M i s

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Revised 08/14
Methods of Measuring for
Irrigation Scheduling—WHEN
Edward C. Martin
Proper irrigation management requires that growers assess
their irrigation needs by taking measurements of various
physical parameters. Some use sophisticated equipment
while others use tried and true common sense approaches.
Whichever method used, each has merits and limitations.
In developing any irrigation management strategy, two
questions are common: “When do I irrigate?” and “How much
do I apply?” This bulletin deals with the WHEN.
Soil Moisture Techniques
One method commonly used to determine when to irrigate
is to follow soil moisture depletion. As a plant grows, it
uses the water within the soil profile of its rootzone. As the
water is being used by the plants, the moisture in the soil
reaches a level at which irrigation is required or the plant
will experience stress. If water is not applied, the plant will
continue to use what little water is left until it finally uses all
of the available water in the soil and dies.
When the soil profile is full of water, reaching what is
called field capacity (FC), the profile is said to be at 100%
moisture content or at about 0.1 bars of tension. Tension is
a measurement of how tightly the soil particles hold onto
water molecules in the soil: the tighter the hold, the higher
the tension. At FC, with a tension of only 0.1 bars, the water
is not being held tightly and it is easy for plants to extract
water from the soil. As the water is depleted by the plants,
the tension in the soil increases. Figure 1 shows three typical
curves for sand, clay and loam soils. As Fig. 1 shows, the plants
will use the water in the soil until the moisture level goes to
the permanent wilting point (PWP). Once the soil dries down
to the PWP, plants can no longer extract water from the soil
and the plants die. Although there is still some moisture in
the soil below the PWP, this water is held so tightly by the
soil particles that it cannot be extracted by the plant roots.
The PWP occurs at different moisture levels depending on the
plant and soil type. Some plants, which are adapted to arid
conditions, can survive with very little moisture in the soil.
With most agronomic crops, PWP occurs when the tension
in the soil is at 15 bars. This means that the soil is holding on
very tightly to the water in its pores. In order for plants to use
Figure 1. A diagram of typical tension and water amounts for sand, clay and
loam. (Taken from the National Engineering Handbook, 210-VI).
this water, they must create a suction greater than 15 bars. For
most commercial crops, this is not possible. At 15 bars, most
plants begin to die. The difference between field capacity and
PWP is called the plant available water (PAW).
Irrigation targets are usually set as a percent depletion of
the PAW. This depletion level is referred to as Management
Allowable Depletion (MAD). The bulk of irrigation research
recommends irrigating row crops such as grain or cotton when
the MAD approaches 50%. For vegetable crops, the MAD is
usually set at 40% or less, because they are more sensitive to
water stress. These defined amounts insure that water stress
will not be so severe as to cause any appreciable yield losses.
Careful monitoring of the PAW needs to be done throughout
the season so that the appropriate point of irrigation can
be anticipated. The following approaches can be used to
determine soil moisture content.
The “Feel Method”
Determining soil moisture by feeling the soil has been
used for many years by researchers and growers alike. By
squeezing the soil between the thumb and forefinger or by
squeezing the soil in the palm of a hand, a fairly accurate
estimate of soil moisture can be determined. It takes a bit of
time and some experience, but it is a proven method. Table
1 gives a description of “how the soil should feel” at certain
soil moisture levels. In this table soil moisture information
is given using inches per foot (in. /ft.). This term (in. /ft.)
refers to how many inches of water are available in a foot
of soil. For example, looking at sand (Table 1, column 1) we
can see that the wilting point is about 1.0 in. /ft. This implies
that sand holds one inch of water per foot of soil. As the soil
dries, it becomes harder to make a soil ball; soon the soil is
crumbling in your fingers. Irrigation should occur somewhere
in the shaded area, earlier for crops sensitive to water stress.
Let’s look at clay loam. At a 0.4 in. /ft. deficit, a ribbon can be
easily made when the soil is squeezed between the thumb and
forefinger. Since the wilting point occurs at about 1.8 in. /ft.,
a 0.4 deficit would equate to a 22% deficit (using Equation 1).
(0.4/1.8) * 100 = 22%
Sandy loam soil makes a good ball at 0.6 in. /ft. deficit
(about 40% deficit) but will not make a ball at all and only
sticks together at 1.0 in. /ft. (about 66% deficit). Once you
become familiar with the feel of the soil, it becomes easier
to estimate soil moisture content. However, it takes time to
become familiar with the feel of the soil and this method
requires a great deal of experience.
Table 1. Description of the soil texture parameters used to determine soil moisture using the feel method.
