How is Distribution Uniformity (DU) calculated from a catch-can test?

• A. DU = (maximum depth) / (minimum depth)
• B. DU = (total water applied) / (area irrigated)
• C. DU = (average of lowest quarter of catch can depths) / (overall average depth), expressed as a percentage.
• D. DU = (average depth in top half) / (average depth in bottom half)

Answer: (C)

Distribution Uniformity measures how evenly water is applied across a sprinkler zone, and it’s estimated from a catch-can test. In that test, you place several catch cans in the area, run the irrigation, and measure how much water ends up in each can.

The correct method for DU uses the average depth of the lowest 25% of catch-can readings and divides it by the overall average depth of all cans, then expresses the result as a percentage. This works because when distribution is uniform, the bottom quarter of readings is close to the overall average, yielding a high percentage. If some spots receive much more water, the lowest-quarter depths fall well below the average, and DU drops.

For example, if the overall average depth is 0.70 inches and the average depth of the lowest 25% of cans is 0.68 inches, DU ≈ 0.68 / 0.70 ≈ 97%.

Other formulas don’t reflect how evenly water is spread across the area: max-to-min depth, total water over area, or top-half versus bottom-half comparisons can be influenced by outliers or ignore distribution patterns entirely, so they aren’t used to assess distribution uniformity.