Paste measurements from catch cans or emitter flow checks. Use consistent spacing and run the test under steady pressure.
A 16-point catch-can test (units in mm). Lowest quarter is the four smallest values.
| # | Value | # | Value | # | Value | # | Value |
|---|---|---|---|---|---|---|---|
| 1 | 12.0 | 2 | 11.5 | 3 | 10.8 | 4 | 9.2 |
| 5 | 12.3 | 6 | 11.9 | 7 | 10.1 | 8 | 8.7 |
| 9 | 12.1 | 10 | 11.7 | 11 | 10.5 | 12 | 9.0 |
| 13 | 12.2 | 14 | 11.6 | 15 | 10.3 | 16 | 8.5 |
With this dataset, DU (low quarter) is about 82.13% using the default settings.
Low-quarter distribution uniformity (DU) compares the driest quarter of observations to the overall average. It is commonly used to judge how evenly an irrigation zone applies water.
- Average (All) = mean of all valid measurements.
- Average (Low Quarter) = mean of the lowest 25% of measurements.
- DULQ (%) = (Average Low Quarter divided by Average All) times 100.
- CV (%) = (Standard Deviation divided by Average All) times 100.
- CU (%) = 100 times [1 - Sum |x - mean| divided by (n times mean)].
- Run your irrigation zone at normal operating pressure.
- Collect uniformity measurements: catch-can depths or emitter flows.
- Paste the values into the Measurements box (any separator works).
- Optionally trim outliers and exclude zeros if data is invalid.
- Press Calculate DU to see results above the form.
- Use exports to track improvements after maintenance or adjustments.
Higher DU means more consistent watering and fewer dry spots. If DU is low, check nozzle wear, clogged filters, pressure differences, spacing, wind effects, and run time.
Why low quarter DU matters for gardens
Low quarter distribution uniformity (DU) highlights how the driest areas perform compared to the zone average. A high DU means fewer dry spots, better plant consistency, and lower risk of overwatering the rest of the area. In practice, DU helps you decide whether to fix hardware, adjust pressure, or change run time.
Recommended sampling and test setup
Use at least 16 measurements for small zones, and 24 to 40 for larger zones or windy sites. Keep spacing even, run the zone long enough for measurable catch depths, and avoid changing pressure during the test. For drip, measure emitter flow at multiple laterals and include end-of-line points.
Interpreting DU bands with actions
Typical DU bands are: 90%+ excellent, 80–89% good, 70–79% fair, and below 70% poor. If DU is fair or poor, inspect clogged filters, stuck check valves, nozzle wear, mismatched nozzles, and pressure loss from elevation or undersized pipe. Retest after changes and compare exports.
Using CV and CU to confirm variability
DU focuses on the driest quarter, while CV and Christiansen CU describe overall spread. A rising CV often signals pressure imbalance, partial clogs, or mixed sprinkler arcs. CU complements DU for sprinklers, while DU remains the stronger scheduling indicator because it protects the lowest-application areas.
Example data and what it tells you
The set below resembles a 16-can lawn test. The lowest four values average 8.85 mm and the zone average is 10.78 mm, giving DU = 82.13%. This is a “Good” result: minor tuning may help, but large repairs are usually unnecessary.
| Test point | Depth (mm) | Test point | Depth (mm) | Test point | Depth (mm) | Test point | Depth (mm) |
|---|---|---|---|---|---|---|---|
| 1 | 12.0 | 2 | 11.5 | 3 | 10.8 | 4 | 9.2 |
| 5 | 12.3 | 6 | 11.9 | 7 | 10.1 | 8 | 8.7 |
| 9 | 12.1 | 10 | 11.7 | 11 | 10.5 | 12 | 9.0 |
| 13 | 12.2 | 14 | 11.6 | 15 | 10.3 | 16 | 8.5 |
1) What does low quarter DU measure?
It compares the average of the lowest 25% of measurements to the overall average. This highlights how the driest areas perform and supports safer irrigation scheduling.
2) How many measurements should I collect?
Use at least 16 points for small zones. For larger or irregular zones, 24–40 points improves confidence and reduces the effect of one unusual measurement.
3) Can I use this for drip irrigation?
Yes. Enter emitter flows from multiple laterals and include end-of-line checks. DU still indicates how uniformly water is delivered across the zone.
4) Should I exclude zeros?
Exclude zeros if they come from a knocked-over can, blocked emitter, or measurement error. Keep zeros if they represent a real “no water” condition you must fix.
5) Why trim outliers?
Trimming removes extreme highs and lows when you suspect bad data. It helps stabilize DU, but do not use it to hide persistent problems like chronic low pressure areas.
6) What DU is considered good for gardens?
Many well-maintained zones can reach 80–90% DU. Below 70% usually indicates hardware or pressure issues that should be corrected before increasing runtime.
7) How often should I retest DU?
Retest after nozzle replacement, filter cleaning, pressure regulation changes, or major landscaping. Seasonal retesting is helpful if wind, clogging, or wear is common.