Good irrigation uniformity is essential for efficient water and fertiliser use
Applying water as evenly as possible from your sprinkler system is important so that all areas of your crop receive similar amounts of water and nutrients. Unfortunately not all areas receive the same amount of water. Sprinkler uniformity is the term used to describe how evenly water is applied by overhead irrigation.
The sprinkler uniformity can be described mathematically by simple terms known as the distribution uniformity (DU) and the coefficient of uniformity (CU). The uniformity of your irrigation system can be measured using catch containers placed in a grid pattern between sprinklers and laterals to measure application rates. The measurements are then used to calculate the DU and CU and can also be transferred to graph paper (see below).
The accepted international design standards for irrigation uniformity are a DU greater than 75% and a CU greater than 85%. The higher the CU and DU, the greater the area receiving the average amount of water and smaller the area that is being over or under-watered.
Below are the results of uniformity tests on two sprinkler irrigated areas. Both areas were tested at the same time within the same set of laterals but using different sprinkler types. The uniformity of the first area (Figure 1) is less than the accepted 85% CU and 75% DU. The key on the side of the graphs shows the hourly application rate in millimetres (mm), with the average application rate shown in orange. The crop area receiving the average amount of irrigation is small compared to areas receiving more or less than the average. The driest area has received only 3.5 to 4.5 mm per hour and wettest area 9.5 to 10.5 mm per hour, almost three times more.
Sprinkler uniformity of the second graph (Figure 2.) is better than the accept standard of 85% CU and 75% DU and clearly shows a larger area is receiving the average or close to it. The driest area received 5 to 6 mm of irrigation and the wettest 8 to 9 mm.
Poor system uniformity results in overwatering in some areas and underwatering in others. Poor uniformity also affects fertiliser distribution throughout the crop, especially where fertiliser is applied with irrigation. If you apply fertiliser through your sprinkler system, poor uniformity can deprive some areas of adequate fertiliser and supply excess to other areas. Nitrogen is highly mobile in soil and is lost when leached passed the root zone with the excess water. If fertiliser is broadcast or banded and your irrigation system has poor uniformity, nitrogen fertiliser will still leach past the root zone in areas that receive too much water.
You can test the uniformity of your system by placing simple catch cans in a grid pattern at regular spacings between sprinklers and laterals and record catch volumes (see details here).