TRANSPIRATION AND EVAPOTRANSPIRATION V1ITh ALEPPO PINE (Pn'ius HALEPENSIS MILL.) SEEDLINGS

TRANSPIRATION AND EVAPOTRANSPIRATION V1ITh ALEPPO PINE (Pn'ius HALEPENSIS MILL.) SEEDLINGS
TRANSPIRATION AND EVAPOTRANSPIRATION V1ITh
ALEPPO PINE (Pn'ius HALEPENSIS MILL.) SEEDLINGS
UNDER VARYING SOIL MOISTURE AND SOLAR RADIATION LEVELS
Howard G. Halverson
A Thesis Submitted to the Faculty of the
DEPARThENT OF WATERSHED MANAGEt"ItENT
In Partial F\1fil1rnent of the Requirements
For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1962
STAT4ENT BY AUTHOR
This thesis has been sulmdtted in partial fuLfillment of
requirements for an advanced degree at The University of Arizona
and is deposited in The University Library to be made available to
borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special
permission, provided that accurate acknowledgment of source is made.
Requests for permission for extended quotation from or reproduction of
this manuscript in whole or in part may be granted by the head of the
major department or the Dean of the Graduate College when in their
judgment the proposed use of the material is in the interests of
scholarship. In all other instances, however, permission must be obtained from the author.
SIGD:
Howard G. Halverson
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
(2
A. .MCCB
Head
Department of Watershed Management
11
ACKGW1LEDG4ENTS
The author wishes to express his appreciation to
Dr. A. L. McCnb for his assistance in setting up the study and
editing this paper.
Professor P. B. Rowe also has the author's
gratitude for his assistance in this study.
Appreciation is rendered to Mr. L. P. HmUton and the
personnel of The Plant Materials Center of the University of Arizona
for generously providing a location and necessary equipuent for this
study.
Mr. H. L. Silvey also has the author's thanks for his assis-
tance in collecting soil and constructing the bench area.
The author would like to express his appreciation to
Dr. H. Tucker for his able assistance in the statistical analysis.
The Institute of Atmospheric Physics must be mentioned for
their cooperation in providing solar radiation data.
111
TABLE OF CONTENTS
Page
INTRODUCTION
1
REVIEW OF LITERkTURE
2
U
U
U
RESEARCH METHODS
Soil Moisture Depletion Study
Soils
Climate
Pot Treatment
Analysis
12
13
15
Wilting Point Study
15
16
Analysis
Soil Moisture and Solar Radiation Study
16
16
17
17
19
Soils
Climate
Pot Treatment
Analysis
20
RESULTS
20
Soil Moisture Depletion Study
Transpiration
Evapotranspiration
Evaporation
Comparison of Transpiration, Evapotranspiration
and Evaporation
20
22
24
26
Wilting Point Study
33
Soil Moisture and Solar Radiation Study
34
43
DISCUSSION
Water Loss Under Different Soil Moisture and Solar
Radiation Levels
43
Wilting Point
4-4
iv
Table of Contents (Continued)
Page
SUMMARY
46
BIBLIOGRAPHY
4$
APPENDIX
51
V
LIST OF TABLES
Table
1
Title
Mean water losses by transpiration for different levels
of incident solar radiation
2 Mean water losses by evapotranspiration for different
levels of incident solar radiation
Page
20
22
Mean water losses by evaporation for different levels of
incident solar radiation
24
Soil moisture percentages at permanent wilting of tree seedlings under two sources of water loss and four levels of
solar radiation
33
5
Analysis of variance of wilting point data
34
6
Average daily rates of water loss in grams by transpiration under different levels of solar radiation and soil
moisture
35
Average daily rates of water loss in grams by evapotranspiration under different levels of solar radiation and
soil moisture
37
Average daily rates of water loss in grains by evaporation
under different levels of solar radiation and soil moisture
39
Analysis of variance of unweighted means of moisture loss
data from soil moisture and solar radiation study
41
3
4
7
9
APPENDIX
1 Soil moisture retention at different atmospheric pressures
for soil used in moisture depletion study
2.
52
Particle size distribution of soil used in soil moisture
depletion study
52
Data on temperature, relative humidity, and solar radiaation from May 15 to August 22, 1961
54
4
Soil moisture retention at different atmospheric pressures
in soil moisture and solar radiation study
57
5
Particle size distribution of soil used in soil moisture
and solar radiation study
57
3
vi
LIST OF FIGURES
Figure
1
2
3
4
5
6
7
9
10
U
1
2
Title
Page
Mean daily water losses in grams by transpiration for
different levels of soil moisture and solar radiation
21
Mean daily water losses in grains by evapotranspiration
for different levels of soil moisture and solar
radiation
23
Mean daily water losses in grains by evaporation for
different levels of soil moisture and solar radiation
25
Mean daily water losses in grains by transpiration,
evapotranspiration, and evaporation at different levels
of soil moisture under 100 percent solar radiation
2g
Mean daily water losses in grams by transpiration,
evapotranspiration, and evaporation at different levels
of soil moisture under
70 percent solar radiation
29
Mean daily water losses in grams by transpiration,
evapotranspiration, and evaporation at different levels
of soil moisture under 49 percent solar radiation
30
Mean daily water losses in grams by transpiration,
evapotranspiration, and evaporation at different levels
of soil moisture under 6 percent solar radiation
31
Mean daily water losses by transpiration, evapotranspiration and evaporation during a soil moisture depletion
study
32
Mean daily water losses in grains by transpiration
under four different levels of soil moisture and
solar radiation
36
Mean daily water losses in grams by evapotranspiration under four different levels of soil moisture
and solar radiation
3
Mean daily water losses in grams by evaporation under
four different levels of soil moisture and solar radiation
40
APPENDIX
Diagram of design of soil moisture depletion study showing
the position of each pot by code number
52
Diagram of design of soil moisture and solar radiation
study showing the position of each pot by code number
vii
57
INTRODUCTION
There have been few basic studies concerning the rates of
transpiration, evapotranspiration, and evaporation as related to soil
moisture and solar radiation.
Such studies are essential to a more
complete understanding of the hydrologic cycle and of its modification
by land and vegetation management practices.
In arid regions water is a limiting factor in almost every
sphere of human activity.
Much interest is centered in obtaining more
water for a variety of human uses and purposes.
The magnitude of the
yields of econcmtLc watershed plants is often determined by the amounts
of water available to these plants.
At the same time the amounts of
water occurring as streuflow is dependent in part upon the kinds and
density of the vegetation occurring on the watershed.
To understand
these reciprocal relationships and to make intefligent watershed
management decisions,the processes relating to water losses must be
studied in both field and laboratory.
The basic purpose of this study was to provide a qualitative
index of the relations among water loss, solar radiation, and soil
moisture.
The study used Aleppo pine as an indicator due to its rapid
establishment when transplanted under the climatic conditions at Tucson,
Arizona.
The study was a fixed model so quantitative applications of
results to field conditions or to other populations are not justifiable.
This study is part of a larger study designed to study plant-water re-
lations in the pine zones of Arizona.
1
REVIEW OF LITERATURE
Knowledge of the processes utilizing soil moisture is important
in watershed managnent.
There is a large amount of reference material
available concerning transpiration and evaporation.
In view of this
fact only the more pertinent material is presented here as a background.
The question of whether soil water between field capacity and
permanent wilting point is equafly available to plants for growth 1as been
disputed for nearly 30 years (Stanhifl, 1957).
Veihmeyer and Hendrick-
son (1955) proposed the view that water is equafly available to plants
at any point between field capacity and the permanent wilting point.
Their work was done using Aleppo pine sealed in a suspended tank
equipped with an automatic weighing device.
Graphical results indicated
that there was no difference between the rates of transpiration immediate-
ly after irrigation and when the soil mass was near the wilting point.
Similar results were obtained in subsequent experiments with other plants.
Slatyor (1957) stated that the permanent wilting point was not a
soil constant but a plant osmotic characteristic.
