Monitoring Microirrigation Performance
By Farouk A. Hassan, Ph.D
operation and management of microirrigation systems require a proactive monitoring
approach to maintain the desired system performance. Two devices for monitoring the
performance of these irrigation systems are presented in this article. The tensiometer for monitoring the moisture
status in the rootzone for sound irrigation scheduling,
and the flowmeter for measuring the system flow rate and recording the
volume of applied water. While the use of
tensiometer enables determining when to irrigate, the flowmeter provides the necessary
information for evaluating the system performance, and for estimating irrigation
parts shown in figure 1 include the ceramic tip, the transparent plastic tube (stem) with
a side port to accommodate a vacuum gauge, the vacuum gauge, and the rubber stopper
attached to the service cap. The
tensiometer is usually installed in the field with
the ceramic tip (ceramic cup) placed where the soil moisture status is to be monitored,
and the plastic tube is long enough so that the stopper and the gauge remain above ground
(see figure 2).
Figure 1. Tensiometer with ceramic tip, plastic
tube and side port
Figure 2. Tensiometer installed in the field
the proper use of tensiometer, the porous cup should be soaked in water overnight to
ensure that the pores in the wall of the cup are saturated.
The tube is then filled with
water so that no air bubbles remain inside it and the service cap is closed to secure the
airtightness of the tensiometer. The
tensiometer is installed in the field with
the porous cup in glovetight contact with the soil. The
saturated pores of the porous cup connect with the soil pores and create a continuous
hydraulic (water) connection between the water in the
soil pores and water inside the porous cup.
irrigation, as the soil dries it exerts tension (or suction effect) on the water in the
soil pores. The developed tension is
transmitted via the established hydraulic connection
to the water inside the porous cup causing some water to be sucked out of the cup leaving
vacuum above the water column in the tube. The
gauge registers the magnitude of the
developed vacuum. Further soil dryness causes more water to move outward from
the porous cup to the soil. This is reflected
by higher gauge readings.
When the field
is reirrigated, suction in soil pores is reduced and the previously created vacuum above
the water column in the stem of the tensiometer causes the water to be drawn back into the
porous cup . This inward water movement reduces the vacuum and lowers the
As soil starts
to dry again, the water moves outward from the porous cup and the above pattern repeats
itself. Evidently any air leakage into the
tensiometer could impair its function as it
destroys the developed vacuum. A good
maintenance routine with regular air removal
from the tensiometer stem and filling it with water could avoid such leakage.
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 scale of the vacuum gauge reads up to 100 cb, a tensiometer can operate 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.
B. The Instrument of Choice
frequency of water application under microirrigation
maintains moisture content near
field capacity with a corresponding low soil-moisture tension in the root zone (well below
80 cb). Tensiometer is most suited for
monitoring soil-moisture depletion under this wetting pattern. It is also relatively inexpensive instrument and
easy to use. Monitoring soil moisture
depletion in the root zone is essential for determining
when to irrigate and an indispensable part of a good irrigation management
When to Irrigate
in the season irrigate vegetables, row crops and newly planted trees when 20% of the
available water in the active root zone is depleted.
Later in the season irrigation frequency
will change with growth stage and local conditions. Irrigation
of well established tree crops should begin when depletion approaches 25-30%. Table 1 provides a guideline for tensiometer
readings at field capacity (F.C.) and at 20-25% depletion of available water in soils of
1: Moisture characteristics of soils of
|Soil Texture at
(inches per foot)
|Tensiometer Reading at
|Tensiometer Reading at
Monitoring Soil Moisture for
Vegetable and Row Crops
tensiometer, 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.
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 centibar (cb) in sandy soils, 25 to 30 cb in clay loam soils and 35 to 40 cb in
heavy clay soils. The deeper 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.
E: Monitoring Soil Moisture for
Orchards and Vineyards
orchards and vineyards the ceramic tip of the tensiometer should be placed in the root
ball. After several weeks, install the
tensiometer near the drip line of the growing tree. Additional
tensiometers should be added to accommodate changes in root distribution over time as the
plants grow. Tensiometer should be placed on
the southwest side of the tree in the northern hemisphere, since this side receives the
hot afternoon sun and tends to dry more quickly. Tensiometers
are installed in the row, between trees or
vines, about 12 inches from the emission
point in sandy soils and about 18 inches from the emission point in clay soils, close to tree drip line.
For most mature
orchards and vineyards tensiometers are installed in sets of three at 12, 24 and 36 inches
deep. For deeper rooted trees (e.g. almond,
walnut) a fourth tensiometer may be installed at a depth of 48 inches. The top tensiometer is in the part of the root
zone that dries out first. Under good
management this tensiometer should read about 25 and 10 centibar (cb) in sandy soils before and after irrigation
respectively and about 35 and 25 cb in clay
soils before and after irrigation respectively.
tensiometer should go down to about 10 and 25 cb range in sandy and clay soils
respectively after irrigation. If the
readings of the deep tensiometer remain significantly
higher than these values, this means that the volume of applied water,
irrigation time, or frequency of application should be increased. On the other hand, if the readings of the deeper
tensiometer do not come up to the 10-25 cb range between irrigations it means that
application is too heavy, too frequent, or drainage is restricted.
