Adjusting irrigation uniformity coefficients for unimportant variability on a small scale

Abstract

Distribution Uniformity (DU) and the Coefficient of Uniformity (CU) are commonly used irrigation performance measures of how evenly water is applied across a field during irrigation. The more DU or CU can be improved, the more water will be conserved and the better the crop will perform. DU and CU are most commonly measured for sprinkler irrigation using catch cans. Gross irrigation amounts are often increased to account for poor DU and CU by dividing the net irrigation required by the DU or CU as a decimal. However, non-uniformity on a very small scale (i.e. less than 30 cm) is less important since the soil can redistribute the water and the crop roots can get water from wetter regions. In this paper, a new method is suggested to measure and adjust DU and CU for the less important irrigation application variability on a small scale. These methods were applied to uniformity evaluations in grower fields at different locations in Washington and Oregon, USA. The results showed that the absolute value of DU and CU might be adjusted up 6–8% and 3–6%, respectively. If gross irrigation depth is being corrected for poor DU or CU, then using this method would result in 9–12.5% less water being applied to compensate for poor DU. This method helps give a DU and a CU that are more representative of what the crops in a field as a whole “feel” and helps prevent over application of irrigation water to correct for variability in water application depths on a small scale.