He further pointed out
that the evidence of Veihmeyer and Hendrickson was based on lysimeter
studies where results may be altered by uneven root distribution.
There-
fore, the measurement of plant responses and the interpretation of these
data may be in error.
Lane and McComb (l94) using several species of plants reported
2
3
that a decrease in transpiration with increasing soil moisture tension
existed.
They also reported that the soil moisture percentage at the
permanent wilting point varied between species.
However, these differ-
ences may have been due to differences in root penetration of the soil
mass together with differences in extent of suberization, rate of
respiration, and protoplasmic differences in the water absorbing area of
the roots of different species.
Stanhill (1957) summarized the results of
0 experiments relating
growth to soil moisture and showed that in 65 cases the greatest yields
were associated with the wetter soil moisture regime.
Only in a carrot
seed crop was the greatest yield associated with a dry regime.
The 14
experiments that showed no response to soil moisture were older experi-
ments in which fruit growth was the yield criteria.
He attributed these
results to the ability of the flowering parts to compete with vegetative
parts for water during periods of moisture stress.
Kramer and Kozlowski (1960) suggest that much of the confusion
relating to water availability and transpiration in relation to soil
moisture has resulted from failing to measure the internal water balance
of the plant and to quantitatively estimate the factors controlling availability of soil water to the plant and the rate of water loss from the
plant.
The work of Richards and Wadleigh (1952) showed that availability
of soil moisture to plants depended on the shape pf the moisture tension
curve.
Gingrich and Russell (1957) found that the growth of corn roots
at equal moisture stress was greater when the stress was developed
4
osmotically than when it was developed as soil moisture stress.
They
attributeI the lesser growth under soil moisture stress to a rapid change
in water transmission rates in their soil as soil moisture stress was
increased.
The effect was most pronounced in the 1- to 3-atmospheres
range.
Slatyer (1957) has attempted to show some of the relationships
among transpiration, soil moisture, soil moisture stress, diffusion pressure deficit, osmotic pressure, and relative turgidity.
He concluded
that since transpiration is primarily a passive plant process it need not
cease but may be reduced as soil moisture stress increases due to stomatal closure and reduced rates of water movement in unsaturated soils.
Transpiration may continue after the death of a plant, limited by the
energy available for evaporation, the resistance to water movement into,
through, and out of the plant, and by the rate of flow of soil water into
the roots.
Kramer (1937) reported that water absorption lagged behind transpiration during the day but exceeded it at night.
He found no differ-
ences in the lag between woody, herbaceous, and succulent species.
In
all cases transpiration changes preceded absorption changes so he concluded
water intake was largely governed by water loss.
Bierhuizen (195) showed that transpiration and evapotranspiration
decreased dth increasing soil moisture tensions under conditions of high
evaporative potentials.
inconclusive.
Under low evaporative conditions the results were
He suggests that some of Veihmeyer's and Hendrickson's
experiments may have been conducted under conditions of low potential
5
evaporation.
Closs (1958) states that two processes may be involved in
determining transpiration.
tial water absorption.
They are: potential transpiration and poten-
The most important one in any given situation
depended upon where along the soil moisture tension curve the plant was
located.
Bierhuizen (1958) suggested that the transpiration rate is greater
at higher light intensities, increases with available soil moisture and
may become constant at high moisture levels.
Recently, Kuiper and Bierhuizen (1958) studied the influence of
several envirorm'iental factors on transpiration of cut leaves in potoineters
and of entire plants grown in soil.
They found that Fick's diffusion law
can be applied to transpiration of cut leaves in potorneters.
They also
presented a formula to estimate the transpiration of plants in soil.
Kuiper and Bierhuizen found the effect of soil moisture levels
on transpiration was great under conditions of high potential transpiration and small under low potential transpiration conditions.
They sug-
gest that soil structure and oxygen content of the soil may become
limiting factors in soil moisture absorption under certain conditions.
Bierhuizen (195$) states that some of Veihmeyer's early work showing soil
moisture to be equally available was done on coarse- to medium-textured
soils.
On clay soils deep-rooted species may show wilting responses to
moisture deficits before the soil moisture reaches the permanent wilting
point.
Kuiper and Bierhuizen (1958) found a linear relationship between
light intensity and transpiration.
At low light intensities (up to
6
17.5
x lO
ergs/cni.2/sec.)
6 percent of the light effect was due to
stomatal changes and decreased diffusion resistance to water.
Less than
14 percent was due to a temperature change and the accompanying increased
vapor pressure deficit.
At high light intensities the linear relationship
was thought due mainly to an increase in leaf temperature and vapor pressure deficit.
They stated that in experiments with plants in soil, the soil
moisture tension affects the total diffusion resistance of leaves independent of light intensity.
The effect of soil moisture on diffusion
resistance may be due partly to incipient drying and partly to an osmotic
stoinatal reaction.
Abd el Rahman, Kuiper, and Bierhuizen (1959) found a linear relationship exLsted between transpiration and incident light intensity
with the exception of the lowest light intensity under controlled temperature and relative humidity conditions.
They attributed the deviation
of transpiration at the low light intensity to morphological differences
in the plants grown at these light intensities.
Tests on 25 sample trees showed that light and soil moisture were
the most prominent factors affecting transpiration (Rothacker, 1949).
A decreased transpiration rate occurred with increased soil dryness.
Plants in sealed containers placed on a tower at the crown levels of a
forest stand and exposed to full solar radiation lost twice as much
moisture as similar plants at the ground level in the same length of time.
Hendrickson (1942) stated that quantitative results from container experiments such as those Rothacker reports cannot be applied to watershed
areas.
7
Wiegand and Taylor (1961) stated that evapotranspiration, the
suni of transpiration plus evaporation, must be analyzed on the basis of
certain plant factors.
Two such factors are the adaptations of land
plants for mad.muni photosynthetic advantage and developuent of xylem
which provides a supply of water to the mesophyll cells of the leaf.
The effectiveness of stomata in control of transpiration is much debated.
Stomata do occur at the only point at which protection against transpiration would be effective, namely at the plant-air interface.
These authors also discuss three stages or periods in the drying
of porous solids by evaporation.
During the constant rate period
evaporation is 1 mi ted by external conditions.
rate periods of drying can be separated.
The two distinct falling
The first is controlled by the
rate of surface evaporation and the second is controlled by the rate of
internal liquid diffusion.
Rowe (194) found that evaporation losses from the upper 12-inch
depth of soil of annually burned plots in California chaparral were great--
er than evapotranspiration losses from undisturbed plots.
He also stated
that bared soil surfaces showed more rapid drying between storms than
undisturbed plots.
Zahner (l95a) found that a hardwood imderstory increased the rate
of soil moisture depletion in pine stands
A chemical treatment of the
understory was more efficient than a controlled burn in preventing moisture loss.
By the end of the growing season the soil in both treated
and untreated areas had reached the wilting point and moisture loss had
almost ceased.
meyer (1955).
These results agree with those of Hendrickson and VeihThese authors stated that the greatest water loss occurred
8
during the growing season.
Water loss decreased rapidly at the end of
the growing season.
Netz and Douglass (1959) working with a 66-inch depth of Piedmont soil found that evapotranspiration losses from a pine plantation
were greater than evaporation losses from a barren area.
These authors
found that evaporation from the surface 5-inches of soil was greater
than evapotranspiration during a 40-day drying period.
that moisture loss is related to soil depth.
They concluded
Under vegetation it is a
reflection of root concentration and from bare soils it is a reflection
of the characteristics of evaporation.
Koshi (1959) found that no differences in soil moisture retained
throughout the growing season existed in the top 12-inches of soil under
an undisturbed oak stand, a thinned oak stand, and a cleared area in
Texas.
In the 12-- to 24-inch horizon the moisture losses on the cleared
plots were distinctly less than for the other two treatments.
Failure to calculate interception losses when estimating soil
moisture depletion by soil sampling under field conditions may alter the
results of an experiment (Lull and Axley, 1958).
