Measurement of soil water for irrigation management

Material Information

Measurement of soil water for irrigation management
Series Title:
Florida Cooperative Extension Service Circular 532
Smajstrla, Allen George
Harrison, Dalton Sidney
University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Place of Publication:
Gainesville, Fla.
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida,
Publication Date:


Subjects / Keywords:
Agriculture ( LCSH )
Farm life ( LCSH )
Farming ( LCSH )
University of Florida. ( LCSH )
Agriculture -- Florida ( LCSH )
Farm life -- Florida ( LCSH )
Spatial Coverage:
North America -- United States of America -- Florida


Florida Historical Agriculture and Rural Life

Record Information

Source Institution:
Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location:
Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management:
All rights reserved, Board of Trustees of the University of Florida
Resource Identifier:
9857926 ( OCLC )
027254709 ( ALEPH )

Full Text


Measurement of Soil Water

For Irrigation Management

Allen G. Smajstrla
Dalton S. Harrison

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Florida Cooperative Extension Service Institute of Food and Agricultural Sciences
University of Florida, Gainesville John T. Woeste, Dean for Extension

Measurement of Soil Water
For Irrigation Management
Allen G. Smajstrla and Dalton S. Harrison*

Good irrigation management requires that the status of soil
water be accurately evaluated. There are direct and indirect
methods to measure soil water content, and several alternative
ways to express it quantitatively. There is no universally recognized
standard method of measurement and no uniform way to compute
and present the results.
Although there are many ways the status of soil water can be ex-
pressed, all can basically be classified into one of two categories: (1)
the amount of water (water content) held in a given amount of soil,
and (2) the potential, or tension with which the water is held by the
soil. These two properties describing the soil water status are
related to each other throughout the entire soil water content range.
This relationship is important because it describes the ability of a
soil to hold water available for plant growth and the forces with
which it is held by the soil as it is depleted. The relationship for a
typical Florida soil, Lake Fine Sand, is given in Figure 1. The rela-


_j 1000 -

Loo -

100 -


10 --
I- I

lo I I I I
0 0.10 020 0.30 0.40

Figure 1. Water Capacity Curve for Lake Fine Sand.

*Associate Professor and Professor, respectively, Agricultural Engineering
Department, IFAS, University of Florida, Gainesville, FL 32611.

tionship shown, called a water capacity curve, is different for each
soil type and must be measured for each individual soil type.

Units of Water Measurement
Water Content
The amount of water that is held by a certain mass or volume of
soil can be expressed as a percentage by weight or percentage by
volume. Soil water content as a percentage by weight gravimetricc
water content) is based on the dry weight of the sample,

Weight percent = weight of water 100% (1)
weight of oven-dry soil

Soil water content as a percentage by volume volumetricc water
content) is based on the volume of the sample,

Volume percent volume of water 100% (2)
Volume percent = (2)
bulk soil volume

The volume percentage is a more useful measure to the irrigator
because it readily allows the amount of water available to a crop to
be calculated. For example, a soil with a water content of 10% by
volume contains (10%)(12 in/ft) = 1.2 inches of water per foot of

Water Potential
The water potential, or tension with which it is held by the soil is
a function of water content. Water molecules are attracted to soil
particles (adhesive forces) and to other water molecules (cohesive
forces) so that they are not readily removed from the soil by plants.
Rather, the attractive forces between the soil and water must be
overcome by the plant in order to obtain water for its use. As soil
water is used by plants, that most readily available is removed first.
Each succeeding increment becomes less readily available as water
is first depleted from the large soil pores and is held more tightly by
the smaller soil pores.
Water potential or tension is measured in a variety of units. A
common unit is the bar (1 bar 1 atmosphere of pressure 14.5
psi). Subunits commonly used are centibars (cb) and millibars (mb).
One millibar is approximately equal to the pressure required to sup-
port a column of water at a height of one centimeter.
Water potentials are negative to reflect the fact that force must
be exerted to extract water from soils. However, because it is im-

plicit, the negative sign is often omitted when water potentials are
referenced. In this manuscript the negative sign will be omitted for
convenience of the reader.
Water is available to plants at tensions from near zero to as
much as 15 bars. However, water is readily available to plants (no
growth reduction) over a much narrower range of tension. Com-
monly, irrigations are scheduled at tensions of less than one bar.
Because of limited available water in sandy soils, they are usually
irrigated when soil water tensions reach 200-300 mb. For these soils,
water is readily available for plant growth only in the range of ap-
proximately 60-300 mb.

