PERFORMANCE OF IRRIGATION SYSTEMS IN SOUTH AFRICA - Management, Performance, and Applications of Micro Irrigation Systems - page 61

FIGURE 1 Irrigation efficiencies.

5.3 UNIFORMITY COEFFICIENTS OF IRRIGATION SYSTEMS
5.3.1 COEFFICIENT OF UNIFORMITY
The coefficient of uniformity (CU) is used to express how evenly the water is spread
over the irrigated area. Uniformity coefficient values are determined by catching dis-
charge from sprinklers on emitters in equally spaced cans and evaluating the catch-
ments mathematically. The values are considered satisfactory, when the CU is greater
than 84%.

n
i

CU = 100 {1 x − ∑ [ ( Ri − m )] ÷ ( m x n )}

(1)
The CU is an indicator of how the individual rain gauge readings deviate from the
mean and it is calculated using Eq. (1), where: R i = Individual rain gauge reading in
mm; m = Rain gauge mean reading (mm); and n = Number of rain gauges. For a center
pivot, we must use the modified form of CU formula, which involves area weighing
(multiplying by the area) of each individual rain gauge reading to its representative
section of the pivot area.

= 1

5.3.2 DISTRIBUTION UNIFORMITY
Distribution uniformity (DU) is defined as a ratio of the smallest 25% accumulated
depths in the distribution to the average depth of the whole distribution. Values of DU
less than 60% are generally considered low and a value of DU greater than 75% is
considered satisfactory. DU can be calculated using Eq. (2), where DU = distribution
uniformity (%); m 25 = mean of the lowest 25% of rain gage readings (mm); and m =
mean for all the rain gages (mm).