Interception losses may
amount to as much as 10 percent of the total rainfall.
This may account
for part of the differences in water loss between forested and bared
plots.
Zahner (1955) found that the rate of water loss between oak and
pine stands was little different for corresponding depths.
The upper
3-feet of soil lost water twice as fast as the layers below three feet.
Gardner and Firan (l95) stated that there are two ma.xiinum
9
soil moisture evaporation rates: one is the potential rate as determined
by external conditions, and the other is determined by the rate at which
water can be transmitted in the soil.
Water will be limited by the lesser
of' the two and may vezynearly equal the lesser.
1easurements of transpiration of intact five-stamen tamarisk
(Tamarix pentandra Pall.) plants by using an infrared gas analyzer
(Decker and Wetzel,
1957)
showed that transpiration increased linearly
with increasing light intensity up to 600 foot-candles.
was reached between 600 and 3,000 foot-candles.
Light saturation
Arizona cypress (Cupres-
sus arizonica Greene) gave results essentially the same as those obtained
with tamarisk.
These authors also found that transpiration decreased
with increasing humidity and increased with increasing temperature.
In a study of diurnal variations of transpiration (Parker,
1957)
the evening drop of transpiration seemed clearly related to decreases in
light intensity.
Occasional periods of cloudiness during the day had no
apparent effect in depressing transpiration rates.
Janes (1954) noted that even though leaves may absorb some water
from a saturated atmosphere, this water did not replace soil moisture in
promoting growth.
He also pointed out that leaves under a partial mois-
ture stress may have reduced rates of photosynthesis.
Peters and Russell (1959) reported that the relative water losses
from field corn by transpiration and evaporation from 1954 through 1957
resulted in the conclusion that transpiration accounted for only 30 to 50
percent of the total water loss.
The remainder was attributed to
evaporation.
Harrold
et al.
(1959) using plastic covered lysimeters reported
10
that evaporation accounted for over half of the total water loss from
field corn.
Holt and Van Doren (1960) used plastic covered plots to estimate the water losses from field corn.
50 percent of the total water loss.
Evaporation accounted for 40 to
The low water loss on bare plots
indicated a low rate of evaporation when the soil surface was dry.
Recent gains have been made in research regarding the relative
magnitudes of evaporation, evapotranspiration, and evaporation.
However,
more research is needed before the effects of vegetation manipulation
and land management practices can be fufly evaluated.
RESEARCH METHODS
Soil Moisture Depletion Study
The first part of this study of the relations among soil moisture, solar radiation, and water loss involved deriving a soil moisture
depletion curve.
The study was designed to investigate the rates of
water loss by transpiration, evapotranspiration, and evaporation over a
range of soil moisture and solar radiation levels.
The results may help
explain the magnitudes of the processes involved in water loss.
Seedlings of Aleppo pine (Pinus halepensis Mill.), two years old,
grown in plastic pots were used for part of these determinations.
Than-
spiration was studied by using pots that contained a tree seedling with
the soil protected from evaporation; evapotranspiration, by pots that con-
tained a tree seedling with the soil exposed; and evaporation, by pots
that contained soil only.
Soils.
Four groups of 15 pots were filled with a forest soil
from the Bear Wallow area of the Santa Catalina Mountains of Arizona.
The soil used in this experiment was collected under a mixed stand of
ponderosa pine (Pinus ponderosa Laws.) and douglas fir (Pseudotsuga
taxifolia (Poir) ]3ritt.).
to remove rocks.
It was screened through a quarter-inch mesh
The soil had a field capacity of 26 percent soil mois-
ture using 1/3-atmosphere of tension as a simulator, and a wilting point
of nine percent soil moisture using 15-atmospheres of tension.
11
Other
12
soil moisture tests were run at 3.4 and 5-atmospheres of tension
(Table 1, Appendix).
The mechanical composition of the screened soil was
48 percent sand, 47 percent silt, and 5 percent clay (Table 2, Appendix).
The oven-dry weight of the soil was determined for each pot and proved to
be in the range of 4000 to 5000 grams.
Climate.
A continuous record of relative humidity and tempera-
ture was obtained during the experimental period by placing a hygrothermograph in the experimental area.
Day temperatures ranged from 96° F. to
over 110° F. and night temperatures from 56° F. to 840 F.
Although some
of the day temperatures were high for plant growth, they did not extend
over long periods of time (Table 3, Appendix).
During the experimental period the daily maximum temperature increased from a high of 100° F. during the last two weeks of May to a
daily maximum of over 110° F. during the period from June 13 until June
27,
1961.
Another period of maximum daily temperatures over 110° F. pre-
vailed from July 7 until July 22,
1961.
After July 27, the daily maxi-
mum temperature dropped to approximately 100° F. for the remainder of
the experimental period.
Daily minimum temperatures averaging 60° F. were recorded from
May 15 until June 8, 1961.
The minimum daily temperature increased to
an average of 800 F. within 10 days and remained at this level until the
end of the experiment on August 22,
1961.
Relative humidity during the experimental period ranged from
5 percent to 100 percent.
A minimum daily relative humidity of less
than 15 percent prevailed from May 15 until June
9, 1961.
The highest
13
minimum relative hwniciity was 42 percent recorded on August 15, 1961.
Madmum relative humidity never exceeded 65 percent between May
15 and June 17, 1961.
After June 17 madnium daily relative humidity
increased steadily until July 28, 1961.
After July 28 relative humidity
values above 85 percent were recorded almost every day until August 22,
1961.
High relative humidity values were obtained during summer convec-
tional storms but lasted for only a short time.
A record of total solar
radiation in gram-calories/cxn.2/day was obtained from the Institue of
Atmospheric Physics, The University of Arizona (Table 3, Appendix).
Pot Treatment.
In this part of the study treatments were repli-
cated five times; each replication consisting of the following three
treatments.
To study evapotranspiration, seedlings of Aleppo pine were
planted in five randomly selected pots out of each group of 15.
To study
transpiration five other pots were treated by planting a seedling in the
pot and then covering the soil surface with 4 mu
neutral polyethylene
film and sealing it to the tree trunk with grafting wax to minimize evaporation.
The remaining five pots in each group of 15 were left as a
bare soil check on evaporation.
For convenience, the pots were numbered
in such an order that the first five pots out of each group of 15 held a
tree with the soil covered with polyethylene, the second five pots held
a tree seedling, and the remaining five pots contained soil only.
Each pot in the experiment was 10-inches in diameter and allowed
78.54 square inches of surface area when filled with soil.
10-inches deep and filled with 7 to 9 inches of soil.
Each pot was
The pot was equip-
ped with a bottom liner of one-inch thickness fiberglass to minimize
14
water loss through the bottom of the pot and two plastic tubes leading
into the soil to allow uniform watering throughout the soil depth.
The
two tubes were placed 3-inches and 6-inches below the surface of the
soil, respectively.
The buried end of each tube consisted of a circular
section with several holes punched along this length to allow uniform
lateral watering throughout the soil mass.
The weight of each pot with
equiinent, oven-dry soil, and tree was determined to the nearest gram and
soil moisture changes were determined by weighing.
Tree seedlings were planted in the plastic pots on February 12,
1961 and allowed an establishment period under field capacity soil inoisture conditions until May
On May 15, 1961
15, 1961.
the pots were moved to The Plant Materials Center
of the University of Arizona.
They were randomly placed by groups on a
prepared bench two and one-half feet above the ground with a roof of
4 mu
neutral polyethylene four feet above the bench surface to prevent
normal summer precipitation from entering the pots.
The sides of the
bench were not enclosed so normal air temperatures prevailed throughout
the measurement period.
To prevent solar radiation from striking the
sides of the pots, thus increasing soil temperatures, a 6-inch border was
constructed around the edge of the bench and the interval between the pots
on the bench was filled with excelsior.
At the start of the measurement period each randomly selected
group of 15 pots was placed under one of four solar radiation levels.
Solar radiation levels were 100 percent, 70 percent, 49 percent, or 6 percent solar radiation.