F: Points of Consideration for Tensiometer Utilization
Deeper tensiometer provide information on whether 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.
Tensiometer placement must be in the zone wetted by the downward and outward movement of
water in the soil. If the tip is outside the
wetted zone, readings will indicate low soil
moisture content resulting in over-irrigation. On
the other hand, if the tip is too close to the emitter,
the high moisture content indicated by the instrument may lead to
Correction of the gauge reading is usually
required to account for the depth of
placement of the tensiometer. The depth of
tensiometers is determined as the distance from the middle of the ceramic tip to the
gauge. Three centibar should be subtracted
from the reading for every foot of depth. A
4-ft tensiometer should require that 12 cb (= 4 feet x 3 cb per foot) be subtracted from
the gauge reading. A gauge reading of 40
centibar on a 4-ft tensiometer actually
indicates 28 cb (= 40 -12) of moisture tension.
4. 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 should be 72 cb.
5. Daily readings
of tensiometer should be done at the same time of the day, preferably early in
the morning. At each reading add water if
the water level in the tensiometer falls more than 1 to 2 inches below the stopper.
6. Recording and plotting of readings on a chart
indicating date, depth of reading, and date of irrigation are necessary to achieve the
full benefit of the tensiometer use for monitoring water
applications and for irrigation management. These records can also be useful for future
7. Irrigation management
decisions should be based on the readings of more than one set of tensiometers. No definite number of tensiometer sets per number
of acres is recommended. However, at least
two sets should be 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.
the flow of irrigation water is an essential
aspect of any efficient irrigation management. The
meter reading may indicate the flow rate, the total flow volume, or both. Most
flow rate indicators report in gallons per minute (gpm) or in cubic feet per second (cfs),
while total flow indicators (totalizers) report in gallons, acre-feet, or cubic feet. Some indicators report in metric units.
The Meter of Choice
flowmeters are the most commonly used for agriculture.
Figure 3 shows an example of propeller meter. Though other metering technologies are in use
propeller meters are the meters of choice for irrigation because they are accurate (?
2.0%), of relatively low cost, require no
external power, and withstand harsh environmental conditions.
Figure 3. Propeller meter.
Points of Consideration for Propeller Flowmeter Utilization
1. The meter should be installed downstream from a
straight unobstructed section of the pipe of eight to ten diameter in length and a
straight section two pipe diameter long immediately downstream from the meter to minimize
turbulence caused by various fittings and
valves. Erratic behavior of the rate indicator
may also be due to the presence of air or gas in the water.
Straitening vanes (six-vane
straightener) can be placed just ahead of the flowmeter to break up most swirls and ensure
2. Inaccuracy of the propeller meter is most likely due to mechanical problems.
propeller meter with mechanical problems will
have an unsteady flowrate reading
bouncing needle) or the rotating propeller may create noise and vibration. Mechanical problems are usually caused by a jam
inside the meter. Once the body that is
causing the jam is removed the meter will work
3. A flowmeter is most accurate and the pressure loss
caused by meter is minimal (less than 1 psi) when used within its flow range. The range is very
large and is expressed as the "turndown" of the meter. The turndown is the ratio of the maximum flow rate
to the minimum flow rate and is often 15 :1. That means that the meter would remain accurate up
to 15 times its minimum flow rate. For
example, a meter with a minimum flow rate of
100 gpm and a turndown of 15 :1 would remain accurate up to a flow of 1,500 gpm.
Benefits of Using Flowmeter
use of flowmeter makes it possible to identify changes in flow rate (measured at the same
pressure) during the season. Excessively
higher than usual flow rates or significantly lower rates may provide early alert to some
potential operational problems as indicated below. Other
benefits of water metering include ensuring that the scheduled amount of water is actually
applied and to avoid over-irrigation and leaching of NO3-N
below the root zone and contamination of groundwater. Flowmeter record keeping is useful for planning, management and water budgeting, assessing the
performance of the system, and estimating irrigation efficiency as described below.
Water Metering and System Performance
flowmeter records together with measuring irrigation time can be used for monitoring system performance.
Actual irrigation time is the elapsed time between the beginning of the
irrigation run and until the reading of the flow meter indicates that the total water
volume scheduled for irrigation is applied. For
monitoring system performance:
Obtain an estimate of irrigation time in hours using the following formulae for different types of emitter lines:
a. For hose line (in-line / on-line
Estimated Irrigation Time (hours ) = [irrigation requirements (inches) x acres
x 452.5] / gpm
acres = .............. number
of acres per irrigated set or field
................irrigation system flow rate in gallons per minute, at average
irrigation requirements for a 20 acres field is 0.3 inch.
The capacity of the surface
drip system used
to irrigate this field is 500 gpm. What
should the irrigation time?
