Monitor the system uniformity from time to time using collection cans. For overhead systems, the can should be the same type and size. For a trickle system, place the emitter into the can. Place the cans in a grid throughout the irrigation zone.
The irrigation system should then operated for a period of time that ensures water delivery to all containers without overflowing any of the cans. Try to do the test during the time of day the zone would usually be watered and for about the same duration, if the cans have enough capacity.
Once water stops flowing, measure the accumulated volume in each can as accurately as possible with a graduated cylinder. Record the values on paper and use them to compute a coefficient of uniformity (Q). To do this:
- Compute the average (A) of all the measurements (n);
- Subtract this average (A) from each measurement, and drop any negative signs so that all numbers are positive (i.e., compute absolute values of the differences);
- Sum these positive numbers (sum of the deviations of each measurement) and divide this sum by A.
- Divide the result by the number of observations (n). The result of this will be a number between zero and one; and
- Subtract this number from 1.0
Q has a maximum possible value of 1.0, which would indicate perfect uniformity. A Q-value of 0.8 or less indicates a poorly performing irrigation system.
Note that the uniformity of subirrigation systems cannot be assessed in this way.