Calculator inputs
Example data table
Use this sample to confirm your setup. Paste the readings into the calculator and compare outputs.
| Reading set | Catch-can depths (mm) | Expected DUlq (approx.) |
|---|---|---|
| Sample A | 13.2, 12.9, 13.4, 11.7, 12.1, 13.0, 12.6, 14.0, 11.9, 12.8, 13.1, 12.0 | ~91% |
| Sample B | 10.0, 9.4, 9.8, 6.5, 7.1, 9.2, 8.9, 11.0, 6.9, 8.6, 9.5, 7.0 | ~77% |
| Sample C | 8.0, 8.1, 7.9, 8.2, 8.0, 8.1, 8.0, 8.2 | ~99% |
Formula used
Low-quarter distribution uniformity (DUlq) measures how well the driest quarter of the area is watered compared with the overall average.
- Mean = average of all catch-can depths.
- Lowest-quarter average = average of the lowest 25% of readings.
- DUlq (%) = (Lowest-quarter average ÷ Mean) × 100.
- CU (%) = 100 × (1 − Σ|xi − mean| ÷ (n × mean)).
- CV (%) = (Sample standard deviation ÷ Mean) × 100.
How to use this calculator
- Place identical catch-cans evenly across the watered area.
- Run the sprinkler zone for a fixed time (e.g., 10–20 minutes).
- Measure water depth in each can and enter the readings.
- Press Calculate uniformity to view DUlq, CU, and CV.
- Download CSV or PDF to keep records and compare adjustments.
- Improve low DU by correcting spacing, pressure, tilt, or nozzle selection.
What DU shows
Distribution uniformity (DU) summarizes how evenly irrigation water lands across a bed, lawn, or container area. Higher DU means fewer dry spots, steadier plant growth, and less disease pressure from soggy patches. Tests often show that the “average” depth looks fine while low-quarter zones receive 20–40% less, forcing longer runtimes and increasing runoff and nutrient leaching. For seedlings, small dry zones can stunt growth early.
Interpreting DUlq ranges
For overhead sprinklers, DUlq above 80% is typically excellent, 70–80% is good, 60–70% is fair, and below 60% signals major improvement potential. Drip and micro-sprays can exceed 90% when pressure is stable. Use CU and CV alongside DUlq to spot random variability versus systematic pattern errors, especially on windy or sloped sites. Record pressure at the valve and farthest head.
Catch-can test setup
Use identical catch-cans and space them uniformly across the irrigated footprint. Keep the runtime constant, record depths to the nearest millimeter, and avoid windy conditions. Level each can, label positions, and note head type and spacing. A minimum of 12–24 cans improves confidence; repeating the test twice and averaging readings reduces one-off splashes or measurement noise. For drip, sample emitters at start, middle, and end.
Common causes of low uniformity
Low DU usually comes from pressure variation, clogged nozzles, mixed nozzle types, incorrect head spacing, tilted risers, or blocked spray arcs from plants or hardscape. Even small leaks can drop pressure on the last heads. For drip systems, emitter plugging, undersized laterals, and elevation changes create unequal discharge; flushing lines and cleaning filters often raises DU quickly. Replace worn nozzles annually to restore matched output.
Using results to tune irrigation
After calculating DU, adjust one variable at a time and retest. Match nozzles to the same precipitation rate, correct spacing to head-to-head coverage, and regulate pressure at the zone. Use the lowest-quarter depth to set runtime: target depth ÷ (low-quarter rate). When DU improves, you can usually shorten watering while still meeting the driest area’s requirement.
FAQs
How many catch-cans should I use for a reliable DU test?
Use at least 12 cans for small zones, and 24 or more for larger areas. More cans improve confidence, especially near edges and overlaps. Keep spacing even and run the same duration each test.
Can I enter readings in millimeters or inches?
Yes. DU, CU, and CV are ratios, so units cancel out. Just use one unit consistently within a test. If you mix units, results will be wrong.
Why does the calculator show both DUlq and CU?
DUlq focuses on the driest quarter, which drives irrigation scheduling. CU measures overall uniformity and penalizes absolute deviations from the mean. Together they help distinguish pattern issues from random scatter.
What should I do if a few readings are zero?
Zero values usually indicate blocked arcs, severe pressure loss, or missed coverage. Keep them in the dataset to reflect real performance. Then fix the cause and retest; DU should increase noticeably.
How often should I retest distribution uniformity?
Retest at the start of each growing season, after nozzle or emitter changes, and whenever plant growth blocks spray. For drip, retest after filter service or flushing if clogging is common.
What DU target is practical for home gardens?
For sprays and rotors, aim for 70–80% DUlq after tuning. Drip and micro can often reach 85–95% with clean filters and stable pressure. If you cannot reach targets, consider redesigning spacing or zoning.
Tip: Repeat tests on calm days and after any hardware changes.