Estimated Irrigation Time =
(0.3 x 20 x 452.5) / 500 = 5.5 hours
b. For drip
Irrigation Time (hours) = 1.04 x tape
spacing (feet) x irrigation requirements
(inch) / tape flow rate
The irrigation requirements of a
tomato field is 0.3 inch. The field is
irrigated with subsurface drip with tape spaced at 5 feet. The tape flow rate is 0.4 gallon per
minute per 100 feet. What should the irrigation time be for this field?.
irrigation time =1.04 x [5(feet) x 0.3 (inch)]/ [0.4
= 4.0 hours
convert the depth of irrigation requirements
in acre-inch to volume of water in
gallons or cubic feet (flowmeter
units), use the following formula:
acre-inch = 27152 US gallons = 3630 cubic
jets and microsprinklers (for trees)
Estimated Irrigation Time (hours) = irrigation requirements (gallons/tree) / emitter
gph = gallons per hour
acres orchard is irrigated with
microsprinkler system, one microsprinkler of 20
gph per tree. The orchard is
irrigated daily with irrigation
requirements of 80 gallons
per tree per day. The tree spacing is 20 ft x 20 ft. Estimate the irrigation time for this
orchard, the volume of water per irrigation, and the inches
of applied water.
Estimated Irrigation Time = 80 (gallons/tree) / 20 (GPH)= 4 hrs
Number of trees in
the field = [43560 (sqr. ft/Ac) x 20 (Ac)] /
400 (sqr. ft/tree) = 2178 trees
Volume of water to
be applied per irrigation = (80 gallons/tree) x (2178 trees) =
174,240 gallons per irrigation
Volume of water to
be applied per acre = 174,240 / 20 = 8712 gallon/acre
Inches of applied
water = 8712 (gal/acre) / 27152 (gal/acre-inch) = 0.32 inches
2. Monitor the flowmeter and the determine the actual
time needed to apply the volume of irrigation requirements in gallons or cubic feet.
the actual irrigation time is greater than the estimated irrigation time by more than 15%,
this may be an indication of a clogging problem in progress, especially if pressure buildup and noticeable reduction in system flow
rate are observed. Check emitters flow rate,
filters differential pressure and performance and take necessary maintenance measures.
A significant drop in the flow rate and discharge
pressure could also be the result of excessive drawdown or the need for pump adjustment or
repair. These possibilities should be examined.
Unnoticed reduction in the system discharge rate would lead to
under-irrigation of the field if flowmeter readings are ignored and irrigation run is terminated after a preset time. This can compromise the benefits of
other hand, If actual irrigation time is significantly less than estimated irrigation time with apparent difficulty in maintaining system pressure, check for leaks and make needed
E. Estimation of
approximate but adequate estimate of irrigation efficiency is the ratio of crop water
requirements to the actual depth of irrigation water
applied to the field to bring up the average moisture content of the rootzone to field
capacity. This estimate can be obtained as
Total volume of water applied to the field in gallons or cubic feet (cf) = (totalized meter reading at the end of irrigation) -
(totalized meter reading at beginning of irrigation)
2. Total volume of applied water per acre (gal/Ac or
cf/Ac) = Total volume of water applied to the field / number of acres irrigated
3. Depth of irrigation water applied in inches (in) = Total volume of applied water per acre (gal/Ac) /
27152 = Total volume of applied water per acre (cf/Ac)
4. Irrigation efficiency (decimal) =
water requirements (in) / depth of irrigation water
5. Irrigation efficiency (%) = Irrigation efficiency
(decimal) x 100
estimated water requirement of a crop is 0.25 inches and the assumed efficiency of the microirrigation system used to irrigate
this 20-acre field is 85%. The readings of
the flowmeter totalizer were 101, 000 gallons and 300, 240 gallons before and after
irrigation respectively. Calculate the
scheduled irrigation application depth based on the assumed irrigation efficiency, and the
actual irrigation efficiency based on the given flowmeter readings.
irrigation application depth = 0.25 / 0.85 =
volume of water applied to the field (gallons) = 300,240 - 101000 = 199, 240 gallons
volume of applied water per acre (gallons/acre) =
199,240 / 20 = 9962 gallons/acre
depth of irrigation water applied (inches) = 9962 / 27152 = 0.40 inches
irrigation efficiency (decimal)= 0.25 / 0.40 = 0.63
irrigation efficiency (%) = 0.63 x 100 = 63%
irrigation efficiency is consistently less than 80%, then necessary measures should be
taken to improve efficiency (examine the possibility of clogging, check filter condition,
method outlined above for estimating irrigation efficiency is not a substitute for
professional irrigation evaluation service, but
it can be used when such service is not readily available.
conclusion, performance monitoring tools are indispensable for a proactive approach of
managing microirrigation. Such an
approach saves money, conserves resources, protects the environment and is a prerequisite
for a successful operation. On the other
hand, lack of monitoring is a characteristic of a reactive approach which costs money and
may not be fast enough to avert unexpected problems at critical times.
A. Hassan is an Irrigation & Soils Consultant with Agro Industrial Management, P.O.Box
5632, Fresno, California 93755, Phone: (559)224-1618, Fax: (559)348-0721, E-mail: email@example.com