Catch-Can Test & Measure Sprinkler Uniformity

CU is the classic measure of how evenly a sprinkler system applies water. It is calculated as CU = 100 × (1 − Σ|xᵢ − mean| ÷ Σxᵢ), where xᵢ are the depths caught in each can. A CU of 100% means every can caught exactly the same depth; lower values mean more variation. The standard interpretation is 90%+ excellent, 84–90% good, 75–84% fair, and below 75% poor.

DU, the low-quarter distribution uniformity, focuses on the driest part of the field: DU = 100 × (mean of the lowest 25% of cans ÷ overall mean). It is the figure that drives scheduling, because the dry quarter is what suffers first. DU is always a little lower than CU for the same data and is the more conservative, decision-relevant number. Bands are 84%+ excellent, 75–84% good, 65–75% fair, below 65% poor.

Use identical containers on an even grid across the wetted area — a minimum of about 16 to 24 cans gives a stable result, more for a pivot. Space them evenly (for example every 3 metres on a solid-set, or along a radial line for a pivot) and run the system for a fixed, timed period. Then type each can's caught depth into the grid above; the tool sizes the grid to match.

The scheduling coefficient is mean depth ÷ low-quarter mean depth. It is always 1 or greater and tells you how much longer you must run the system so the driest quarter still receives the target depth. A scheduling coefficient of 1.3, for example, means you must run 30% longer — and the rest of the field is over-watered by that amount, which is wasted water and a runoff risk.

It depends on the system. A well-set-up center pivot with low-pressure spray often reaches CU 92% / DU 88%; impact pivots run a little lower; solid-set sprinklers around CU 84% / DU 75%; and a travelling gun is inherently less uniform at roughly CU 75% / DU 65%. The tool shows the benchmark your result sits closest to so you can judge whether the system is performing as it should.

Because a sprinkler system can only be as good as its driest quarter. If you irrigate to make sure the dry spots get enough, every other part of the field gets too much — that surplus drains away or runs off, carrying nutrients with it. Improving uniformity lets you cut total run-time while still satisfying the dry quarter, which is the cheapest water you will ever save.

The usual culprits are worn or mismatched nozzles, incorrect or failed pressure regulators, wrong sprinkler spacing, wind during the test, leaning risers, and clogged screens. A pattern in the grid is a clue: a consistently dry corner points to pressure loss at the end of a line, while scattered dry cans suggest nozzle wear or wind. Fix the cause before you simply run the system longer.

Yes — wind is the single biggest source of error in a sprinkler catch-can test. It pushes droplets downwind, creating apparent dry and wet zones that are really a weather artefact. Run the test in calm conditions (ideally under about 1.5 m/s wind) and repeat on a still day if results look unusually poor. The formulas themselves are unaffected; only the depths you feed them matter.

Yes. The CU and DU formulas are method-agnostic — they work on any set of caught depths. For a pivot, lay cans along a radial line out from the pivot point and enter those depths; for solid-set or hand-move, use a grid between sprinklers. The interpretation bands and the system benchmark list cover pivots, solid-set, travelling guns and micro-sprinklers alike.

Any consistent unit. CU and DU are ratios, so millimetres, millilitres caught, or inches all give the same percentages as long as every can uses the same unit. The mean and low-quarter figures will be in whatever unit you typed. For the run-time correction, enter the target application depth in the same unit you used for the cans.

Always. A catch-can test is cheap and fast, so re-run it after any nozzle change, pressure adjustment or regulator replacement to confirm the fix worked. Uniformity drifts over a season as nozzles wear, so an annual test — or one each time performance seems off — is good practice and quickly pays for itself in water saved.

No. The entire calculation runs locally in your browser using the Christiansen and low-quarter formulas. Nothing you type is sent to a server or stored, so your field data stays on your device.