Longer sprinkler operation increases the average water applied, but it does not automatically make uneven coverage uniform. A set of equal catch cans helps separate those two questions: how fast water is arriving and how consistently it reaches the measured area.
Enter the depths in Sprinkler Catch Can Calculator, along with the test duration and desired average depth. Each row records one can. The result includes average depth, application rate, an estimated runtime for the target average and a low quarter uniformity measure.
Four readings from a twenty minute test
Suppose four cans contain 3, 4, 5 and 4 mm after 20 minutes. Their average is 4 mm. At that rate, average application is 12 mm per hour. Reaching a 10 mm average depth would therefore take 50 minutes under the same constant application conditions.
The lowest quarter in this four can example is the single 3 mm reading. Dividing that by the 4 mm overall average gives 75% low quarter distribution uniformity. A 10 mm average target does not mean every location will receive 10 mm.
The low quarter rule is explicit
For an arbitrary number of readings, this worksheet takes the lowest ceil(number of cans divided by four) measurements. It averages those and divides by the overall mean. That rounding convention matters when the count is not divisible by four.
Use equal cans and comparable depth readings. Mixing millimetres of depth with millilitres from containers of different opening sizes makes the input rows inconsistent. Record where the cans were placed separately so a later test can examine the same coverage rather than an unrelated pattern.
If all entered catches are zero, the tool reports zero average depth and no finite runtime for a positive target. A missing or empty catch should not be changed into a made up positive reading just to obtain a time result.
Review coverage before chasing the average
The tool does not diagnose pressure, nozzle problems, wind or the placement responsible for uneven readings. It also does not predict runoff or how much water enters the root zone. Those need observation of the actual test and garden.
A rate measured under one set of operating conditions should not be reused automatically when heads, pressure or the operating zone change. Repeat the observations and preserve the test duration with the depths when saving the worksheet.
For a broader crop water estimate, Crop Water Budget Calculator works with entered ETo, crop coefficient and effective rain. Its net root zone requirement is not identical to an average catch can depth, so keep the application and storage assumptions visible when moving between those calculations.