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Monitoring
Microirrigation
Part 2: The Flowmeter
By Farouk A. Hassan, Ph.D.
American Editor’s note: In the second of a two-part series, the author explains how to use the flowmeter to evaluate system performance and estimate irrigation efficiency.
In PART one of this two-part article
discussed the utilization of the
tensiometer as a tool for monitoring
the moisture status in the root zone to
determine when to irrigate. The flowmeter
is presented here as a tool for evaluating
system performance and estimating irrigation
efficiency.
Monitoring the flow of irrigation water
is an essential aspect of any efficient irrigation
management program. The meter
reading may indicate the flow rate, the
total volume of flow, or both.
 Propeller flowmeters are the most commonly
used for agriculture. The details
of a propeller meter are shown in Figure 1. Though other metering technologies
are in use, propeller meters are the meter
of choice for irrigation because they are
reasonably accurate (within 2.0%), of
relatively low cost, require no external
power, and withstand harsh environmental
conditions.
Using Propeller Flowmeters
1. Install the meter downstream from a
straight unobstructed section of the pipe of
eight to ten pipe diameters in length, and
immediately upstream from a straight section
two pipe diameters long, 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. Straightening vanes
(six-vane straightener) can be placed just
ahead of the flowmeter to break up most
swirls and ensure accurate measurement.
2. Inaccuracy of the propeller meter
is likely due to mechanical problems. A
propeller meter with mechanical problems
will have an unsteady flow rate reading (a
bouncing needle) or the rotating propeller
may create noise and vibrate. 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 again.
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Commonly used for agriculture, propeller flowmeters are
accurate, inexpensive, and
require no external power. |
3. A flowmeter is most accurate and the
pressure loss caused by the 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 1500 gallons per minute (gpm).
In conclusion, performance monitoring tools are indispensable
for a proactive approach of managing microirrigation.
Such an approach saves money and is a prerequisite for
a successful operation. On the other hand, lack of monitoring
is characteristic of a reactive approach which costs money
and may not be fast enough to avert unexpected problems at
critical times.
Estimating Efficiency
An 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 the average moisture content of the
root zone to field capacity. Estimate efficiency as follows:
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 the beginning of irrigation)
Total volume of applied water per acre (gal/Ac or cf/Ac) =
Total volume of water applied to the field
÷ -number of irrigated acres
Depth of irrigation water applied in inches (in) =
Total volume of applied water per acre (gal/Ac) ÷ 27,152 =
Total volume of applied water per acre (cf/Ac) ÷ 3630
Irrigation efficiency (decimal) =
water requirements (in) ÷ depth of irrigation water applied (in)
Irrigation efficiency (%) =
Irrigation efficiency (decimal) x 100
If irrigation efficiency is consistently less than 80%, necessary
measures should be taken to improve efficiency (examine
the possibility of clogging, check filter condition, etc.). The
method outlined above for estimating irrigation efficiency is not
a substitute for professional irrigation evaluation services, but it
can be used when such services are not readily available.
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Water Metering And 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:
1. Obtain an estimate of irrigation time in hours using the
following formulae for different types of emitter lines:
Hose line
(in-line/on-line emitters)
Estimated irrigation time (hours) =
[irrigation requirements (inches) x acres x 452.5] ÷ gpm
where:
acres = number of acres per irrigated set or field
gpm = irrigation system flow rate in gallons per minute, at
average operating pressure
Drip tape
Estimated irrigation time (hours) =
1.04 x tape spacing (feet) x irrigation requirements (inches)
tape flow rate (gallon/minute/100 feet)
To convert the depth of irrigation requirements in acre-inch to
volume in gallons or cubic feet (flowmeter totalizer units), use the
following formula:
One acre-inch = 27,152 U.S. gallons = 3630 cubic feet
2. Monitor the flowmeter and determine the actual time
needed to apply the volume of required irrigation water.
3. If 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 also
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 at a preset
time. This can compromise the benefits of microirrigation.
On the 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 repairs.
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Hassan is an irrigation and soils consultant with Agro Industrial
Management: Fresno, CA; fahassan@aol.com.
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