Soil Texture Classification
Moisture Deficiency
(Loamy Sand)
(Sandy Loam)
(Clay Loam)
(Field Capacity)
(Field Capacity)
(Field Capacity)
(Field Capacity)
Leaves a wet outline
on hand when
Leaves wet outline on Leaves wet outline on hand; will ribbon out
hand; makes a short hand; will ribbon out
about 2 inches
about 1 inch
Appears moist
Makes a hard ball
Makes a weak ball
Sticks together
Moisture Deficiency
Forms a plastic ball,
Slicks when rubbed
Makes a good ball.
Will slick and ribbon
Makes a thick ribbon
Slicks when rubbed
Very dry; loose, flows
through fingers
Makes a weak ball
Forms a hard ball
Makes a good ball
Wilting point
Sticks together
but will not ball
Forms a good ball
Will ball but won’t
ribbon. Small clods
Forms a weak ball
Wilting Point
Clods crumble
A “Ball” is formed by
squeezing a handful
of soil firmly
A “Ribbon” is formed
between thumb and
Wilting Point
Wilting Point
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Figure 2. Diagram of a neutron moisture gauge (neutron probe).
Neutron Probe
The neutron probe has been used extensively in research
situations to determine soil moisture. A neutron probe or
neutron moisture gauge contains a radioactive source that
sends out fast neutrons. These fast neutrons are about the size
of a hydrogen atom, a critical component of water. When fast
neutrons hit a hydrogen atom, they slow down. A detector
within the probe measures the rate of fast neutrons leaving
and slow neutrons returning. This ratio can then be used to
estimate soil moisture content. However, because every soil
has some background hydrogen sources that are not related
to water, calibration is important for each soil. To measure soil
moisture with a neutron probe, an access tube is installed into
the ground. Then, the probe (which contains the radioactive
source and the detector) is lowered to the desired depth (Fig. 2).
Probes are quite expensive (approximately $6,400), and because
they contain radioactive material, require an operating license.
Figure 3. Diagram of resistance blocks. Here, three blocks are anchored by a
stake in the field.
Electrical Resistance
Another method that has been used for several years to
determine soil moisture content is electrical resistance. Devices
such as gypsum blocks and Watermark sensors use electrical
resistance to measure soil moisture. The principle behind
these devices is that moisture content can be determined by
the resistance between two electrodes embedded in the soil.
The more water in the soil, the lower the resistance. In the early
stages of development, it was discovered that a salt bridge
can form between the two electrodes, giving false readings.
Today, electrodes are embedded in more stable material and
are not as susceptible to salt bridging. The practical use of
these devices is limited as they operate best in the high range
of soil moisture. To measure soil moisture, the blocks are
buried in the ground at the desired depth, with wire leads
to the soil surface. A meter ($200-$300) is connected to the
wire leads and a reading is taken (Fig. 3). Retrieval of these
instruments is difficult in clay soils, but they are relatively
inexpensive (approximately $25 ea.).
Figure 4. Diagram of a tensiometer. In some cases, the gauge is replaced
with a connection for a transducer that measures suction.
Soil Tension
As previously mentioned, as soil dries out, the soil particles
retain the water with greater force. Tensiometers measure how
tightly the soil water is being held. Most tensiometers have a
porous or ceramic tip connected to a water column.
The tensiometers are installed to the desired depth (Fig. 4).
As the soil dries, it begins to pull the water out of the water
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column through the ceramic cup, causing suction on the water
column. This force is then measured with a suction gauge.
Some newer models have replaced the suction gauge with
an electronic transducer. These electronic devices are usually
more sensitive than the gauges. Tensiometers work well in
soils with high soil-water content, but tend to lose good soil
contact when the soil becomes too dry. Like the resistance
blocks, they are difficult to remove from clay soils. Costs range
from $30 for small tensiometers with gauges to $2000 for the
electronic meters (reads multiple sites).
Capacitance and TDR
New devices and methods become available to growers every
year. Two new techniques for soil moisture determination are
instruments using Time-Domain Reflectometry (TDR probes)
and Capacitance (C-Probes, Frequency-Domain Reflectometry
TDR instruments work on the principle that the presence of
water in the soil affect the speed of an electromagnetic wave
(slows it down). The TDR sends an electromagnetic wave
through a guide (usually a pair of parallel metal spikes) placed
into the ground at the desired depth. It then measures the
time it takes the wave to travel down the guide and bounce
back (reflect back) up the guide. The time is recorded and
converted to a soil moisture reading. The wetter the soil, the
longer it takes for the electromagnetic wave to travel down
the guide and reflect back.