For convenience, the pots were numbered in groups
15
of 15.
Pot numbers 61 to 75 were placed under six percent solar radiation,
76 to 90 were placed under 49 percent solar radiation, 91 to 105 were
placed under 70. percent solar radiation, and 106 to 120 were left under
full incident solar radiation.
For a diagram of the design see Figure 1,
Appendix.
The soil in each pot was watered to 26 percent which corresponded
to field capacity as determined by 1/3-atmosphere of tension.
Daily water
losses were determined by weighing each pot to the nearest gram from the
time the soil was brought to field capacity until the seedlings were permanently wilted.
Analysis.
This required about 30 days.
Daily water losses were not related to leaf area but
were left on a per pot basis due to the difficulty of determining the
actual internal leaf area involved in transpiration and to aflow a direct
comparison of transpiration, evapotranspiration, and evaporation (Decker,
1955).
Wilting Point Study
t'hen the plants were permanently wilted at the termination of the
soil moisture depletion study the moisture percentages in each pot were
determined from the total pot weight.
The wilting behavior of the
Aleppo pine seedlings was noted on the experimental plants by a change
in hue and a dessicated and curled appearance of the juvenile needles and
from the daily water loss values.
These served as guides to determine
when permanent wilting had occurred.
The purpose of this work was to determine the soil moisture percentage when the tree seedlings reached permanent wilting over a range
16
of incident solar radiation levels.
The results may help explain failures
in tree reproduction under restricted light conditions.
Analysis.
Evaluation of this part of the experiment was supplenien-
ted by using analysis of variance.
Soil Moisture and Solar Radiation Study
The second major section of this study of the relations among
soil moisture, solar radiation, and water loss involved establishing
daily water loss averages under different pot treatments, soil moisture,
and solar radiation combinations.
The results may add some information
on the processes involved in soil moisture loss.
Similar two-year-old seedlings of Aleppo pine (Pinus halepensis
Mill.) were used for this work.
Transpiration studies were conducted
by planting a tree in a plastic pot and covering the soil surface with
polyethylene.
Evapotranspiration was studied by using a tree seedling
with an exposed
8oil
surface contained in a plastic pot.
Plastic pots
containing soil only were used to study evaporation.
Soil.
Four groups of 15 plastic pots were filled with a forest
soil from the Bear Wallow area of the Santa Catalina Mountains of Arizona..
The soil used in this part of the study was collected under a mixed stand
of ponderosa pine (Pinus ponderosa Laws.) and douglas fir
tadio1ia (Foir) ]3ritt.).
mesh to remove rocks.
(Pseudotsuga
It was also screened through a quarter-inch
Moisture tension analysis of this soil at 1/3-
atmosphere revealed a "field capacity" moisture content of 22 percent.
A similar test at 15-atmospheres of tension showed the permanent wilting
point to be at nine percent soil moisture.
Other soil moisture
17
values were determined at
Appendix).
3.4-
and 5-atmospheres of tension (Table
4,
Iechanical analysis of the screened soil showed 50 percent
sand, 46 percent silt, and
4 percent clay (Table 5, Appendix).
The oven-
dry weight of the soil in each pot ranged from 4000 to 5000 grams.
Climate.
This section of the experiment was run concurrently.
with the other sections so climatic conditions were the same (Table 3,
Appendix).
Treatment.
Each of three treatments in this part of the study
was replicated five times.
Evapotranspiration was studied by planting
seedlings of JLleppo pine in five randomly selected pots out of each group
of 15.
Five other pots received a treatment to study transpiration con-
sisting of planting a tree seedling in the pot and covering the soil
surface with a 4 mil neutral polyethylene film and sealing the polyethy-
lene to the seedling stem with grafting wax to minimize water loss by
evaporation.
The remaining five pots of each group were left as bare soil
surfaces to study evaporation.
The same pot numbering scheme was again
adopted as was previously employed.
The first numbered five pots of each
group of 15 contained a tree seedling with the soil covered, the second
numbered five pots held a tree seedling, and the remaining numbered pots
held soil only.
The pots used in this experiment contained fiberglass
bottan liners and watering tubes identical with those described earlier.
Tree seedlings were transplanted to the plastic pots on February
2 and February
3, 1961
and allowed an establishment period under field
capacity soil moisture conditions until May 15,
1961.
On May 15, 1961
these post were also transferred to The Plant Materials Center and placed
18
on the bench that was previously described.
At the start of the measurement period each group of 15 pots was
brought to a selected soil moisture level (22 percent, 18 percent, 14 percent, or 10 percent) as indicated by pot weight and placed under full
solar radiation conditions in a completely randomized design.
1 to 15 were watered to 22 percent soil moisture, 16 to
to 18 percent soil moisture,
moisture, and
46
Pot ninnbers
30 were watered
31 to 45 were watered to 14 percent soil
to 60 were watered to 10 percent soil moisture.
sign of the experiment is shown in Figure 2, Appendix.
combination was continued for 16 days.
The de-
This measurement
Pots were weighed to the nearest
grain daily and were rewet to the selected soil moisture level each day by
adding water to the pots as described hereafter.
After each pot was weighed and the proper amount of water measured
into a graduated cylinder the water was added to the pot by thirds.
One
third was placed into the tube leading 6-inches below the soil surface,
one third was placed into the tube leading 3-inches below the surface and
the remainder was added evenly to the surface of the soil.
This method
was used to insure that the soil mass in each pot was watered equally
throughout the soil depth.
After the full solar radiation measurements had been completed
various screens were erected over groups of pots to allow various incident
solar radiation (70 percent, 49 percent, and 6 percent of the total) levels
and measurements including watering were continued in the same manner and
for the same length of time as previously described.
At the end of the
experiment measurements had been recorded for all soil moisture and solar
radiation combinations with the exception of the groups of pots having
19
10 percent soil moisture and an incident solar radiation level of 70 percent and 49 percent of total solar radiation.
The experimental seedlings
did not survive at this low soil moisture level.
Tree growth during the period was not measured but appeared to be
negligible.
Analysis.
Analysis of this section of the experiment was supple-
mented by using analysis of variance of unweighted means.
RESULTS
Soil Moisture Depletion Study
Transpiration.
To obtain a measure of the rate at thich tran-
spiration was depleting soil moisture each pot was weighed daily.
correction was made for the green weight of the pine seedling.
A
The
average weights of soil moisture lost for five replicates during the
first eight days of the wilting period, during the remaining II to 24
days before the tree seedlings were pernianentr wiJted, and during the
entire period are presented in Table 1.
Moisture losses in relation to
soil moisture are displayed graphically in Figure 1.
1Jhen solar radia-
tion treatnents are listed according to the amounts of water lost during
the first eight days, from the largest to the smallest, they show the
following order:
100 percent, 70 percent,
49 percent and 6 percent.
The results showed that transpiration decreased as soil moisture decreased under all levels of incident solar radiation.
However, after
the first eight days or when the soil moisture had reached approdmate-
ly 22
percent the decline was fairly constant regardless of the solar
radiation level.
Table 1.
Mean water losses by transpiration for different levels of
incident solar radiation
Solar Radiatio
cal./cm. /
percent
day
100
70
49
6
775
542
30
47
First
grams
240
22
132
109
20
days
Remaining
Period
grams
days
145
195
194
11
11
24
24
24
Total
grams
35
423
326
290
50
100 % SOLAR RADIATION .
0__
70 %
'I
49 0/
- _O
6%
40
(I)
S
4
Q
I 30
>-
4
0
7
Lii
/
a-
U)
U)
0
/ 4/
-J
/',+
/-/.-' +
w
I.4
- _+_
12
24
20
16
SOIL MOISTURE
Figure 1.
-+
+
0
0
a
28
PERCENT
Mean daily water losses in grams by transpiration for different levels of soil moisture and
solar radiation.
21
22
Evapotranspjratjon.
The same procedures were used in measuring
evapotranspiration as described for transpiration.
