Distribution Uniformity or DU in irrigation is a measure of how uniformly water is applied to the area being watered, expressed as a percentage. The distribution uniformity is often calculated when performing an irrigation audit. The DU should not be confused with the coefficient of uniformity (CU) which is often preferred for describing the performance of overhead pressurised systems.
The most common measure of DU is the Low Quarter DU, which is a measure of the average of the lowest quarter of samples, divided by the average of all samples. The higher the DU, the better the performance of the system. If all samples are equal, the DU is 100%. If a proportion of the area greater than 25% receives zero application the DU will be 0%. There is no universal value of DU for satisfactory system performance but generally a value >80% is considered acceptable.
One alternative is the Christiansen's Uniformity Coefficient (CU). This is calculated by taking the taking one minus the total each sample deviates from the mean divided by the total of all samples.
Another less common alternative to the standard DU is the absolute distribution uniformity, calculated by dividing the minimum depth applied to any point by the average of all samples.
Distribution uniformity is useful when determining the total watering requirement during irrigation scheduling. For example, an irrigator might want to apply not less than one inch of water to the area being watered. If the DU were 75%, then the total amount to be applied would be the desired amount of water, divided by the DU. In this case, the required irrigation would be 1.33 inches of water, so that only a very small area received less than one inch. The lower the DU, the less efficient the distribution, and thus the more water that must be applied to meet the minimum requirement.
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