.Measurement of Soil Water Content

Several methods can be used to provide an indication of the
volume of water contained in a volume of soil. These include a)
gravimetric sampling, b) neutron scattering, c) gamma ray attenua-
tion, d) carbide method, and e) thermal conductance. A summary of
the various methods, as well as advantages and disadvantages of
each, is given in Table 1.

Gravimetric Method
The standard method of determining the amount of water in soil
consists of physically collecting a soil sample from the field,
weighing it, and oven drying it to constant weight at 105 C. The
weight difference before and after drying is considered to be water
removed. Water contents can be calculated on a weight basis using
equation (1), or on a volume basis from equation (2) if the soil volume
or bulk density was measured when the sample was taken.
The measurement of sample volume or bulk density is difficult
and subject to errors, particularly in the field. The gravimetric
Table 1. Methods of measurement of soil water content
Method Advantages Disadvantages
Gravimetric Method Little specialized equipment Laborious and time consuming
required process
Independent bulk density
measurement required
Destructive sampling required
Neutron Scattering Rapid measurements Field calibration required
Volume rather than point soil Large equipment cost
measurement Radioactive licensing required
Nondestructive measurements
Gamma Ray Rapid Measurements Variations in soil bulk density
Attenuation Nondestructive measurements strongly influence results
Thin layer resolution obtainable Radiation hazard
Large equipment cost
Carbide Method Rapid, in-field measurements Destructive sample required
Small samples limit accuracy
Specialized equipment and
reagents required
Thermal Conductance Nondestructive measurements Large equipment cost
Automated data collection Field calibration required

method itself is subject to inherent errors because it depends on
sampling, transporting, and repeated weighing. It is also laborious
and time consuming, since a period of at least 24 hr is usually re-
quired for complete drying. The standard method of oven drying is
also arbitrary. Some clays may still contain appreciable amounts of
adsorbed water even after drying. Conversely, some organic matter
may oxidize and decompose at 105 C so that the weight loss may
not be due entirely to the evaporation of water.
The errors incurred with the gravimetric method can be reduced
by increasing the size and number of samples. However, the sam-
pling method is destructive and may disturb a location sufficiently
to distort the results.
For all of the previously given reasons, most irrigators prefer in-
direct methods of moisture measurement. Most indirect methods
permit frequent or continuous measurements to be made at the
same locations. Also, once the equipment is installed and calibrated,
measurements can be made with much less time and labor.
Neutron Scattering
The neutron moisture meter (Fig. 2) is an instrument which
allows the nondestructive measurement of soil water content in the
field. It allows rapid and periodically repeatable measurements in
the same location and depth of soil. An access tube must be in-

Figure 2. The Neutron Mositure Meter for Soil Water Content Measurements.
a. Field measurement with the neutron moisture meter.
b. The neutron probe and soil access tubes.

stalled in the soil to allow a probe to be lowered to the desired soil
The neutron meter operates on the principle of nuclear ther-
malization. Fast neutrons are emitted from a radiation source and
lose energy (and velocity) as they collide with hydrogen ions. The
slow neutrons are counted by a detector on the instrument. The
number of slow neutrons is an indirect measure of the quantity of
water in the soil because of the hydrogen ions present in the water
molecule (H+ OH-). Because only hydrogen ions (and not water
molecules) are counted, the neutron meter must be calibrated for
each specific soil type on which it will be used. This is especially true
if the quantity of organic matter is variable between soil types
because readings will reflect the large number of hydrogen ions
present in organic matter.
A spherical volume of soil is sampled as neutrons are emitted in
all directions and counted as they are reflected to the detector. The
radius of that volume varies from only a few inches in a wet soil to a
foot or more in a very dry soil.
The neutron meter is primarily a tool used by researchers, rather
than by individual irrigators in commercial enterprises, because of
the meter cost ($5,000-$7,000) and complexities in calibration and
use. Also, because a radioactive source is used, state laws require
licensing and operating only by trained personnel.
Gamma Ray Attenuation
Moisture changes in soils can be determined by measuring the
amount of gamma radiation energy lost (attenuated) as a radiation
beam is directed through a soil. This technique depends on the fact
that gamma rays lose part of their energy upon striking another
substance. As the volume of water in a soil changes, the amount of
attenuation will also change, and it can be related to volume of soil
water from a previous calibration. If the soil density changes with
water content, it must be measured independently because these
changes also affect radiation attenuation.
Three types of gamma radiation instruments are now commer-
cially available. One type detects radiation backscatter. Both the
radiation source and detector are located on a probe which is in-
serted into the soil using access tubing. Radiation is emitted from
the source and radiation reflected from the surrounding soil is
A second type of instrument has the source and detector in-
stalled on separate probes which are inserted into parallel access
tubes. This instrument measures the attenuation of radiation by the
soil and water between the source and detector. The instrument
focuses the radiation into a narrow beam to study 1 cm thick soil