The average weights of soil moisture lost during the first eight
days of the wilting period, during the remaining 17 to 20 days before the
seedlings were permanently wilted, and during the entire period are presented in Thble 2.
lJhen the solar radiation treatments are listed accord-
ing to the amounts of water used during the first eight days, frcn greatest
to smallest, they fall in the order 100 percent, 49 percent, 70 percent
and 6 percent.
After the first eight days of the wilting period the soil
moisture losses were approdniate1y the same regardless of incident solar
radiation.
Therefore, after the soil moisture had dropped to 19 percent,
solar radiation did not affect water loss.
Moisture losses in relation
to soil moisture percentage are displayed in Figure 2.
The curves did
not follow the same pattern as did the tabulated water loss values.
This effect was due to high water losses during the first two days under
70 percent solar radiation.
Water loss decreased rapidly during the next
The curves showed that water loss decreased both with decreas-
six days.
ing soil moisture and decreasing solar radiation during a period when no
water was added.
Table 2.Mean water losses by evapotranspiration for different levels of
incident solar radiation.
Solar Radiation
cal./cm.2/
day
perceift
100
70
49
3go
6
47
775
542
First
grains
448
333
346
274
days
Remaining
Period
grams days
225
229
234
264
18
17
17
20
Total
grams
673
562
580
538
100 % SOLAR RADIATION '
70%
I 00-
I,
49 0/
/
6
------
"
"
+
+
80U)
4
a:
0'
---
o
7
- -+
0
0
12
SOIL
Figure
2.
20
16
MOISTURE
24
28
PERCENT
Mean daily water losses in grams by evapotranspiration for different levels of soil moisture
and solar radiation.
23
24
Evaporation.
With the exception of correcting water loss values
for the weight of a tree seedling the same procedures were used as described
for transpiration.
The average weights of soil moisture lost during the first eight
days of the moisture depletion period, the remaining 24 days, and for
the entire period are presented in Table 3.
Water loss under different
solar radiation treatments,when listed from the largest to the smallest
over the first eight days follows an order of 70 percent, 100 percent,
49 percent, and 6 percent incident solar radiation.
followed for total water loss.
The same order is
The differences are probably not large
enough to be significant.
Moisture loss in relation to soil moisture percentage is shown
graphically in Figure 3.
Moisture losses by evaporation decreased with
decreasing soil moisture but were not related to incident solar radiation.
The curves and data show the rate at which soil moisture was depleted by
evaporation during the period when no water was added.
Table 3.Mean water losses by evaporation for different levels of solar
radiation.
Solar Radiatio
cal./cm. /
percent
day
First 8 days
grains
Remaining
Period
grams days
Total
grams
100
775
426
239
24
665
70
542
463
254
24
717
49
380
382
206
24
588
6
47
377
194
24
571
100 0/, SOLAR RADIATION .
70 0/,
o
160-
49 %
rj
01
F,
+
+
/
;
'I
80-
0
(I)
Cl)
0
-J
0
U 40
F-
4
0
'
D_
B-+'j. - - q.
-
O
0
I2
SOIL
Figure 3.
20
16
24
28
MOISTURE - PERCENT
Mean daily water losses in grams by evaporation for different levels of soil moisture
and solar radiation.
25
26
Comparison of Transpiration, Evapotranspjratjon, and Evaporation.
In aU solar radiation treatments evaporation exceeded either transpiration or evapotranspiration at high soil moisture percentages.
Evapora-
tion remained the greatest cause of water loss until the soil moisture
dropped to l
percent where evapotranspiration became the greatest.
Al-
though evaporation losses were the greatest at all levels of soil moisture
under 70 percent solar radiation this was probably not significant due to
experimental error.
l
The high evaporation losses at soil moistures above
percent were probably due to direct absorption of radiation and heat
and to the high temperatures and low relative hiunidities under which the
experiment was conducted.
The temperature effects could be more pro-
nounced when small pots containing only a small volume of soil were used
such as in this experiment.
In all cases evaporative losses decreased
sharply with decreases in available soil moisture indicating that the
surface soil moisture was governing water loss (Wiegand and Taylor, 1961).
Evapotranspiration losses were intermediate between evaporation
and transpiration losses at soil moisture percentages above l percent.
This may have been clue to a change in the microenvironment at the soil
surface in the pot brought about by the addition of a tree seedling.
At
soil moisture percentages below l percent evapotranspiration became the
greatest method of water loss except at the 70 percent solar radiation
level.
Evapotranspiration did not decrease as sharply as evaporation in-
dicating that moisture removal from the soil by transpiration became more
important as evaporation was restricted by a dry surface layer.
27
Transpirationa]. water loss was the lowest of the three and in
all cases did decrease as soil moisture decreased.
Low transpirational
water losses under low levels of incident solar radiation may have been
due partly to depletion of food supplies within the plant and eventual
starvation (Abd el Rahnian, Kuiper and Bierhuizen, 1960) but mainly to
less available energy.
Transpiration, evapotranspiration and evaporation under different
levels of incident solar radiation are compared in Figures 4, 5, 6, and
7.
A comparison of transpiration, evapotranspiration and evaporation
averaged over all incident solar radiation levels is presented in
Figure
.
In this case evaporation was the greatest source of water loss
at high soil moisture levels but assumes a position intermediate between
transpiration and evapotranspiration after the eighth day of measurement.
The eighth day of measurement corresponds to about l percent soil moisture.
The effect of solar radiation appeared to be greatest on transpiration and least on evaporation.
This may have been caused by direct
absorption of solar energy by tree seedlings which were highly sensitive
to this form of energy.
Evaporation may have been controlled by air
temperature to such an extent that the effects of solar radiation were
obscured.
Evapotranspiration would show both effects.
160EVAPORATION
TRANSPIRATION
EVAPOTRANSPIRATION ---- -
4
a
w
80
a-
U)
Cl)
0
-J
,,
40-
/
/0/
/
/
/
/
/
/
/
w
I4
0
12
20
16
SOIL
24
28
MOISTURE - PERCENT
Figure 4. Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation
at different levels of soil moisture under
100 percent solar radiation.
2
60EVAPORATION
TRANSPIRATION
E VA POTR ANS PIR AT ION
C,,
4
20
>-
4
a
o
U
/
/
80-
/0
a-
/
U)
C,,
0
-J
U
40-
0,'
0/
oo___
4
0-9--
9-o___
-
0. -S
0
0
0
2 12
SOIL
Figure 5.
20
16
24
28
MOISTURE - PERCENT
Mean daily water losses in grains by transpiration, and evaporation at different levels
of soil moisture under 70 percent solar
radiation.
29
EVA POR ATION
TRANSPIRATION
EVAPOTRANSPIRAT ION
c 80
p
a(I)
U)
0
-J
40w
o__
o-cr
0
12
20
16
SOIL
o
24
28
MOISTURE - PERCENT
Figure 6. Mean daily water losses in grains by transpiration,
evapotranspiration, and evaporation at different
levels of soil moisture under 49 percent solar
radiation.
30
160-
EVAPORATION
TRANSPIRATION - -
-
EVA POT RAN S P1 RAT I ON
C,)
$20C,
>-
4
80w
0.
U)
U)
0
-J
0
40-
- -0
w
I-
4
0
0
- .----.- -.0
$2
20
$6
24
-,
28
SOIL MOISTURE - PERCENT
Figure
7.
Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation
at different levels of soil moisture uxxler
6 percent solar radiation.
31
160-
EVAPORATION
TRANSPIRATION
EVAPOTRANSPIRATION
eO3Cl)
U)
0
-J
c
40-
LU
I4
0
0
I
I
8
16
TIME
Figure
.
24
32
DAYS
Mean daily water losses by transpiration,
evapotranspiration, and evaporation during
a soil moisture depletion study.
32
33
Wilting Point Study
The percentages of soil moisture at permanent wilting for the
indicator plants used in the moisture depletion study are presented in
Table
4.
