Testing sprinkler uniformity
What is the Distribution Uniformity (DU)?
The distribution uniformity of an irrigation system is a measure of how uniformly water is applied across the area irrigated. The DU can be calculated from catch can data collected within the sprinkler pattern by dividing the average reading of the lowest one-quarter of catchments by the average reading of all catchments, and multiplying the answer by 100. A good DU is 75% or above.
How to measure DU
To test sprinkler uniformity a grid of evenly spaced catch cans or rain gauges are placed between sprinklers as shown in the diagram below. Around 20-24 cans are enough, but the more cans, the better you will see the pattern of water application.
Run the system for at least 30 minutes and measure the amount of water in mm that has been collected in each can. If you are using rain gauges the measurements are on the side. If you are using catch cans that don’t taper in or out you can use a metal rule to measure the depth or a measuring cylinder to measure the volume and work out the depth.
An example of a the catch can readings (in mm) from a uniformity test are below:
Follow the following steps to calculate the DU of your system.
Step 1. Divide the number of cans by 4 to get a 25% or ¼ of the cans. 25/4 = 6.25 If you don’t get a whole number, round down to the closest whole number, in this case 6
Step 2. Put the readings in order from lowest to highest or circle the lowest 25% of values.
3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 7, 8 and 9
Step 3. Work out the average of the lowest 25% of cans (in this case 6 cans) by adding up the first six values From the numbers you have put in order from lowest to highest or the six circled values and dividing them by the number of can in the lowest quarter.
So 3+3+3+3+4+4 = 20
then 20/6 = 3.34 (write this number down)
Step 4. Add up all the values and divide them by the number of cans or rain gauges used. This will give you the average amount of water applied during the test.
So 3+3+3+3+4+4+4+4+4+4+4+4+5+5+5+5+6+6+6+6+6+6+7+8+9 = 118
then 118/25 = 4.72 (write this number down)
Step 5. DU = (Average of the Lowest 25% of applied depths / Average of all applied depths) x 100
the DU is 3.34/4.72 x 100 = 70.7%
The DU in this example is below the acceptable standard and improvements need to be made to the system to increase the DU to greater than 75%.
Because conditions (eg wind) will be different each time a uniformity test is conducted it is important to run the test in a range of conditions. The aim is to have a DU greater than 75 % in the majority of conditions.
The Coefficient of Uniformity (CU) is an estimate of the uniformity of the sprinkler pattern based on the average of application rate of the area tested. Calculating your CU is slightly more complicated than calculating DU. Please contact the Department of Agriculture and Food if you require further information.