Evaluation of Emission
Uniformity For Efficient Microirrigation
By Farouk A. Hassan, Ph.D
An efficient microirrigation system must apply (emit) water
uniformly throughout the field. Low emission uniformity (EU) will necessitate applying
more water to satisfy the need of plants receiving less than their water requirements.
Consequently, plants previously having too little water will get enough while the rest
will receive too much. If irrigation efficiency is defined as the percentage of the
applied water that is stored in the root zone, then poor EU will lead to over irrigation
resulting in low efficiency and excessive energy consumption at the pump. It will also
result in contamination of ground water and inefficient use of fertilizer as it will be
leached below the root zone by the excessive amount of applied water. Evidently, high
emission uniformity is a prerequisite for efficient irrigation.
High EU is achieved by maintaining a limited variation in
discharge rate among system emitters. Proper maintenance of filters is also vital for
preserving system EU. Emitter clogging and uneven pressure distribution are the major
factors contributing to disparity in discharge rate and poor uniformity. Upgrading EU to
90% could save on water, power and fertilizer bills, improve irrigation efficiency and
crop yield, preserve the environ- ment, and enhance grower's net profit. Annual evaluation
of EU is recommended for monitoring system performance and pinpointing problems. It is
also advisable to evaluate newly installed systems to establish a baseline for future
evaluations. A simple method for the evaluation of emission uniformity is described below.
The equipment needed are readily available on most farms.
- Pressure gauge, 0-50 pound per square inch (psi) range, with
attachment for a shrader valve and fittings for the end of the lateral (hose/drip tape)
- Stop watch or a watch with a second hand.
- Plastic cup about 250 ml and one-gallon plastic bucket.
- Two feet of 1 inch garden hose.
- Two plastic graduate cylinder, 100 ml and 1000 ml capacity.
- Plastic trough 3 feet long for measuring discharge from 3-foot
length of drip tape. A 2-inch PVC pipe cut in half lengthwise makes a good trough.
- Measuring tape 100 feet long.
A: System Preparation and Adjustments
a. Flush system piping and emitter laterals thoroughly, start
with the larger pipes then the smaller ones.
b. Clean screen filters at the manifold and emitter lateral
c. Inspect pressures at pump discharge, across the main
filter and at the inlet to the mainline and submain. Secure these pressures as per design.
d. Adjust the pressure regulators at the inlet to all
manifolds in the field to the same value as per design.
B: Discharge Measurements
Locate four emitter laterals (hose/drip tape) along an
operating manifold or in a block of laterals controlled by one valve. One lateral near the
water inlet to the manifold, the second at about 1/3 of the manifold length, the third at
2/3 of the length, and the fourth near the end. For hoses with individual emitters, e.g.
drip emitters, micro-jets, or spray heads, collect discharge from four emitters on each
hose, one emitter near the water inlet to the hose, the second at about 1/3 of the hose
length, the third at 2/3 of the length, and the fourth near the end. This makes a total of
16 discharge measurements, 4 per each hose. Use the 3-foot trough to collect discharge at
sixteen 3-foot sections of drip tape, located as described above. Collect the discharge of
individual drip emitters and drip tape sections in the 250 ml plastic cup and measure its
volume using the 100-ml graduate cylinder. Slip the one-inch hose on the micro-jets or
spray heads to collect their individual discharge in the one-gallon bucket and use the
1000-ml graduate cylinder to measure its volume. Collect discharge for one minute for all
types of emitters. The measured volume equals the emitter discharge rate in ml per minute.
Divide this value by 63 to convert it to gallon per hour.
The volume of one minute discharge of an emitter = 95 ml
Emitter discharge rate = 95 ml/minute/63 = 1.5 gallons per
For drip tape count the number of orifices per 100 feet and
per 3 feet of the tape. The ratio of these two numbers multiplied by the measured
discharge rate of the 3-foot section gives the discharge in ml per minute per 100 feet.
A drip tape with 6 orifices per 3 feet, 200 orifices per 100
feet and discharge rate of 57 ml/min per 3-foot section, the measured discharge rate of
the 3-foot section of the tape =
57 x (200/6) =1900 ml/100 feet/min = 1.9 liter/100 feet/min.
One US gallon = 3.785 liters
The flow rate of the 3-foot section of the tape = 1.9
liter/100 feet/min./3.785 = 0.5 gallon/100 feet/min.
C: Pressure Measurements
Measure the water pressure at the inlet and at the end of
each of the four selected emitter laterals and the manifold, for each of tested blocks,
using the pressure gauge and the shrader valve attachment or the appropriate fittings. If
feasible use the same gauge for measuring pressure at all locations in the block/field to
eliminate the effect of difference between individual gauges on the readings.
