Irrigation Efficiency
(Uniformity)

In order to determine how much irrigation water to apply, you need to estimate your irrigation efficiency. The irrigation efficiency data collected is used to determine the Coefficient of Uniformity (CU) for the irrigation system being used. The results are given as percent uniformity and are used in calculations for determining how much water to apply. Systems that approach 100% are considered very uniform, or highly efficient in delivering water to the target crop. Statewide averages run around 80%.

Materials Needed:

1. Graduated Cylinder or other container marked such that you can see when the water reaches that point.
2. Stopwatch
3. Datasheet and pencil


Procedure: (prepare to get wet)

1. Irrigation system is turned on and allowed to come to full pressure
2. If possible, sprinkler is inverted into the graduated cylinder and the stopwatch started. For fixed risers one can use a plastic container (2 liter soda) with a hole cut in the bottom through which you insert the sprinkler. A second hole should be provided to funnel the water collected into the marked container.
3. When the cylinder or container is filled to the predetermined amount, the time is recorded in seconds to the nearest 100th second and the sprinkler returned to its original position.

Notes:

1. Be sure to collect from various portions of the field. Especially high and low points, or areas that seem to receive too much or too little water.
2. The larger the number of sprinklers sampled, the greater the confidence in the results.


Irrigation Efficiency Calculation

Once the data is collected, uniformity is determined using the following formula based on the times collected:

Sd = standard deviation
x = mean
Example: You've collected the times on 10 sprinklers (use a spreadsheet to do the calculations)
Sprinkler
Seconds
1
36.62
2
46.06
3
46.78
4
41.69
5
39.75
6
43.09
7
37.05
8
38.56
9
38.28
10
39.97
Mean:
40.78
Sd:
3.561
Uniformity = (1-(3.561/40.78))x100

Uniformity = 91.27%

For Further Information

Irrigation Scheduling, a guide for efficient on-farm water management. University of California Division of Agriculture and Natural Resources Publication #21454. 1989.