layers. It can be more accurate than the backscatter instrument if
the access tubes are parallel and if the distance between them is ac-
curately known.
A third instrument consists of a stationary detector at the soil
surface and a radiation source on a probe which is inserted into the
soil near the ground surface. This instrument has the advantage
that the relationship between source and detector is accurately
known. Thus, changes in density can be accurately measured.
Because the detector is located at the soil surface, measurements
are limited to the surface one foot of the soil profile. This instrument
is available with a neutron detector as a combination instrument
Gamma radiation instruments typically use Cesium 137 as the
radiation isotope for the source of radiation. For this reason they
must be licensed by state law as the neutron instruments are. They
can be a health hazard and should be used carefully by trained per-
sonnel. Because of the nature of gamma radiation, lead shields must
be used so that the isotopes can be handled safely.
Because of their complexity of operation, need for calibration,
and potential health hazards, the gamma radiation attenuation
technique is normally used only as a research tool. Instrument cost
is similar to the neutron probe, in the range of $5,000-$7,000.

Carbide Method
As water is added to calcium carbide, a chemical reaction occurs
during which CO, gas is emitted. A measurement technique has
been devised which uses this principle for the determination of soil
water content. The technique consists of combining a small (approx-
imately 26 gram) wet weight soil sample with calcium carbide in a
sealed container. The amount of gas produced is indicated on a
pressure gauge. The pressure reading is converted to gravimetric
moisture content by using the calibration curve provided with the
instrument. Test time is approximately one to three minutes per
Independent measures of bulk density must be made to convert
to volumetric moisture contents. A commercially available instru-
ment provides the capability of measuring both gravimetric and
volumetric moisture contents in the field, but extreme care must be
exercised in its use because soil samples are small. An instrument of
this type can be purchased for approximately $350. The calcium car-
bide reagent is expended in the test. A container with a sufficient
amount for approximately 50 tests can be purchased for less than

Thermal Conductance

The rate of heat dissipation in a porous material of low heat con-
ductivity is sensitive to water content. Therefore, the water content
of a porous material can be measured by applying heat at a point
centered within the material and by measuring the temperature rise
at that point. The water-dependent rate of change in temperature
must be calibrated for individual soils. With calibration, this tech-
nique will allow measurement of water contents in the water tension
range of 0.1-15 bars.

An instrument designed to measure water contents using this
technique is commercially available. Individual sensors are priced
at $250-$300 and a portable data display unit at approximately
$1,500, making it generally too expensive for individual irrigators
for scheduling purposes.
This instrument has the capability of allowing automated data
collection, does not disturb the soil during data collection, and
measures soil water content (or potential, if so calibrated) only. Its
cost and complexity make it primarily a research tool at this time.

Measurement of Soil Water Potential (Tension)

Several methods can be used to determine the soil water poten-
tial or tension with which the water is held against plant extraction
in the soil. These included) tensiometersb) soil psychrometers, c)
electrical resistance blocks, and d) freezing point determination. A
summary of these methods, including advantages and disadvan-
tages of each, is included in Table 2.

Table 2. Methods of measurement of soil water potential (tension)
Method Advantages Disadvantages
Tensiometers Low cost Requires periodic servicing
Direct water potential reading for Operable to only 80 cb of tension
scheduling irrigation
Continuous measurements at
same location
Soil Psychrometers Laboratory calibration only is Specialized equipment required
necessary Not accurate at water potentials
Rapid measurements below 1 bar
Measurement of osmotic and
capillary water potential
Electrical Resistance Low cost Field calibration required
Blocks Repeated nondestructive Very sensitive to salinity
measurements at same place Calibration changes with time
in soil Short life of sensors
Rapid measurements
Freezing Point Measures osmotic and capillary Not accurate at water potentials
Depression water potential below approximately 3 bars
Specialized equipment required
Independent measure of salinity

A tensiometer (Fig. 3) measures the potential or tension of water
in soil. The instrument consists of a closed tube with a ceramic cup
on the end inserted into the soil and a vacuum gauge at the surface
(Fig. 3a). The tube is filled with water and the ceramic cup is in-
stalled in the active root zone of the soil. As the soil dries, water is
pulled through the ceramic cup and tension is registered on the
gauge. After a period of time, the vacuum gauge reads a tension
equivalent to the soil moisture tension. In this manner, the ten-
siometer indicates the force exerted by the soil to extract water
from the ceramic cup. This is also the force that a plant would need
to exert to extract water from the soil. Therefore, a tensiometer
measures the energy status, or availability, of water to a plant
rather than quantity of water in the soil.
A tensiometer left in the soil for a long period of time tends to
follow the changes in the tension of soil water. As soil moisture is

Figure 3. Tensiometers for Soil Water Potential Measurements.
a. Action of Tensiometer in Soil.