D: Computation of EU Value
Add up all measured emitter discharge rates and divide the
sum by the number of measurements to obtain the average discharge rate. Select the lower
25% of the measured discharge rate, i.e. if 16 measurements were made take the lowest 4
and calculate their average. This is the average of the lowest quarter.
Average discharge rate = 2.7 gph
Average of the lowest quarter = 2.2 gph
EU = [(average of the lowest quarter)/(average discharge
EU = (2.2 / 2.7) x 100 = 81%
General criteria for EU values for systems in operation for
one or more seasons are: excellent, greater than 90%, good, 80% to 90%, fair, 70% to 80%,
and poor, less than 70%.
INTERPRETATION OF FINDINGS
Systems operating for more than one year with EU in the
excellent and good ranges testify to a satisfactory maintenance practice. However, poor or
near poor EU (lower 70's % or less) usually indicates clogged, gradual clogging, or
deteriorating emitters, or problems with pressure regulation. Symptoms and remedies of
these ailments are discussed next.
A: Emitter Clogging and Deterioration
Random variations in the values of measured emitter discharge
rates throughout the field is an indication of emitter clogging. A measured average
discharge rate less than the manufacturer specified value by 15% or more could mean
gradual clogging. This should be confirmed by close inspection of samples of emitters at
various locations in the field for the presence of precipitates and deposits in the
orifices. If the clogging problem is severe, the volume of applied water indicated by the
totalized readings of the flow meter could be significantly less than the scheduled
irrigation requirements for the evaluated block/field.
Remedies of clogging include more frequent flushing of
irrigation system pipes and emitter laterals and chemical treatments. If the contaminant
in the water flushed out of laterals is in the form of precipitate, e.g. white calcium
carbonate or reddish brown iron oxide, acidification would be necessary. Flushed microbial
growth, e.g. bacterial slime, indicates the need for chlorination. Acidification and
chlorination will be needed if both types of contaminants are observed. The presence of
particulates, e.g. sand, in the flushed water calls for more close inspection and
maintenance of the main filter as well as more frequent cleaning of the lateral and
manifold screen filters. Though manual cleaning of some partially clogged emitters may
improve their performance, plugged ones should be replaced. If the desired emitter
cleaning was not achieved repeated treatment is necessary. A post treatment evaluation is
If close examination of emitters reveals no evidence of
clogging (e.g. after implementing required treatments to alleviate clogging) but EU
remains low, emitter deterioration is suspected. Compare the calculated average discharge
rate of emitters in the post treatment evaluation to the manufacturer specified discharge
rate at the same operating pressure. A deviation of 15% or more from the manufacturer
listed value is an indication of emitter deterioration. If this is confirmed by repeated
testing at different locations in the block/field, partial emitters replacement would be
necessary to improve EU.
B: Pressure Variation
When pressure regulation is the culprit, emitters with low
discharge rate are observed near the end of laterals or towards the downstream end of
manifolds. The pressure variation throughout the block, i.e. between the inlet to the
manifold and the end of the far most lateral on the manifold, should not exceed 20% and
10% for turbulent and laminar flow emitters respectively to maintain high EU. This will
result in variation in discharge rate of 10% for both types of emitters.
If pressure at manifold inlet = 20 psi,
20% of the pressure at manifold inlet = 20 x (20/100) = 4
psi, and pressure at the end of the far most lateral on the manifold should not be less
than 16 psi (= 20 - 4) to maintain high uniformity.
When repeated measurements confirm that pressure variation
exceeds the indicated limits, excessive lateral length and/or undersized manifold pipe are
suspected. Pressure differences larger than the design values between the inlet and the
end of laterals and/or manifolds validate the above mentioned suspicion. Consult with the
system designer for necessary corrective measures to restore system EU.
Efficiency is defined as the ratio of output to input. If the
input is the applied water, energy, fertilizer, other chemicals, and management, while the
output is the quality and quantity of crop yield, the quality of the environment, and the
conserved resources, then emission uniformity is a sound indicator of the efficiency of
microirrigation. High uniformity coupled with good management enables the conservation of
inputs and provides the opportunity for optimizing the outputs. Periodic evaluation of
uniformity as part of a well planned maintenance program is essential for securing
efficient system performance and for reaping the benefits of the microirrigation
Farouk A. Hassan, Ph.D. is an irrigation and soils consultant
with Agro Industrial Management, Fresno, California. This article contains information
from the new book "Drip and Microirrigation Management and Maintenance" and the
software program "Benefits of Emission Uniformity Improvement", both available
from Agro Industrial Management.
Phone: (209) 224-1618, Fax: (209) 348-0721, E-mail: email@example.com