Overall statistical analysis of these data (Table 5) showed
that differences, significant at the five percent level, existed between transpiration and evapotranspiration with evapotranspiration reducing the soil moisture percentage to a lower level in all cases.
Solar
radiation levels were also significant.
Soil moisture was reduced to a lower level by evapotranspiration
than by transpiration alone.
These data are mean values for five ex-
periniental plants each having the same treatment.
With the exception of
the 49 percent solar radiation treatment with polyethylene covered soil
all means decrease, aligned in an order of increasing solar radiation.
Comparisons among individual wilting percentages showed that in
both transpiration and evapotranspiration the 6 percent and
49 percent
incident solar radiation levels gave permanent wilting points significantly higher than the wilting points under greater levels of solar radiation.
This may have been due to restricted root growth, water absorption, and
photosynthate production.
Table
4.
Soil moisture percentages at permanent wilting of tree seedunder two sources of water loss and four levels of solar
radiation.
lings
Source
100
Solar Radiation (Percent)
70
6
49
Total
Average
Transpiration
15.78
16.15
20.39
19.25
71.57
17.89
Evapotranspiration
U.96
12.69
14.81
15.31
54.77
13.69
34
Table
5.
Analysis of variance of wilting point data.
Source
Degrees of
Freedom
Sum of
Mean
Squares
Square
F
Pot treatment
1
173.98
173.98
35.72*
Solar radiation
3
106.68
36.23
7.44*
Interaction
3
12 81
4.24
32
155.91
4.87
39
451.38
Error
Total
*
0.87
Significant at the five percent level.
Soil Moisture and Solar Radiation Study
Measurements were made of the rates at which soil moisture was
depleted by transpiration, evapotranspiration, and evaporation under four
different levels of solar radiation and four soil moisture percentages.
Each value reported is an average of from 29 to 75 indisrLdual observations.
Soil moisture exerted a definite influence on daily transpiration-
al losses but the role of solar radiation was obscure although greatest
loss appeared to occur at full solar radiation.
As soil moisture became
limiting transpiration decreased in the same manner as in the moisture
depletion study.
The decrease was present under all levels of incident
solar radiation.
The actual incident solar radiation probably did not
greatly influence the average daily water loss due to the high temperatures and low relative humidities of the environment under which the
determinations were made.
Data for mean daily water losses by transpiration under different
35
levels of soil moisture and solar radiation are presented in Table 6 and
9.
Figure
6.
Table
Average daily rates of water loss in grams by transpiration
under different levels of solar radiation and soil moisture.
Solar
Radiation
Soil Moisture (Percent)
22
18
10
14
(Percent)
100
70
49
6
Total
Average
52
37
40
41
34
27
36
24
12
18
19
5
13
6
170
121
30
62
16
22
42
6*
5*
Total
103
88
100
84
Average
26
22
25
21
6
* Measurements not taken, values added for computation assuming that
actual observations would be intermediate to other observed values at
10 percent moisture.
Evapotranspirational water losses foflowed the same pattern as
transpirational losses.
Soil moisture levels exerted the greatest in-
fluence on daily water loss.
The results showed a decreasing rate of
water loss as soil moisture was depleted under constant solar radiation.
This was the same result pattern as was obtained in the soil moisture
depletion study.
A solar radiation effect was present between fufl
solar radiation and the reduced levels of solar radiation at all soil
moisture levels.
The 100 percent solar radiation level allowed a much
greater rate of daily water loss than did the lower levels of solar
radiation.
This may have been partiJ]y due to the experimental procedure
of taking all the observations under 100 percent solar radiation during
the same period of time.
The observations of evapotranspiration under
100% SOLAR RADIATION
50-
70%
49 %
6%
40U)
2
4
30-
20
0
0
5
10
SOIL
1igure 9.
15
20
MOISTURE - PERCENT
Mean daily water losses in grams by transpiration under four different levels of soil
moisture and solar radiation.
36
37
the other three levels of solar radiation were taken over the next two
months.
Thus a variation in climatic conditions over the measurement
period may have altered the results.
At low solar radiation levels the
effect may have been obscured by high temperatures and low relative humidities.
Daily evapotranspiration losses under different levels of soil
moisture and solar radiation are presented in Table 7 and Figure 10.
Table 7.
Average daily rates of water loss in grams by evapotranspiration under different levels of solar radiation and soil moisture.
Solar
Radiation
(Percent)
100
70
49
6
Total
Average
Soil Moisture (Percent)
22
18
14
10
209
139
139
343
177
83
26
93
87
73
39
30
36
23*
20*
630
158
430
108
188
47
86
22
17
Total
Average
495
294
276
269
124
74
69
67
* Measurements not taken, values added for computation assuming that
actual observations would be intermediate to other observed values at
10 percent moisture.
Daily water losses by evaporation were related to soil moisture and
solar radiation as in the soil moisture depletion study.
Evaporative
losses were definitely influenced by decreasing soil moisture but followed no definite pattern with regard to incident solar radiation.
soil moisture became limiting evaporation was retarded.
As
A dry soil sur-
face was not present in this study because water losses were replaced
daily by watering.
The reduction in evaporative loss was due to an
100 0/, SOLAR RADIATION
70%
200
-
49%
6%
160p
/
120-
80
U)
U)
0
-J
c
LU
40-
I-
0
0
5
10
SOIL
15
20
MOISTURE - PERCENT
Figure 10. Mean daily water losses in grams by evapotranspiration under four different levels
of soil moisture and solar radiation.
3
39
increased resistance to evaporation as soil moisture decreased (Table 8
and Figure II).
Table 8.
Average daily rates of water loss in grains by evaporation under
different levels of solar radiation and soil moisture.
Solar
Radiation
Soil Moisture (Percent)
(Percent)
22
100
70
49
18
14
10
99
6
68
54
51
22
21
34
4
388
218
161
20
97
55
40
5
175
45
62
106
6
Total
Average
30
5*
5*
Total
Average
334
131
157
165
84
33
39
41
* Measurements not taken, values added for computation assuming that
actual observations would be intermediate to other observed values at
10 percent soil moisture.
Analysis of variance of unweighted means of all soil moisture
loss data are presented in Table
9.
The pot within treatment sum of
squares was divided by the harmonic mean to obtain a testing term.
The
F test showed that the interaction was non-significant but that pot
treatments and soil moisture-solar radiation levels were significant
at the five percent level.
A compariaon of the data showed that water
losses by transpiration, evapotranspiration, and evaporation were significantly different.
The order exhibited by the water losses, from largest
to smallest, was evapotranspiration, evaporation, and transpiration.
In this section the magnitudes of water loss by evapotranspiration and
evaporation were reversed from those found in the moisture depletion
study ; evapotranspiration was the greater in this case.
A comparison
too o, SOLAR RADIATION
70Db
49%
6%
(,)160
4
I
>-
1'
H
120
4
C)
o80
Ui
C/)
C/)
/
0
J
/
40-
0
0
-ft
;/..7
I
I
I
5
10
15
SOIL MOISTURE
20
PERCENT
Figure 11. Mean daily water losses in grams by evaporation under four different levels of soil
moisture and solar radiation.
40
41
of the mean water losses under the soil moisture-solar radiation treatments showed that only two significant groups of water loss values
existed.
The water losses attributed to transpiration, evapotranspiration,
and evaporation under all incident solar radiation levels at 22 percent
soil moisture and under 100 percent and 49 percent incident solar radiation
at 18 percent soil moisture were significantly greater than the rest of
High soil moisture and incident solar radiation will increase
the means.
water loss.
Comparisons of individual means were not performed since the
interaction was not significant.
Table
9.
Analysis of variance of unweighted means of moisture loss data
frcn soil moisture and solar radiation study.
Degrees of
Freedom
Source
Sum of
Squares
Mean
Square
F
2
29,149
14,575
7.36*
Soil moisture-Solar
radiation treatment
13
70,140
5,375
2.72*
Interaction
26
22,024
847
151
1,291,857
Pot treatment
Pot within treatment
0.42
1,979#
* Significant at the five percent level.