A Removable Cap
BIT Vacuum Gauge

/ /

depleted by drainage or plant use, or as it is replenished by rainfall
or irrigation, corresponding readings on the tensiometer gauge
occur. Because of the hydraulic resistance of the cup and the
surrounding soil, and of the contact zone between the cup and soil,
the tensiometer response will lag behind tension changes in the soil.
The tensiometer porous cups are permeable to both water and
solutes (salts). Thus, solute concentrations in the cup equilibrate
with those in the soil, and the tensiometer does not measure osmotic
Most tensiometers have a vacuum gauge graduated from 0 to
100 centibars, cb (0-1000 mb). A reading of 0 indicates a saturated
soil. The useful limit of tensiometers is about 80 cb (800 mb). Above
these tensions air enters through the ceramic cup or the column of
water in the tensiometer breaks, causing the instrument to fail to
operate. This limitation is not serious, especially for crop production
on sandy soils. Irrigation should be commenced on the sandy soils
of Florida when the tensiometer vacuum gauge reading is in the
range of 20-40 cb, depending upon the soil characteristics, crop re-
quirements, and irrigation system capabilities.
Tensiometers are sold in multiples of 6" lengths. Often two are
required at each site to describe the soil moisture status in the plant
root zone (Fig. 3b). For shallow-rooted crops such as vegetables, one





Figure 3. Tensiometers for Soil Water Potential Measurements.
b. Placement for Irrigation Control.

may be installed at a 6" depth and one at a 12" depth; whereas for
field crops, it may be necessary to have one at a 12" depth and
another at a 24" to 36" depth. Tensiometers should be installed at
several sites in large fields rather than relying on only one or two
locations. Commonly, one bank of tensiometers should be used for
each 5 acres to be monitored. At this density, system cost would not
be prohibitive, because each tensiometer costs approximately $30
and should be serviceable for several crop years.
In the sandy soils of Florida, field capacity ranges from 10% to
12% moisture by volume, while the permanent wilting point may
range from 2% to 6%. Therefore, the percentage of available
moisture is normally only 6% to 8%. The critical timing of irrigation
caused by the small volumes of available water makes the ten-
siometer a valuable instrument for determining when to irrigate.
The amount of water to apply at each irrigation depends upon the
tension that will be allowed before irrigation, and upon the soil
hydraulic properties. These values can be approximated from the
SCS Irrigation Guide or Soil Survey available from local Soil Con-
servation Service offices and then refined through field experience.
Despite their shortcomings, tensiometers are practical in-
struments, available commercially, and, when operated and main-
tained by a trained worker, are capable of providing reliable data on
the status of soil water and its changes with time for irrigation
scheduling purposes.
Soil Psychrometers
Soil psychrometers are useful instruments for the measurement
of soil water tension in the range of 1-15 bars. Soil psychrometers
operate by cooling a thermocouple junction to cause the condensa-
tion of water on it, and then measuring the junction temperature as
the water is allowed to evaporate. In drier soils, the water
evaporates more rapidly, causing a greater temperature depression
at the thermocouple junction. Thus, temperature depression can be
related to soil water tension. The response is not linear and only
small responses are obtained at potentials below one bar, making
this instrument unsuitable for irrigation scheduling on sandy or
medium textured soils where irrigations would typically be sched-
uled at potentials of much less than one bar.
The soil psychrometer would be a suitable instrument for irriga-
tion scheduling on clay soils at water tensions of greater than one
bar. It also measures osmotic potential and would be suitable for
determining solute effects on water potential in saline soils. A major
advantage of this system is that the sensors are calibrated in the
laboratory using known osmotic solutions. Thus, no field calibration
is necessary.

Disadvantages include system cost and complexity. Individual
sensors cost $20-$30, and the data acquisition meter costs
$1,000-$1,500. In addition, techniques for use are complex and
several minutes are required for each measurement.