C-Probes and FDRs use an AC oscillator to form a “tuned”
circuit with the soil. After inserting probes that are either
parallel spikes or metal rings into the soil, a tuned circuit
frequency is established. This frequency changes depending
on the soil moisture content. Most models use an access tube
installed in the ground (similar to the neutron probe).
TDR, FDR and C-Probes have all worked well, but have
their limitations. They read only a small volume of soil
surrounding the guides or probes. FDR and C-Probes are
also sensitive to air gaps between the access tube and the
soil. Many of these newer instruments require professional
installation to operate properly. In soils where caliche and
other hard pan layers exist, installing these probes may be
difficult. This type of problem is compounded when the soil
is dry. Cost for the probes range from $5,000-$10,000.
Plant Indicators
Also useful in determining WHEN to irrigate are plant
indicators. Plant indicators enable the grower to use the
plant directly for clues as to when to irrigate, not an indirect
parameter such as soil or evaporative demand. Observing a
plant characteristic can a good indication of the status of the
soil’s moisture content.
Infrared/Canopy Temperature
An infrared (IR) thermometer measures the thermal
temperature of the plant leaves or a crop canopy. Similar to
humans perspiring to keep cool, plants transpire through
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Figure 5. Diagram of an infrared sensor.This is a hand-held model.
openings called stomata. Once plants go into water stress, they
begin to close their stomata and cease to transpire, causing the
plant to “heat up” and the canopy temperature to rise. Infrared
readings can detect this increase in plant temperature.
When using this method, baseline temperatures need to
be taken prior to measurements. The baseline temperature
should be taken in a well-watered field, free of water stress.
On days when the air temperature is very high, some plants
will stop transpiring for a brief period. If infrared readings
are being taken at that time, they may read that there is a
water stress when, in fact, it is just a normal shutdown period.
Compare readings with the well-watered readings to make
a decision. IR also requires taking temperature readings on
clear days at solar noon. This normally occurs between noon
and 2:00 p.m. This is to assure that the measurement is taken
at maximum solar intensity. During the monsoon season,
this may be difficult to achieve due to cloud cover. Early in
the season, IR readings will often measure soil temperature
when canopy cover is sparse. These readings usually result
in higher temperature readings since the soil tends to heat up
quickly. Figure 5 is a diagram of a hand-held IR gun.
Computerized Irrigation Scheduling
The use of computer programs to help schedule irrigation
was introduced in the 1970’s. However, only recently with
the introduction of fast, personal computers have they
begun to gain wider acceptance. Several methods can be
used to determine crop water use and help growers schedule
irrigation. The most common is to use an equation to calculate
the water use or evapotranspiration (ET) for a reference crop
and relate that to other crops. ET refers to water loss from
soil evaporation and plant transpiration. In the beginning of
a crop’s growing season, the plants are small and most of the
Table 2. List of equations used to calculate reference ET.(Jensen et al., 1990).
Time Step
Reference Crop
Reference Crop Type
Penman Monteith FAO 56
Hourly or Daily
Grass Reference (ETo) and
Alfalfa Reference (ETr)
Depends on surface
roughness and canopy
ASCE Standardized Equation
Hourly or Daily
Grass Reference, ETo
A hypothetical reference crop
Modified-Penman, FAO-24
Grass Reference, ETo
Well-watered grass, 3-6 in. tall
Jensen Haise
5 days
Alfalfa Reference, ETr
Well-water alfalfa 11.8-19.7 in. tall
10 days
Grass Reference, ETo
Well-watered grass, 3-6 in. tall
Monthly/5-10 days
Grass Reference, ETo
Well-watered grass, 3-6 in. tall
FAO-24 Pan
5 days
Grass Reference, ETo
Well-watered grass, 3-6 in. tall
Kimberly-Penman (1982)
Alfalfa Reference, ETr
Full cover alfalfa
Inches of Water
12 -96
19 -96
26 -96
02 -96
09 -96
16 -96
23 -96
01 -96
08 -96
15 -96
22 -96
29 -96
05 -96
12 -96
Figure 6. Reference evapotranspiration (ETo) and measured crop evapotranspiration (ETc) for dry onions in 1996, Maricopa, AZ.
water loss is through soil evaporation. As the plants grow and
a canopy develops, the soil becomes shaded and most of the
water loss is through plant transpiration.
Reference equations include alfalfa-based equations (ETr)
and grass-based equations (ETo). There are several equations,
each with its own advantages and disadvantages. In Arizona,
the Modified-Penman equation is widely used. This equation
uses weather data to predict the water use of grass. Other
equations used with some success are the Blaney-Criddle,
Jensen-Haise, Hargreaves and more recently the FAO 56
Penman-Monteith (Allen et al., 1998) and the Standardized
Reference ET equation (ASCE-EWRI, 2005).
