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Irrigation—Principles & Practices

Unit 1.5 | Part 1 – 249

Appendix 3 (cont.): Calculating Distribution

Uniformity (DU)

Appendix 3: Calculating Distribution Uniformity (DU)

To calculate DU:

Lay out an evenly spaced grid of catch buckets or rain gauges between irrigation pipes or along

a drip line (see diagram on page I-248); note that if you use the Taylor style rain gauges that

measure precipitation/irrigation in inches, you can simultaneously test for application rate as well

as uniformity

Run irrigation for 5 minutes

Measure and record the volume of water in each catch bucket or rain gauge

Rank the volume of water collected in each bucket or rain gauge, from lowest to highest

Calculate the average volume collected in the lowest 25% of catch buckets or rain

gauges, and divide that number by the average volume of all the samples collected to

get DU (measured as a percentage)

Example:

Average volume of lowest 25% of catch buckets = 4 inches

Average volume of all samples collected = 5 inches

DU = 4 divided by 5 = 80%

A low DU percentage (less than 80%) indicates poor distribution uniformity, i.e., one area of the

field or bed is receiving significantly more irrigation water than other areas. Sources of poor

uniformity can include malfunctioning or clogged sprinkler heads, differences in nozzle orifice

sizes across a field, improper pipe spacing, improper operating pressure (too high or too low),

windy conditions, and differences in pressure due to slope.

A similar DU test can be done for drip irrigation systems:

Once the system is brought up to pressure, collect water for a set amount of time (e.g., 5 minutes)

in shallow containers placed beneath emitters, evenly spaced along drip lines at a number of

locations in the field. Bury the trays to grade level so that they do not create undulations that

might impact distribution uniformity

Make sure that all the containers are under the emitters for the same length of time

Measure and record the volume of water in each container

Use the “low quarter DU” method to calculate distribution uniformity. Note that using a

number of containers/data points divisible by four will make the calculations easier.

Drip system uniformity can also be tested by taking pressure measurements using Shrader valves

throughout the field. See a presentation of this method at:

http://www.agwaterquality.org/toms%20presentation%20DU%20in%20Drip%20and%20Sprinkler.pdf

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