Irrigation Audit Calculator
Calculate Distribution Uniformity (DU) and Christiansen's CU from catch can test data to evaluate irrigation system performance.
About this calculator
A sprinkler or pivot system rarely applies water evenly across a field, and this calculator turns a catch-can field test into the two standard industry metrics for how bad that unevenness is. Distribution Uniformity (low quarter) is simply the average catch of the driest 25% of cans divided by the overall average catch — the classic measure of how the worst-served part of the field is doing relative to the average. Christiansen's Uniformity Coefficient, first defined by J.E. Christiansen in his 1942 University of California study *Irrigation by Sprinkling*, takes a different angle, estimating standard deviation from the range of readings (max minus min, divided by 4, a normal-distribution approximation) and computing CU = 1 − (std dev / mean) × 0.7979, where 0.7979 is the mean-absolute-deviation-to-standard-deviation ratio for a normal distribution.
From DU the calculator derives a scheduling coefficient (1/DU) — the multiplier you'd apply to target irrigation depth so even the driest 25% of the field gets adequate water — and reports the over-irrigation percentage and gallons-per-hour wasted that this compensation implies, plus potential savings if uniformity were improved to a 90% DU benchmark. The catch depth in mL is also converted to inches per hour using can diameter and test duration, giving you an actual application-rate reading, not just a relative uniformity score. One real limitation: CU here is estimated from just min/max/mean rather than computed directly from all individual catch-can readings (the textbook Christiansen formula uses every reading's deviation from the mean), so it's an approximation that works reasonably for roughly normal distributions but can miss skewed or bimodal patterns a full can-by-can CU calculation would catch.
Inputs
Results
Distribution Uniformity (%)
80
Christiansen CU (%)
83.6
Figures current as of 1942. Source: J.E. Christiansen, Irrigation by Sprinkling, University of California, Berkeley, 1942 (Christiansen's Uniformity Coefficient); low-quarter Distribution Uniformity is the companion USDA-originated metric — both are standard sprinkler/pivot uniformity measures computed from catch-can field test data.
How to Use This Calculator
- Enter Number of catch cans, Mean catch volume (mL), and Low-quarter average (mL).
- Set Minimum reading (mL), Maximum reading (mL), and Can diameter (in).
- Adjust Test duration (min), System flow rate (GPM) as needed.
- Review Distribution Uniformity (%) and Christiansen CU (%).
- Use Coefficient of Variation (%) and Application rate (in/hr) to inform your decision.
How the result changes with Mean catch volume (mL)
| Mean catch volume (mL) | Distribution Uniformity (%) | Christiansen CU (%) |
|---|---|---|
| 43 | 158.1 | 67.5 |
| 64 | 106.3 | 78.2 |
| 128 | 53.1 | 89.1 |
| 213 | 31.9 | 93.4 |
What each input means
- Number of catch cans
- Total catch cans placed in the test grid.
- Mean catch volume (mL)
- Average volume collected across all catch cans. Feeds the Distribution Uniformity quality rating below, so this value is not unit-converted to keep the 90/80/70% rating cutoffs stable.
- Low-quarter average (mL)
- Average volume of the lowest 25% of catch can readings. Feeds the Distribution Uniformity quality rating below, so this value is not unit-converted to keep the 90/80/70% rating cutoffs stable.
- Minimum reading (mL)
- Lowest single catch can volume.
- Maximum reading (mL)
- Highest single catch can volume.
- Can diameter (in)
- Inside diameter of the catch cans used.
- Test duration (min)
- How long the system ran during the test.
- System flow rate (GPM)
- Total system flow rate for waste calculations.
- Field size (acres)
- Field area being evaluated.
What each result means
- Distribution Uniformity (%)
- DU low quarter — industry standard uniformity measure. >85% is good.
- Christiansen CU (%)
- Christiansen's Uniformity Coefficient. >80% is acceptable.
- Coefficient of Variation (%)
- Statistical variability in application. <15% is good.
- Application rate (in/hr)
- Average application depth per hour.
- Scheduling coefficient
- Multiply target depth by this to ensure low areas receive enough water.
- Rating (1=Excellent, 4=Poor)
- 1=Excellent (>90%), 2=Good (80-90%), 3=Fair (70-80%), 4=Poor (<70%).
- Over-irrigation factor (%)
- Extra water applied to compensate for non-uniformity.
- Savings if improved to 90% DU (%)
- Water savings potential from improving uniformity to 90%.
- Water wasted (gal/hr)
- Gallons wasted per hour due to over-application for uniformity.
- Mean catch depth (in)
- Average catch depth converted to inches.
- Max/Min ratio
- Ratio of highest to lowest catch — ideally close to 1.0.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersNumber of catch cans = 16, Mean catch volume (mL) = 85, Low-quarter average (mL) = 68, Minimum reading (mL) = 45 = 9 input(s) provided
- Calculate Distribution UniformityDistribution Uniformity = du * 10080 = 80
- Calculate Christiansen CUChristiansen CU83.6 = 83.6
- Calculate Coefficient of Variation20.6 = 20.6
- Calculate Application rate1.078 = 1.078
Figures and sources
- Distribution Uniformity (low quarter) and Christiansen's Uniformity Coefficient (1942) — J.E. Christiansen, Irrigation by Sprinkling, University of California, Berkeley, 1942 (Christiansen's Uniformity Coefficient); low-quarter Distribution Uniformity is the companion USDA-originated metric — both are standard sprinkler/pivot uniformity measures computed from catch-can field test data.
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between Distribution Uniformity and Christiansen's CU?
Distribution Uniformity (low quarter) focuses only on the worst-served part of the field — it's the average catch of the driest 25% of cans divided by the overall average, which is why it's the more conservative, "what's my weak spot" metric. Christiansen's CU instead reflects the spread across all readings, estimated here from the range between the min and max catch (divided by 4, a normal-distribution approximation) relative to the mean. Two systems can have the same CU but different DU if their variability is distributed differently.
Why does a scheduling coefficient of 1.25 mean applying 25% extra water?
The scheduling coefficient is simply 1 divided by DU — if the driest 25% of the field is only getting 80% of the average catch (DU = 0.80), you'd need to multiply your target application depth by 1.25 to guarantee those dry spots still get enough water. That extra 25% overshoots the rest of the field, which is exactly the over-irrigation the calculator reports as wasted water and lost efficiency.
How does the calculator turn catch-can mL readings into an application rate in inches per hour?
It converts the mean catch volume to a depth in inches using the can's cross-sectional area (volume ÷ area, after converting mL to cubic inches), then divides that depth by the test duration and multiplies by 60 to annualize it to an hourly rate. So a wider can or a longer test duration will change the calculated application rate even if the same volume was collected, because the formula is depth-based, not volume-based.
How accurate is the Christiansen CU estimate compared to a full catch-can calculation?
The textbook CU formula sums each individual can's absolute deviation from the mean across every reading, but this calculator only has summary statistics (mean, min, max) rather than all the raw catch-can values, so it estimates standard deviation from the range instead. That works reasonably well when readings are roughly normally distributed, but it can understate or overstate real variability for skewed or bimodal catch patterns that a full can-by-can CU calculation would catch.
Do Number of catch cans and Field size affect the results?
No. Every DU, CU, application-rate, and water-savings figure is computed from the mean, low-quarter, min, and max catch volumes plus can diameter, test duration, and system flow rate. Number of catch cans is recorded for your own reference and Field size isn't used in any formula here, so changing either one on its own won't move any output.
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