# Pot within treatment sum of squares was divided by the harmonic mean
of 4.322 to obtain a testing term.
A comparison of the results of this experiment with the results
from the soil moisture depletion study showed one difference.
In this
study evapotraflsPJJ'at10fll water loss exceeded the water loss by evapora-
tion alone.
This reversal of results was attributed to the experimental
42
procedure of watering each pot to a selected soil moisture level each day.
The constant wetting of the soil surface allowed some water to remain in
a more available position for water loss by evapotranspiration as evaporation could take place directly from the soil surface as the tree seedlings
removed water from the deeper portions of the soil.
Under the conditions
imposed by the depletion study a dry soil surface was allowed to restrict
evaporative losses and may have depressed transpiration by drying out the
soil around the seedling roots in the top portion of the soil.
rational water losses were the lowest in both studies.
Transpi-
DISCUSSION
Water Loss Under Different
Soil Moisture and Solar Radiation Levels
The results of the soil moisture depletion study and the soil
moisture-solar radiation study indicate that soil moisture exerts a
definite influence on water loss but that the influence of solar radiation is obscure.
Loss by transpiration decreased with decreasing soil moisture
over the entire depletion curve.
This would indicate that the rates
of transpiration are influenced by the availability of soil moisture to
the plant roots.
Evapotranspiration and evaporation were also related to soil
moisture.
While soil moisture was abundant and soil moisture tension
was increasing slowly, the rate of water loss was strongly influenced by
moisture levels.
At lower moisture levels water losses did not decrease
as rapidly but a downward trend was still present.
The failure of solar radiation to be clearly a major factor in
controlling the rate of water loss may be due to the uncontrolled
temperature and relative humidity conditions in the experiment.
A
linear relationship has been observed by some workers between water
loss and incident light intensity (Kuiper and Bierhuizen, l95).
How-
ever, these workers used controlled temperature and relative huii dity
conditions.
In this experiment the combination of high soil moisture
and high solar radiation levels did produce significantly higher mean
43
daily water losses.
A comparison of the rates of water loss by transpiration,
evapotranspiration, and evaporation revealed that transpiration depleted
soil moisture most slowly.
In all cases it was significantly slower than
evapotranspiration or evaporation.
The apparent reversal of the rates of
evaporation and evapotranspiration in the two studies dealing with the
average daily water loss has been attributed to the experimental procedure.
It is probably safe to assume that evaporation from a bare soil
surface would be greater than evapotranspiration from a similar soil with
cover when both soils are at or above field capacity in the surface layer
(Rowe,
l94).
Direct evaporation from a bare soil surface would not
require an expenditure of energy to move water through the plant.
Under
field conditions in arid areas where the soil surface is rapidly dried
evapotranspiration would soon assume a leading role in soil moisture depletion.
This would be especially true on deep soils during extended
periods without precipitation.
Wilting Point
The results of the wilting point study showed that water loss
by evapotranspiration was significantly greater than water loss by
transpiration alone.
The differences were probably due to evaporation
from the upper layers of the soil decreasing the average soil moisture
percentage in the pot below the wilting point of the seedlings.
The results also indicate that statistically significant differenôes in wilting percentages may be obtained under different levels of
solar radiation.
This effect may have been influenced by several outside
45
factors.
One is that plants under low solar radiation and high temper-
atures may not produce enough photosynthate to sustain the plant, root
growth may be restricted, and water absorption lowered.
The eventual
depletion of food supplies within the plant may hasten permanent wilting.
However, with only one exception, higher incident solar radiation
levels did induce permanent wilting at lower soil moisture percentages
than did low solar radiation levels.
These differences are reasonable
even though errors may have been made in judging permanent wilting.
The
one exception occurs in the transpiration experiment where trees under
six percent solar radiation wilted at a lower soil moisture value than
those under 49 percent.
It is possible to conclude from this study that tree seedlings
grown under high incident solar radiation levels will survive under lower
soil moisture conditions than tree seedlings grown under restricted solar
radiation.
Moisture tolerances within a species do vary according to the
amount of solar radiation received.
silviculture.
This has important implications in
SU14MARY
A study was made of the rates of water loss by transpiration,
evapotranspiration, arid evaporation from soil in 10-inch plastic pots.
Transpiration and evapotranspiration were studied using Aleppo pine
(Pinus halepensjs Mill.) in sealed and non-sealed pots.
The effects of
different levels of soil moisture and solar radiation were measured.
In a soil moisture depletion study the water loss from the tree
seedlings wider covered and exposed soil conditions was related to
incident solar radiation and soil moisture.
Moisture loss decreased
both with decreasing soil moisture and decreasing solar radiation.
Evaporation from a bare soil surface was not clearly related to incident
solar radiation.
Evaporation was related to soil moisture and decreased
with decreasing soil moisture.
when the soil was at or near field capacity evaporation was the
greatest source of water loss.
As soil moisture became less available
evapotranspiration assumed the leading role in rate of water loss.
The soil moisture at the permanent wilting points of the tree
seedlings were significantly higher under covered soil conditions than
under exposed soil conditions.
Also, statistically significant differ-
ences existed supporting the conclusion that tree seedlings which received greater incident solar radiation wilted at lower soil moisture
percentages.
46
47
A second study related transpiration, evapotranspiration, and
evaporation to four fixed levels of soil moisture and solar radiation.
Statistically significant differences exLsted which supported the conelusion that greater soil moisture allowed greater rates of water loss.
The effects of incident solar radiation were obscure but an effect may
have been present between full solar radiation and reduced levels of
solar radiation.
BIBLI GRAPHY
Bierhuizen, J. F.
1958. Some observations on the relation between transpiration and
soil moisture. Netherlands Jour. Agric. Sc!. 6(2):94-98.
Reprinted in Tech. Bull. No. 4 (1959). Institute for Land
and Water Management Research, Wageningen, The Netherlands.
Closs, R. L.
1958.
Transpiration from plants with a limited water supply.
Climatology and Microclimatology Canberra Symposium. UNESCO.
Pp. 168-171.
Decker, J. P.
1955. The uncommon denominator in photosynthesis as related to
tolerance. For. Sci. 1:88-89.
Decker, J. P. and B. F. Wetzel
1957. A method for measuring transpiration of intact plants under
controfled light, humidity and temperature. For. Sc!. 3:
350-354.
Gardner, W. R. and N. Fireman
1958. Laboratory studies of evaporation from soil columns in the
presence of a water table. Soil Sc!. 85:2i-249.
Gingrich, J. R. and N. B. Russefl
1957. A comparison of effects of soil moisture tension and osmotic
stress on root growth. Soil Sc!. 84:185-194.
Hendrickson, A. H.
1942. Determinations of the losses of moisture by evaporation from
soils in a watershed area. Trans. Amer. Geophys. Union 23:
471-477.
Hendrickson, A. H. and F. J. Veihmeyer
1955. Daily use of water az depth of rooting of almond trees.
Proc. Amer. Soc. Hor. Sc!. 65:133-138.
Harrold, L. J., D. B. Peters, L. B. Dreibelbis and J. L. NcGuinness
1959. Transpiration evaluation of corn grown on a plastic covered
lysimeter. Soil Sc!. Soc. Amer. Proc. 23:174-178.
Holt, R. F. and C. A. Van Doren
1960. Water utilization by field corn in western Minnesota.
Agron. Jour. 53:43-45.
48
49
Janes, B. E.
1954.
Absorption and loss of water by tomato leaves in a saturated
atmosphere. Soil Sci.
7:l9-197.
Koshi, P. T.
1959.
Soil moisture trends under varying densitites of oak overstory.
Southern For. Expt. Sta. 0cc. Paper 167. New Orleans, La.
Kramer, P. J.
1937. The relation between rate of transpiration and rate of absorption of water in plants. .Ainer. Jour. Dot. 24:10-15.
Kramer, P. J. and T. T. Kozlowski
1960. Physiology of trees. 642 pp.
New York, Toronto, London.
