What does distribution uniformity (DU) measure in a catch-can test?

• A. The pressure in the irrigation mainline.
• B. The total water applied to the zone.
• C. The uniformity of water distribution across the zone, comparing the lowest quarter to the average.
• D. The rate of evaporation from the soil.

Answer: (C)

Distribution uniformity is about how evenly water is applied across the irrigation zone. In a catch-can test, you measure how much water each can collects and then look at the average depth in the lowest 25% of cans and compare it to the overall average depth. The ratio of the lowest-quarter average to the overall average (expressed as a percentage) shows how uniform the distribution is: higher values mean more even coverage, lower values indicate uneven application. This concept is separate from pressure, total water applied, or evaporation, which are different aspects of irrigation performance.