In addition to using equations to calculate a reference ET,
evaporation pans are used to determine a reference ET which
is then related to the crop ET. Also, there are energy equations
and several other approaches to determining reference ET.
Table 2 gives a list of popular methods.
Figure 7. Crop coefficient curve for dry onions developed from ETo and ETc
data from Fig. 6 and two other years of data from Maricopa, AZ.
As previously stated, in Arizona the Modified-Penman
equation has been used for several years with success. Figure
6 shows a graph of the calculated reference ET (ETo) using the
Modified-Penman equation for dry onions grown in Central
Arizona in 1996. Figure 6 also shows the measured crop water
use for the crop (evapotranspiration of the crop - ETc). Using
the following equation:
ETc = ETo * Kc
the crop coefficient (Kc) can be calculated. Using several
years of weather data and crop water use data, crop coefficient
can be determined and a specific crop curve can be developed
(Fig. 7). Using thermal time (Heat Units), these crop curves
can be used in areas where daily temperatures differ.
Equally as important as the crop curve in irrigation
scheduling are the soil water parameters. The PAW of the soil
must be known as well as the FC.
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In its simplest form, irrigation scheduling is similar to a
checkbook balancing system. For most crops in Arizona, the
soil is at or very near 100% moisture at planting time or just
aft irrigation. At those times, using ETo equations with crop
coefficient daily crop water use can be determined. This is
subtracted from the total water in the soil and a new soil water
content can be determined. This continues until the amount of
depletion of PAW in the soil reaches a predetermined setting
(the MAD). For many crops, the MAD is set to 40-50% in the
rootzone of the crop. However, some crops, such as vegetable
crops, are more sensitive to large fluctuations of soil moisture
and the MAD are set to lower levels.
Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop
evapotranspiration: guidelines for computing crop water
requirements. FAO Irrigation and Drainage Paper no.
56, Rome, Italy.
National Engineering Handbook, Part 652, Irrigation. 1997.
The most common irrigation scheduling methods used by
growers are: scheduling according to the calendar (number
of days since the last irrigation), looking at the crop for color
change or digging in the field and feeling the soil to estimate
soil moisture. Calendar scheduling does not take into account
weather extremes, which may cause problems from yearto- year. Looking at the crop requires experience and a good
eye—some growers have it, some do not. Even when you
have a good eye, by the time the plant shows visable signs
of stress, a yield loss has already occurred. Feeling the soil
can give good estimates, but is often too time consuming for
many growers. Also, when using this technique, one needs
to take into account the soil profile of the active rootzone.
Estimating rootzone depth can be difficult.
In this paper, we discussed some of the options available to
assist growers in determining WHEN to irrigate. Whichever
method is decided on, choosing a definite approach is always
wise. Guessing can lead to unnecessary frustration, yield
loss or excess water costs by the end of the season. Take
your time and do some investigation before you invest in
any new soil moisture measuring system. An excellent place
for information is on the Internet. A site called http://www.
sowacs.com contains information on many of the instruments
described in this publication. The site hasn’t been updated
recently, but it still contains some good links and information
and is worth a visit.
ASCE-EWRI, 2005. The ASCE Standardized Reference
Evapotranspiration Equation. Technical Committee
report to the Environmental and Water Resources
Institute of the American Society of Civil Engineers from
the Task Committee on Standardization of Reference
Evapotranspiration. ASCE-EWRI, 1801 Alexander Bell
Drive, Reston, VA 20191-4400, 173 pp.
Jensen, M.E., R.D. Burman and R.G. Allen. 1990. Evaporation
and irrigation water requirements. ASCE Practice No.
70. ASCE, NY, NY.
Martin, E.C., A.S. de Oliveira, A.D. Folta, E.J. Pegelow and
D.C. Slack. 2001. Development and testing of a small
weighing lysimeter system to assess water use in shallow
rooted crops. Transactions of the ASAE. 44(1):71-78.
The University of Arizona
College of Agriculture and Life Sciences
Tucson, Arizona 85721
Edward C. Martin
County Extension Director
Maricopa County Cooperative Extension
Professor And Irrigation Specialist
Department Of Agriculture & Biosystems Engineering
Edward C. Martin
[email protected]
This information has been reviewed by University faculty.
Originally published: 2009
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do not imply endorsement by The University of Arizona.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Jeffrey
C. Silvertooth, Associate Dean & Director, Extension & Economic Development, College of Agriculture Life Sciences, The University of Arizona.
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