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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.

EQUIPMENT NEEDED

  1. 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) measurements.
  2. Stop watch or a watch with a second hand.
  3. Plastic cup about 250 ml and one-gallon plastic bucket.
  4. Two feet of 1 inch garden hose.
  5. Two plastic graduate cylinder, 100 ml and 1000 ml capacity.
  6. 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.
  7. Measuring tape 100 feet long.

PROCEDURE

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 inlets.

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.

Example:

The volume of one minute discharge of an emitter = 95 ml

Emitter discharge rate = 95 ml/minute/63 = 1.5 gallons per hour.

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.

Example:

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.

Example:

Average discharge rate = 2.7 gph

Average of the lowest quarter = 2.2 gph

EU = [(average of the lowest quarter)/(average discharge rate)] x100

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 recommended.

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.

Example:

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 technology.


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: weaim@aol.com