What a catch cup audit is for
Set out containers across a zone, run it for a known time, and measure what landed in each one. That is the whole test. It takes half an hour and it replaces every assumption in an irrigation schedule with a measurement, which matters because the assumptions are usually wrong in the same direction: nozzle charts describe new nozzles at rated pressure on a windless day, and none of those conditions apply to a system that has been in the ground for six years.
The audit answers two separate questions. The first is how fast the zone applies water, which sets run time. The second, and the more valuable, is how evenly it applies it, which decides whether any run time can be correct at all. A zone can have a perfect average and still be unwaterable if half of it gets twice what the other half gets.
Use identical containers with straight sides so that depth is depth. Purpose-made catch cups are cheapest bought as a set, but tuna cans, straight-sided food tins and cut-off bottles all work if they match each other. If the sides taper, the depth reading lies, so measure the volume instead and let this page convert it using the throat diameter — that conversion is exact regardless of what the container does below the rim.
Distribution uniformity and why it uses the low quarter
Distribution uniformity is the average of the driest quarter of the readings divided by the average of all of them. A perfectly even zone scores 1.00, or 100 percent. A zone where the driest quarter gets 64 percent of the average scores 0.64.
The low quarter is deliberate. You could measure unevenness with a standard deviation, or with the Christiansen coefficient, which averages every reading's distance from the mean. Both are legitimate and both are gentler than DU, which is why the CU figure on this page always comes out higher than the DU figure for the same data. But neither answers the question an irrigator actually has, which is how long to run the valve. That is decided by the ground that gets the least, because that is the ground that goes brown and generates the complaint. The low quarter is a robust stand-in for "the dry part" that does not swing wildly on one unlucky cup the way the single minimum would.
The practical consequence is the scheduling multiplier: one divided by DU. At DU 0.64 the multiplier is 1.56, so bringing the driest quarter to half an inch means running the zone long enough to put 0.78 inches on the average ground. Fifty-six percent more water, every watering, for as long as the system stays as it is. Over a season that surcharge is usually larger than the cost of fixing whatever caused it.
| Low quarter DU | Multiplier | What it usually indicates |
|---|---|---|
| 0.80 and above | 1.25x or less | Well designed and well maintained, or a drip zone |
| 0.70 to 0.80 | 1.25 to 1.43x | A sound rotor layout in reasonable condition |
| 0.55 to 0.70 | 1.43 to 1.82x | Common on older residential systems, usually fixable |
| Under 0.55 | Over 1.82x | A layout or pressure fault, not a scheduling problem |
Those bands are orientation for reading your own result, not a standard anyone is graded against. Spray zones and rotor zones behave differently, wind flatters nobody, and a zone on a slope will read worse than the same hardware on the flat.
Running the test properly
Wind is the enemy of a repeatable audit. Test at dawn or in the evening when the air is still, and if it is genuinely blowing, wait for another day rather than recording a result you will not trust. Run long enough to get a readable depth in the driest cup — fifteen minutes suits most spray zones, thirty or more may be needed on rotors that apply slowly, and you can always run longer and divide.
Place cups on a grid across the zone rather than where you already suspect trouble. Deliberately sampling the bad patch produces a bad number that tells you nothing you did not know. Include the corners, include the strip along the boundary, and include at least one cup close to a head and one midway between heads, because those two positions are where the overlap logic either works or does not. Sit the cups on the ground, not on anything raised, and keep them clear of anything that would shelter them.
Six cups is the smallest sample that produces a meaningful low quarter, and it is thin. Twelve to twenty-four is where the numbers settle down. Record which cup was where as you empty them, because the pattern of low readings is what points at the cause.
Reading the pattern, not just the number
Once you have the DU, look at where the low readings sat. The location tells you the fault.
Low readings midway between heads, high readings near heads, across the whole zone: spacing is too wide for the throw radius. That is a layout problem, and the head spacing calculator shows how much ground a stretched grid leaves under a single pattern. Low readings clustered at one end of the zone: pressure is falling along the lateral, usually from undersized pipe or too many heads on the valve. Low readings in one wedge around one head: a blocked or worn nozzle, a misadjusted arc, or a head sunk below grade so the turf blocks the stream. High readings along the perimeter with a starved middle: half-circle nozzles flowing the same as the full-circle nozzles rather than half as much.
Uniformly mediocre readings everywhere, with no pattern at all, most often mean the operating pressure is wrong for the nozzles — too low and the streams break up short, too high and they atomise into mist that drifts. Both look like bad coverage and neither is fixed by moving anything.
What the audit cannot do
It measures water arriving at the surface. It says nothing about what happens next. A zone with excellent uniformity on soil that seals under the applied rate still loses water downhill, and that is a job for the precipitation rate calculator and cycle-and-soak scheduling. It says nothing about root depth or how often to water, which the watering depth and interval calculator covers, and nothing about how the season changes demand, which is the seasonal watering adjustment calculator.
It also has nothing to say about drip zones, where cups have nothing to catch. Drip uniformity is checked by measuring emitter output into a container at several points along the line, near the valve and at the far end, which the drip emitter calculator deals with. If the audit has convinced you the water bill is larger than it should be, the water bill calculator and the water saving guide put the irrigation share in context.
Questions people ask
How many catch cups do I need?
Six is the practical minimum and twelve to twenty-four is where the answer stops moving around. The reason is the low quarter: with six cups the driest quarter is two readings, so one cup that happened to sit in a shadow drags the whole DU down and one that sat next to a head props it up. With twenty cups the driest quarter is five readings and a single odd one barely registers. Spread them on a grid across the zone, corners included, rather than clustering them where you already think there is a problem.
What is a good distribution uniformity?
For a residential rotor or spray system, 0.70 and above is a sound result and 0.80 is very good. Under 0.55 is a fault to chase rather than a number to schedule around, because the multiplier is over 1.8 and you are paying nearly double for the water the dry ground needs. Drip systems audited properly score higher than sprinklers because there is no throw pattern to go uneven, which is much of why they are efficient. These bands are for interpreting your own result over time — the useful comparison is against your own previous audit, not against anyone else.
My cups are tapered. Can I still use them?
Yes, but measure the volume rather than the depth. A tapered container collects rain through its rim opening, so the depth that would have fallen on flat ground is the volume caught divided by the area of that opening. Pour each cup into a kitchen measuring jug, note the millilitres or fluid ounces, measure the throat diameter with a tape, and enter those. The conversion this page does is exact. Reading a ruler against the inside of a tapered cup, by contrast, gives a number that is wrong by whatever the taper happens to be.
Why does the Christiansen number look so much better than the DU?
Because they measure different things. Christiansen uniformity averages how far every reading sits from the mean, so readings above the average partly cancel the effect of readings below it, and the result flatters a zone with a few dry spots. Low quarter DU looks only at the driest quarter, which is the part that decides the run time. A zone at DU 0.64 typically reads around CU 0.79 on the same data, as the worked example on this page does. CU is useful for comparing zones. DU is what you schedule with.
Does a low DU mean I should replace the system?
Rarely, and not before checking the cheap causes. In rough order of how often they turn out to be the answer: a head sunk below grade so the turf blocks it, a partly blocked nozzle, an arc that has crept out of adjustment, a replacement head with a different nozzle from its neighbours, a leaking seal wasting pressure, and pressure at the far end of the lateral too low for the nozzles. Every one of those is an afternoon and a few parts, and any of them can move DU by ten points. Only after those have been ruled out is the answer spacing, which does mean moving heads and adding pipe.