It is difficult to get uniform watering with a single line of nozzles.
Photo: John W. Bartok, Jr.

Knowing irrigation system application uniformity is important to reduce over or underwatering plants. It can also conserve water and reduce energy costs. Uniformity is hardest to obtain with overhead irrigation systems, but drip systems should also be checked. Boom systems, trough, and ebb & flood benches and floors have become popular because they provide the greatest uniformity.

Uniformity is affected by nozzle selection, spacing and height, water pressure, pipe size and maintenance of the system. Distribution uniformity (DU), measured in percentage, is an easy method of checking and provides a visual picture of how the water is being applied. The following are common test methods for overhead and drip systems.


The method consists of placing 16 or more straight-sided catchment cans evenly spaced throughout a representative section of the crop. Coffee or soup cans work well. It is best to locate the cans between sprinklers and along the lateral lines and near the perimeter of the growing area.

Prepare a chart on graph paper noting the location of the cans. Run the system until there is at least an inch of water in the cans. Record the operating time. Pour the water from each cup into a graduated cylinder or beaker and mark the amount on the chart. Most graduated cylinders are calibrated in milliliters (ml). Total up the quantity of the lowest ¼ of the cans and then divide by the number of cans to get the average. Next, total up the volume collected for all the cans and divide this by the number of cans to get an average water application rate.

The DU is the average of the lowest cans divided by the average of all the cans multiplied by 100. This will give the DU in percentage.

The irrigation industry has developed the following DU ratings. Greater than 90% = excellent, 80–90% = good, 70–80% = fair and less than 70% = poor.

A drip system can provide uniform water and save up to 75% in water use.
Photo: John W. Bartok, Jr.

Drip system

The DU method for drip systems consists of sampling the output from a number of emitters. Depending on the application and manufacturer, emitters can have outputs from ½ to 16 gallons/hour (gph). Collection containers should be sized to collect 1 minute of system operation. In one minute, a 1 gph emitter will put out about 1/8 pint (60 ml) whereas a 16 gph emitter will put out about 1 quart.

Three or four sampling locations should be chosen for each lateral line in the zone, one near each end and one or two evenly spaced in the center. Place a container under each emitter. Turn the irrigation system on for one minute then turn it off.

Prepare a data sheet to record the amount of water from each emitter. Use a graduated cylinder or beaker to measure the amount.

Next rank the emitters in order of how much water they put out. Now add up and record the output for the lowest 25 percent. Divide this by the number of emitters. To calculate the DU divide this number by the average rate for all the emmitters and multiply by 100. Refer to the industry DU ratings above to see how your system compares.

To calculate the amount of water in gph that is put out by any emitter or by the average, multiply the milliliter reading by 0.0159. For example, 60 ml/min x 0.0159 = 0.95 gph.

If the DU rating is fair or poor, clean and check filters/screens, nozzles or emitters that have the lowest output.

With the system in operation, check pump pressure with in-line gauges against the pressure at the beginning and end of lateral lines. Low pressure at the ends may indicate leaks, too small supply pipes or too many nozzles/emitters. Water supply should be limited to about 4 gallons per minute (gpm) for ½” pipe, 8 gpm for ¾” and 12 gpm for 1” pipe to keep friction loss at an acceptable level.

You may need to add new nozzles, replace worn nozzles or change to closer spacing to get greater uniformity of overhead systems.

More uniform watering results in more uniformity in plugs, seedlings and mature plants. Checking your system will help to see where changes should be made.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.