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Application uniformity testing for center pivot irrigation systems

Irrigation uniformity testing
Application uniformity is being tested using catch cans.

Identifying water application problems when irrigating using a sprinkler irrigation system, such as center pivots, is vital.

Over and under irrigation can have an adverse effect on crop yield that can be attributed to poor soil aeration, increased disease incidence and leaching of the agricultural chemicals to the groundwater especially in sandy soils, which are highly porous and cannot hold much water.

Consequently, periodic checking of irrigation system uniformity is important and is the first step in improving water use efficiency and yield and reducing the energy costs.

Test procedure

Uniformity testing can be done by setting the catch cans (collectors) along the length of the pivot or perpendicular to the direction of the travel and letting the system pass over these cans.

  • The system pressure should match the pressure used to design the sprinkler package on the machine.
  • Record the amount of water collected in each can and can distance from the center of the pivot.
  • Using this information, a coefficient of uniformity (CU) can be calculated.
  • The coefficient of uniformity (CU) is usually expressed as a percentage.

The formulas and interpretation of CU are discussed in the later section. The step by step procedure is described below:

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How to analyze and interpret your data

Use the Center pivot application uniformity spreadsheet to calculate the coefficient of uniformity (CUH).

The coefficient of uniformity (CUH) for the center pivot system is calculated using the modified formula of Heermann and Hein:
 
Math formula for coefficient of uniformity
where CUH is the Heerman and Hein uniformity coefficient;
N in the number of collector cans;
i is the ith can from the pivot point with i=1 being the can closest to the pivot;
Si is the distance of the ith can from the center of the pivot; Vi is the volume of water collected in the ith can; and
Vp is the weighted average of the volume of water caught. It is calculated with the following:
math formula for weighted volume average

Interpreting your results

Once you've calculated the uniformity coefficient, check the table below to see if you need to take additional actions.

Uniformity coefficient interpretation.

Uniformity coefficient Interpretation
90-100% Excellent; no changes are required
85-90% Good; no changes are required
80-85% Fair; no changes required at this time, but the system should be monitored closely
Below 80% Poor; improvements are required

Additional tips

  • Finding a kit: Irrigation uniformity testing kits are usually available to check-out at your local Natural Resource Conservation Service (NRCS) and Soil and Water Conservation District (SWCD) offices. Their staff may also help you perform the test and calculations. If you do not have access to a testing kit, 16-ounce drink cups, rain gauges or other containers of uniform size and shape may be used to perform a uniformity test.
  • Note any issues: It is very important to make notes of any leaks or if some sprinklers are not operating properly. These notes can help you interpret the data and make recommendations for improvements.
  • Wind speed: The test should not be performed if wind speeds exceed 2.25 miles per hour. Tests are invalid if winds are greater than 11 mph. To minimize the effect of evaporation, perform the test at night or during early daylight hours.

Watch this video to see a test being performed: Irrigation uniformity testing

Vasudha Sharma, Extension irrigation specialist 

Reviewed in 2019

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