Guide · 7 min read
How to run a catch-can test
A catch-can test is the field measurement behind every irrigation audit number. You place open containers across a zone, run the zone for a timed interval, and measure how much water landed in each container. From those readings come the three figures a schedule depends on: the precipitation rate, the low-quarter distribution uniformity (DU), and the Christiansen coefficient of uniformity (CU).
The test is simple, but the details decide whether the result means anything. Too few cans, a runtime too short to fill them measurably, or cans placed only in the healthy-looking middle of the zone will all produce numbers that look precise and are not.
What you need
You need a set of identical catch devices, a way to read them, and a timer. Purpose-made catch cans with a printed scale read directly in inches or millimetres. Straight-sided containers such as tuna tins work too if you measure depth with a ruler. Graduated cylinders paired with any container work as well, as long as you know the container's throat area, since depth equals collected volume divided by throat area.
Identical devices matter more than fancy devices. Mixing container shapes inside one test changes the effective collection area from can to can and corrupts the uniformity statistics.
- 8 or more identical catch devices per zone; 16 to 24 gives a steadier low-quarter figure
- A timer for the zone runtime
- A notebook or the Irrigaudit workspace to record each reading in order
- For mL readings: a graduated cylinder and the throat diameter of your containers
Placing the cans
Set cans in a regular grid across the whole irrigated area, not just where coverage looks good. Common practice places cans roughly a half spacing from heads and from edges, then fills in the middle of the zone at even intervals. Include the areas that prompted the audit: dry corners, slopes, and spots shaded by shrubs.
Keep cans level and clear of grass blades that would deflect spray into or away from the opening. On slopes, push a small terrace flat for each can rather than letting it lean.
Running the test
Run the zone long enough for the average can to collect a comfortably measurable depth. For fixed sprays that is commonly 10 to 15 minutes; for rotors and rotary nozzles, which apply water more slowly, 20 to 30 minutes or one or more full rotation cycles. Record the exact runtime; the precipitation rate divides by it directly.
Read every can, including the empty ones. A zero is data: it says part of the zone received nothing during the test, and dropping it silently inflates both uniformity figures.
Turning readings into results
The precipitation rate is the average catch depth divided by the runtime, scaled to inches per hour. The low-quarter DU is the average of the lowest quarter of readings divided by the overall average. Christiansen CU is based on the average absolute deviation from the mean. The Irrigaudit workspace computes all three as you type readings in, flags the low-quarter cans, and grades the DU against ranges commonly published for the head type.
With fewer than four cans there is no meaningful low quarter, so treat any DU from a tiny test as an anecdote rather than a measurement. The workspace says so explicitly when the sample is that small.
Frequently asked questions
How many catch cans do I need for a valid test?
Use at least 8 identical cans per zone; 16 to 24 gives a steadier low-quarter figure. Below 4 cans there is no meaningful low quarter at all, because DU averages the lowest quarter of readings and a quarter of 3 readings is a single can.
How long should the zone run during a catch-can test?
Long enough for the average can to hold a measurable depth: commonly 10 to 15 minutes for fixed sprays and 20 to 30 minutes for rotors and rotary nozzles. Record the exact runtime, because the precipitation rate is the average catch divided by that time.
Do I count catch cans that stayed empty?
Yes. An empty can means part of the zone received no water during the test. Dropping zeros inflates both DU and CU and hides exactly the problem the audit exists to find.
Can I use kitchen containers instead of purpose-made catch cans?
Yes, if every container in the test is identical and straight-sided. Read the depth with a ruler, or measure the collected volume with a graduated cylinder and divide by the container's throat area to get depth.