Electrical Resistance Blocks
The electrical resistance of porous bodies placed in the soil and
left to equilibrate can be correlated with soil water potential. Such
electrical resistance blocks usually contain a pair of electrodes
embedded in gypsum, nylon, or fiberglass (Fig. 4).
The electrical conductivity of most blocks is due primarily to the
solution contained within it rather than the solid matrix. Thus, it
depends upon the electrolytic solutes present in the fluid as well as
upon the volume content of the fluid. Blocks made of inert materials
such as fiberglass are highly sensitive to even small variations in
salinity of the soil solution. Thus, they are usually not useful for irri-
gation scheduling.
Blocks made of gypsum maintain a nearly constant electrolytic
concentration corresponding primarily to that of a saturated solu-
tion of calcium sulfate. This tends to mask the effects of variations
in soil solution concentrations such as those due to fertilizers or low
levels of salinity. However, since gypsum is soluble, these blocks
eventually deteriorate in the soil. They are normally serviceable for
only one year and may need to be recalibrated more frequently.

Figure 4. A Portable Meter and Electrical Resistance Blocks for Soil Water
Potential Measurements.

Soil resistance blocks are affected by hysteresis as soil moisture
levels change. They tend to lag behind the soil moisture changes
because of the hydraulic properties of the blocks and because of the
contact resistance with soils. They are not uniformly sensitive over
the entire range of soil moisture, and they are more accurate at low
water contents rather than at water contents near field capacity.
Resistance blocks are also temperature sensitive. Temperatures
must be measured and compensated, therefore, accuracies depend
upon the ability to accurately measure temperatures.
Advantages of the resistance blocks include; a) they can be con-
nected to a recorder for a continuous record of water potential, and
b) they do not require frequent servicing as tensiometers do.
Electrical resistance blocks are fairly inexpensive. They can be
purchased from several suppliers for $5-$8 each. Alternating cur-
rent resistance bridges to read the blocks can be purchased for
$150-$300 each.

Freezing Point Depression
From saturation to a total tension (capillary plus osmotic) of 2-3
bars, the freezing point of water changes only slightly. From 3 bars
to about 25 bars there is a pronounced change in the freezing point.
For saline soils this technique could theoretically be used to deter-
mine the water content of soils near saturation if an independent
measure of salinity was made. However, equipment required is com-
plex and not commercially available for field irrigation scheduling
This technique is not suitable for general field use or for irriga-
tion scheduling, especially for non-saline sandy soils. For those soils
this technique is not sufficiently accurate in the range required for
irrigation scheduling.

Good irrigation management requires that the status of soil
water be accurately evaluated. To allow irrigators to evaluate
techniques available, methods for the measurement of soil water
content and soil water potential were presented. Advantages and
disadvantages of the use of each technique were discussed. Approx-
imate costs of each were also estimated. Greatest emphasis was
placed on those techniques most adaptable to irrigation scheduling
by individual irrigators under Florida conditions.


1. Hagan, R. M., H. R. Haise, and T. W. Edminster, eds. 1967. Irrigation
of Agricultural Land. American Society of Agronomy. Madison, Wis-
consin. 1180 pages.
2. Hillel, Daniel. 1971. Soil and Water, Physical Principles and Processes.
Academic Press. New York. 288 pages.
3. Kohnke, Helmut. 1968. Soil Physics. McGraw-Hill Book Co. New York.
224 pages.
4. Pair, C. H., ed. 1975. Sprinkler Irrigation. The Irrigation Association.
Silver Spring, Maryland. Fourth Edition. 615 pages.
5. Smajstrla, A. G., D. S. Harrison, and F. X. Duran. 1981. Tensiometers
for Soil Moisture Measurement and Irrigation Scheduling. Circular
487. Fla. Coop. Ext. Service, IFAS. Univ. of Fla., Gainesville. 15 pages.
6. Soil Conservation Service Engineering Staff. 1964. Soil-Plant-Water
Relationships. Chapter 1, Section 15, SCS National Engineering Hand-
book. 72 pages.

This publication was promulgated at a cost of $1,491, or 16.6 cents per
copy, to inform growers of methods of measuring soil water for irriga-
tion management.

Tefertlller, director, in cooperation with the United States Department IFAS
of Agriculture, publishes this Information to further the purpose of the
May 8 and June 30, 1914 Acts of Congress; and Is authorized to pro-
vide research, educational Information and other services only to Indi-
viduals and Institutions that function without regard to race, color, sex or national orl-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulk
rates or copies fdr out-of-state purchasers Is available from C. M. Hinton, Publications
Distribution Center, IFAS Building 664, University of Florida, Galnesvllle, Florida
32611. Before publicizing this publication, editors should contact this address to deter-
mine availability.