THE CATCH-CAN GUIDE
A measured rate,
then a modelled time.
A catch-can test records the water collected at chosen positions during a timed sprinkler run. Seedcount uses those depths to calculate a simple average rate. You supply the target separately.
Use depth readings from one timed test
Enter the actual test duration in minutes. All cans must describe the same duration and sprinkler zone. Use readings that are already in millimetres of collected depth. A reading in millilitres is a volume and cannot be entered as millimetres without accounting for the collection opening.
For a simple unweighted average, use comparable collection openings and positions that represent the area you are testing. A straight-sided container can show depth directly when its water surface and opening have the same area. A tapered vessel needs a suitable calibrated depth scale or a volume-to-opening-area conversion.
The tool accepts one to 30 readings. One reading gives a rate for that sampled position only. More positions can reveal differences across the zone, but the calculator does not design a sampling layout or certify irrigation performance. Use the separate readings, minimum and maximum as context for the average.
The arithmetic
Average depth (mm)
Sum of all entered depths ÷ number of readings
Application rate (mm/hour)
Average depth × 60 ÷ test duration in minutes
Modelled runtime (minutes)
Your chosen target depth × test duration ÷ average depth
Depth divided by elapsed time gives a rate. Multiplying by 60 expresses a per-minute rate per hour. The runtime formula scales the original test by the ratio between your chosen depth and its measured average. It keeps the same average application rate.
Target depth means the modelled mean depth collected at the sampled positions. It is not a statement of plant need, soil moisture, watering frequency or an amount every position will receive. Choose your target using information relevant to your own situation.
A worked example
These are demonstration inputs, not a watering recommendation.
A 15-minute test gives four can readings: 6, 8, 10 and 8 mm. Their sum is 32 mm, so the average is 32 ÷ 4 = 8 mm. The measured hourly rate is 8 × 60 ÷ 15 = 32 mm/hour.
For a user-chosen target of 12 mm, the modelled time is 12 × 15 ÷ 8 = 22.5 minutes, or 22 minutes 30 seconds. The minimum and maximum test readings remain 6 and 10 mm; the mean does not erase that variation.
Zero, blank and an all-zero test
An entered zero is included in the average. For example, 0 and 8 mm average to 4 mm. Leaving a field blank is different: the calculator asks you to enter a value or remove that can. Negative readings are rejected.
If every depth is zero, there is no positive application rate to scale. The tool stops instead of dividing by zero or displaying an infinite runtime. Check the test and the measurements before calculating again. If the average is positive and your chosen target is zero, the result is zero minutes.
What the result can describe
- One rate from one test. The projection assumes the same sprinkler operation, pressure and conditions continue. A later run can differ.
- Collected water, not absorbed water. The calculation does not measure soil absorption, root-zone moisture, evaporation, runoff, rain or other losses.
- An average, not uniform coverage. Equal weighting does not correct for an uneven sampling layout. This tool does not calculate distribution uniformity or adjust runtime to give the driest position a target.
- Timing, not a schedule. The result does not choose a depth or number of runs. Check the relevant local requirements before operating irrigation.
Precision and the saved plan
Input values can have up to six decimal places, with a maximum of 1,000,000 for each duration or depth. These are arithmetic limits, not suitable test values. The calculation keeps the entered decimals through averaging and scaling before rounding results for display.
On screen, results use up to four decimal places. A very small positive result can therefore display as 0; the downloaded and printed summary includes up to eight places for calculated values and up to six for inputs. The minutes-and-seconds line rounds to the nearest second and is a convenience display, not an automatic controller setting.
Changing an input marks the displayed plan as out of date and disables export until you calculate again. The download is a text file created on your device. Closing or refreshing the page clears the calculator inputs; no project is saved online.
Method reference
The NSW Department of Primary Industries WaterWise catch-can evaluation worksheet (PDF, “Calculating MAR”, page 13) describes averaging collected depths and converting a timed test to a mean hourly application rate. Its broader farm-system evaluation includes checks beyond this small calculator. Seedcount takes depth readings already in millimetres and does not reproduce that full assessment.
For a calculator correction, contact seedcount.snugly156@aleeas.com. Include the inputs and expected arithmetic if helpful. See the site's privacy information for hosting, site services, downloads and email handling.