McGraw-Hill Book Company, Inc.
Ku.iper, P. J. C. and J. F. Bierhuizen
195g.
The effect of some environmental factors on the transpiration
of plants under controlled conditions. Mededelingen Van De
Landbouwhogeschool Te Wageningen, Nederland
Reprinted in Tech. Bull. No. 4 (1959). Institute for Land and
Water Management Research, Wageningen, The Netherlands.
5(ll):l-16.
Lane, R. B. and A. L. McComb
194g. Wilting and soil moisture depletion by tree seedlings and grass.
Jour. For. 46:344-349.
Lull, H. W. and J. H. .Axley
Forest soil-moisture relations in the coastal plain sands of
For. Sd. 4:2-19.
southern New Jersey.
l95.
Metz, L. J. and J. E. Douglass
1959. Soil moisture depletion under several Piedmont cover types.
Tech. Bull. 1207. USDA. Washington, D. C.
Parker, J.
1957.
The cut-leaf method and estimations of diurnal trends in
transpiration from different heights and sides of an oak
and a pine. Dot. Gaz. 119 :93-101.
Peters, P. B. and M. B. Russell
1959. Relative water losses by evaporation and transpiration in
field corn. Soil Sci. Soc. Amer. Proc. 23:170-173.
Raliman, A. A. Abd el, P. J. C. Kuiper and J. F. Bierhuizen
1959.
Prelirn.inary observations on the effect of light intensity and
photoperiod on transpiration and growth of young tomato plants
under controlled conditions. Mededelingen Van De Landbouwhogeschool Te Wageningen, Nederland 59(11):1-]2; Reprinted in
Tech. Bull. No. 16 (1960). Institute for Land and Water
Management Research, Wageningen, The Netherlands.
50
Richards, L. A. and C. H. Wadleigh
1952. Water and plant growth. (In Soil PhysLca1 Conditions and
Plant Growth. Pp. 73-251. Academic Press, Inc., New York).
Rothacker, J. S.
1949. Sumnary of the exploratory phase of the Clear Creek transpiration study. TVA Div. For. Relations. Norris, Tenn.
Rowe, P. B.
1948.
Influence of woodland chaparral on water and soil in central
California. Calif. Dept. of Natural Resources, Div. of Forestry.
70 pp.
Slatyer, R. 0.
1957. The influence of progressive increases in total soil moisture
stress on transpiration, growth and internal water relations
of plants. Australia Jour. Biol. Sci. 10:320-336.
Stanhill, G.
1957.
The effect of differences in soil moisture status on plant
growth; a review and evaluation. Soil Sci. 84:205-214.
Veihmeyer, F. J. and A. H. Hendrickson
1955. Does transpiration decrease as soil moisture decreases?
Trans. Amer. Geophys. Union 36:425-L49.
Wiegand, C. L. and S. A. Taylor
1961. Evaporative drying of porous media.
Agric. Expt. Sta. Spec. Report 15.
Utah State University.
Zahner, R.
1955.
Soil water depletion by pine and hardwood stands during a
dry season. For. Sci. 1:258-264.
Zahner, R.
1958.
Hardwood understory depletes soil water in pine stands.
For. Sci. 4:178-184.
A P P E N D. I X
51
52
Table 1.
Soil moisture retention at different atmospheric pressures
for soil used in moisture depletion study.
Tension
atmospheres
Table 2.
Soil Moisture
percent
15 00
9.30
5.00
13.35
3.40
14. 8
0.33
26.31
Particle size distribution of soil used in soil moisture
depletion study.
Particle
size
Cuposition
sand
4
silt
47
clay
5
percent
Total
100
74
75
66
6i
67
70
72
71
Figure 1.
65
62
69
98
99
104
105
94
93
91
103
101 102
100
96
92
97
95
81
77
83
89
90
78
79
87
88
85
76
80
82
84
86
106
111
110
109
119
107
117
108
120
112
113
116
115
118
114
Diagram of design of soil moisture depletion study shodng the position of each
pot by code number.
64
63
73
6
54
Table
3.
Data on temperature, relative humidity, and solar radiation
from May 15 to August 22, 1961.
Temperature
Date
1961
May
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Max.
Mm.
102
105
106
109
109
102
96
104
106
106
102
108
109
106
102
110
106
57
62
57
59
60
56
57
57
58
107
109
100
104
10].
110
56
56
60
64
61
60
63
57
59
61
Relative Humidity
(percent)
Mm.
Max.
32
48
46
40
44
49
52
45
42
48
46
46
43
45
6
5
8
9
7
7
9
15
12
9
9
12
12
12
11
15
50
46
13
40
II
44
47
45
10
15
14
15
13
10
12
13
16
18
20
15
12
14
25
23
21
20
18
22
14
Solar Raiation
cal./cm./day
802
768
723
813
770
825
678
808
812
815
720
681
813
761
625
812
804
June
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
no
110
110
no
109
106
no
no
110
no
no
108
nO
no
no
57
62
57
62
61
65
67
68
69
69
70
70
81
80
72
78
80
81
80
56
42
40
46
48
47
47
49
49
39
46
49
74
63
59
48
68
821
828
815
810
807
712
816
819
798
774
782
714
777
788
781
747
626
754
772
771
756
Continued
55
Continued
Table
3.
Temperature
Date
Solar Rad2iation
Mm.
22
23
24
25
26
27
28
29
110
110
110
110
109
110
105
106
108
70
58
84
83
81
77
82
81
77
77
48
22
18
56
54
20
20
63
22
51
61
75
76
21
29
31
20
751
754
754
637
760
744
727
683
732
1
109
106
102
104
108
107
110
110
110
110
110
110
109
110
110
109
110
110
110
110
110
110
110
107
107
109
106
100
79
76
67
64
26
27
26
14
15
15
20
22
22
17
17
21
26
21
22
26
25
24
20
22
23
33
24
27
20
16
22
26
33
33
28
679
481
578
780
772
745
746
775
775
747
740
755
730
753
742
726
751
741
741
726
715
449
719
641
748
667
636
570
498
570
690
30
July
(percent)
Max.
1961
June
Relative Hunddity
°F.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
97
99
102
71
73
83
77
80
80
82
85
85
82
78
80
79
81
80
78
80
75
72
75
75
76
78
74
76
76
80
76
Max.
97
93
54
38
53
65
6
81
58
68
51
50
72
60
84
69
69
70
76
99
95
92
96
95
88
97
74
80
87
92
Mm.
cal./cm. /day
Continued
56
Continued
Table 3.
Temperature
Date
°F.
1961
Max.
Mm.
106
102
109
96
106
109
110
110
105
104
107
110
102
101
96
98
103
105
110
106
109
75
74
75
72
77
78
78
76
79
76
76
76
80
74
74
Relative Humidity
(percent)
Max.
Mm.
Solar Radiation
cal./crn. /day
August
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
73
74
76
76
84
80
82
83
100
100
75
85
73
90
100
94
70
99
70
100
100
97
100
100
100
49
80
20
26
19
25
19
15
18
20
22
27
23
20
22
24
42
24
21
24
20
20
22
544
576
605
519
729
694
706
706
690
480
571
665
573
573
573
573
621
589
651
672
672
57
Table
4.
Table 5.
Soil moisture retention at different atmospheric pressures
in soil moisture and solar radiation study.
Tension
atmospheres
Soil 1Ioistue
percent
15.00
9.20
5.00
11.47
3.40
13.69
0.33
22 00
Particle size distribution of soil used in soil moisture
and solar radiation study.
Particle
size
Composition
percent
sand
50
silt
46
clay
4
Total
5g
20
30
39
2
9
6
26
19
34
32
37
43
44
47
23
56
42
25
49
40
22
10
5
24
15
27
4
33
12
51
52
13
53
54
57
46
7
31
41
17
U
14
35
55
45
59
60
29
36
21
58
38
16
18
50
3
1
Figure
2.
2
4a
Diagram of design of soil moisture and solar radiation study
showing the position of each pot by code number.
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