supply of water. Areas
that have already been upgraded have seen an increase in cropping
intensity of anything up to 80 per cent.
In all the small-scale irrigation projects labour-intensive
methods are
used. Excavation, even of long tunnels, is by pick and shovel; waste
is carried away in straw baskets on the head. Masonry is used instead
of concrete; and the stones are collected and broken up on or near the
site. One Balinese weir was getting the final touches when I saw it workers were handsetting
thousands of tiny pebbles into mortar to
provide an attractive finish.
Small Scale Irrigation
152
trunk inlet regulatas only the width of water, not the height. In the
heavy rains of November and December flash floods often sweep aw,*
weirs and break down the bundhs between fields. Farmers usually delay
planting for as much as two months, until water supply is more even.
That delay costs an extra crop of rice in most areas.
In Bali the small-scale irrigation effort has gone into upgrading the
village systems - building permanent, solid weirs and primary canals
and providing control gates that can regulate the supply of water. Areas
that have already been upgraded have seen an increase in cropping
intensity of anything up to 80 per cent.
In all the small-scale irrigation projects labour-intensive
methods are
used. Excavation, even of long tunnels, is by pick and shovel; waste
is carried away in straw baskets on the head. Masonry is used instead
of concrete; and the stones are collected and broken up on or near the
site. One Balinese weir was getting the final touches when I saw it workers were handsetting
thousands of tiny pebbles into mortar to
provide an attractive finish.