Monitoring
Microirrigation
Part 1: The Tensiometer
By Farouk A. Hassan, Ph.D.
Editors note: In the first of a two-part
feature, Hassan explains how to use the tensiometer for irrigation scheduling
under microirrigation.
 In SUCCESSFULLY operating and
managing microirrigation systems
require a proactive monitoring
approach. Two devices for monitoring the
performance of these irrigation systems
are the tensiometer for sound irrigation
scheduling, and the flowmeter for evaluating
the system performance and estimating
irrigation efficiency. The tensiometer
is presented here, and next month the
flowmeter will be discussed.
Operating The Tensiometer
The details of the tensiometer are
shown in Figure 1. It is installed in the
field with the ceramic tip (porous cup)
placed where the soil-moisture condition
is to be monitored, and the tube is long
enough so that the stopper and the gauge
remain above ground (see Figure 2).
The porous cup is soaked in water overnight
to ensure that the pores in the wall of
the cup become saturated. The tube is then
filled with water -- void of any air bubbles -- and the service cap is closed. When the
tensiometer is installed in the field with
the porous cup in glove-tight contact with
the soil, the saturated pores of the ceramic
tip connect with the soil pores and create
a continuous water pathway between the
water in the soil pores and water inside the
porous cup.
 As soil dries after irrigation, it exerts
tension (or suction effect) on the water in
the soil pores. This tension exerted on soil
water is transmitted, via the established
water pathway, to the water in the porous
cup causing it to be sucked out, leaving
a vacuum above the water column in the
tube. The gauge registers the magnitude of
the developed vacuum. Its reading reflects
the magnitude of the soil-water tension.
After irrigation, the tension exerted
on the soil water is reduced and water is
sucked back from the soil into the porous
cup by the previously created vacuum.
This reduces the vacuum and the gauge
reading decreases, indicating the increase
in soil wetness.
The tensiometer gauge is calibrated in
units of centibar (cb). A bar is about one
atmosphere, and a centibar is 1⁄100 of a bar.
Though the gauge scale reads up to 100
cb, the operating range for the tensiometer
is from 0 to 80 cb. Readings from 0 to 5
cb indicate a saturated soil in which the
plant roots will suffer from lack of oxygen,
while a reading of 80 cb reflects the dry
end of the scale.
The Instrument Of Choice
Due to the frequency of water application
under microirrigation and the maintained
moisture content in the root zone
near field capacity, tensiometers are best
suited for monitoring soil moisture depletion
under these systems. Tensiometers
are also relatively inexpensive, easy to use
and maintain, do not require calibration,
and give a direct measure of the availability
of soil water for plant use (i.e., the
higher the tension reading on the tensiometer gauge, the lower the water availability
to the plant and vice versa).
Since the accuracy of the tensiometer
reading depends on maintaining the developed
vacuum as described above, the main
limitation of tensiometers is the loss of vacuum.
This could be caused by air leaking into
the tensiometer at the gauge connection with
the stem, from around the sealing stopper due
to tiny cracks in the porous cup, or because a
large amount of dissolved air was released in
the water used to fill the tensiometer. However,
with daily readings and inspection a
leak can be spotted, air can be removed, and
the faulty parts can be located and repaired or
replaced.
When To Irrigate
Early in the season,
irrigate vegetables and
row crops when 20% of the available water
in the active root zone is depleted. Later, irrigation
frequency will change with growth
stage and local conditions.
Table 1 provides a guideline for tensiometer
readings at field capacity and at
20% to 25% depletion of available water in
soils of different texture.
| Table 1 |
Moisture Characteristics Of Soils Of Different Texture |
| |
| Soil Texture |
Available Water at Field Capacity
(Inches per foot) |
Tensiometer Reading (cb) at Field Capacity |
Tensiometer Reading (cb) at 20-25% Depletion |
| |
| Sand |
0.50-1.00 |
10-15 |
20-25 |
|
| Loam |
1.00-1.75 |
15-20 |
20-30 |
|
| Clay |
1.75-2.25 |
20-25 |
25-25 |
|
Monitoring Moisture For Vegetables
Install tensiometers, in sets of two, in the
row between plants. The ceramic tip of the
first tensiometer should be placed at about
6 inches deep in the soil for shallow-rooted
crops (e.g. lettuce, celery) and at 12 inches
for deep-rooted crops (e.g. tomatoes, melons).
The second tensiometer should be
installed about 12 inches deeper than the
first one.
The shallow tensiometer monitors the
moisture status of the active root zone.
Irrigation should begin when the shallow
tensiometer readings are in the ranges of
20 to 25 cb in sandy soils, 25 to 30 cb in
clay loam soils, and 35 to 40 cb in heavy
clay soils. The deep tensiometer should
read about 10 cb between irrigations. Much
higher readings show insufficient irrigation,
while lower readings may indicate
too heavy or too frequent irrigations or poor
drainage.
Tensiometer Tips
THERE are several other points to consider when using a tensiometer.
1. Deep tensiometers also provide information on whether soil water is flowing upward in the root zone or being lost to deep percolation. A downward movement of water may be desirable if salt leaching is intended.
2. Correction of the gauge reading is usually required to account for the depth of placement of the tensiometer. Three centibar (cb) should be subtracted from the reading for every foot of depth. The depth of the tensiometer is determined as the distance from the middle of the ceramic tip to the gauge. A 2-foot tensiometer should require that 6 cb (= 2 feet x 3 cb per foot) be subtracted from the gauge reading.
3. The upper limit of gauge reading at elevations up to 1000 feet above sea level is 80 cb. At higher elevations, this limit should be reduced, due to the decrease in atmospheric pressure, by 3 cb per 1000 feet increase in elevation; i.e., at 3500 feet above sea level the upper limit of gauge reading is 72 cb.
4. Daily readings of tensiometer should be done at the same time of the day. At each reading, add water if the water level in the tensiometer falls more than 1 to 2 inches below the stopper. Keep records of readings for future planning.
5. Irrigation management decisions should be based on at least two sets of tensiometers installed in each management unit of the field that differs in crop, soil texture, profile depth and stratification, cover crop, other cultural practices, and the desired degree of characterization.
|
Farouk A. Hassan is an irrigation and soils
consultant with Agro Industrial Management,
Fresno, CA; fahassan@aol.com.
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