How this calculation works
Convert equal-area volume catches to depth when needed. Sort depths to select the lowest quarter, compare its mean with the overall mean and separately summarize absolute deviations.
DU (%) = lower-quarter mean / overall mean × 100; CU (%) = (1 − sum absolute deviations / (count × mean)) × 100A worked example
Catches of 8, 10, 10 and 12 mm average 10 mm. The lowest-quarter mean is 8 mm, so DU is 80%; CU is 90%. A 30-minute test gives a mean rate of 20 mm/hour.
Good to know
Instructional comparison, not an audit pass/fail or watering recommendation. Use representative sampling; real audits generally need more than four cans. No wind, evaporation or soil correction.
Reference: University of Minnesota Extension: auditing irrigation systems (opens in a new tab) ↗
Use equal collection weights
Every entered can has equal weight in the averages. Volume mode assumes the same opening area for all collectors, because one millilitre over different opening sizes represents different water depths.
Millilitres are cubic centimetres. Dividing by opening area gives centimetres of depth; multiplying by ten gives millimetres. Do not enter a diameter where the calculator asks for area.
The sprayer water catch calibration calculator adds convert equal-time water catches into nozzle flows, broadcast L/ha, population variation and individual deviations without calculating chemical dose. The drip irrigation emitter runtime calculator adds convert entered net water depth to gross litres and emitter runtime, split equal cycles and compare active time with an operating window.
The lower quarter is an explicit subset
The calculator requires a multiple of four catches so the lowest quarter contains a whole number of collectors. Sort all depths, including zeros, and average that lower group before dividing by the overall mean.
A zero catch is data rather than a missing value. Removing it would inflate uniformity. An all-zero set has no positive mean, so the ratio is undefined and the input is rejected.
Two coefficients describe different features
DU focuses on the lowest quarter. Christiansen’s coefficient uses absolute deviations across every catch; it can become negative for extreme distributions, and this page does not clip that mathematical result.
The mean application rate scales the observed depth by test duration. A target-runtime result refers to the mean only and does not promise that the driest spot reaches that target or that soil receives the collected water.
Use the result with the next measurement
The sprayer water catch calibration calculator examines nozzle outlet flow and coverage arithmetic. This catch-can comparison instead concerns water arriving at sampled positions, so its uniformity values answer a different question.
Instructional comparison, not an audit pass/fail or watering recommendation. Use representative sampling; real audits generally need more than four cans. No wind, evaporation or soil correction.
A few questions, answered
Are zero catches removed?
No, all entered readings are retained.
Why must the count be a multiple of four?
It makes the equally weighted lower quarter unambiguous.
Is a four-can example a field audit?
No, it illustrates arithmetic; representative audits need appropriate sampling.