Irrigation audit field guide: uniformity, precipitation rate, scheduling

What an irrigation audit is, and what it proves

An irrigation audit measures two things about each zone and turns them into a schedule. The first is distribution uniformity, how evenly the zone spreads water across the ground. The second is precipitation rate, how fast it lays that water down in inches per hour. Once you have both, the run time is arithmetic. Without them, the controller is set by feel, and feel always overwaters.

Here is the mechanism the audit exposes. Coverage is never perfectly even. There is a dry quarter of every zone, the part that gets the least water, and the plants in that quarter are the ones that brown out first. So the operator cranks the run time until the dry quarter looks alive. Now the rest of the zone, the part that was already getting plenty, is soaked. The whole zone overwaters to keep the worst spot green, and the water bill pays for the gap between the best and the worst.

That is what the audit proves: where the gap is, how big it is, and what the schedule should be once you account for it. It separates a coverage problem you fix with a wrench from a scheduling problem you fix with the controller. Most zones have some of both. The audit tells you the ratio.

The work itself is not exotic. Catch cups, a pressure gauge, a tape, a watch, and a way to record the numbers. The skill is in reading what the numbers mean and not stopping at the average, because the average is exactly the number that lies to you here.

Distribution uniformity and the lower quarter

Distribution uniformity, DU, is how evenly a zone applies water across its area, expressed from 0 to 1 or as a percentage. The version that matters in landscape work is the lower-quarter distribution uniformity, DULQ. It is the average depth caught in the driest 25 percent of your cups divided by the average depth across all the cups. It is the metric the Irrigation Association teaches and the one EPA WaterSense and the ASABE/ICC 802 standard reference.

Why the lower quarter and not a plain average? Because plants do not water themselves to the average. The driest spots set the run time, since those are the plants that die if you schedule to the mean. DULQ asks the only question that controls the schedule: how starved is the worst quarter compared to the zone as a whole.

A DULQ of 1.0 would mean perfectly even coverage, which no spray or rotor zone reaches. Common practice treats roughly 0.70 and up as good for spray and rotor turf zones, and ASABE/ICC 802 references a lower-quarter floor around 0.65 for installed sprinklers. Drip can run higher. The number to remember is that low uniformity does not just waste water, it forces the overwatering. A zone at 0.55 has to run far longer to keep the dry quarter alive than a zone at 0.75 watering the same plants.

The cost of poor uniformity compounds. Every minute you add to rescue the dry quarter is a minute the wet quarter floods, runs off, and grows disease. You are paying twice for the same bad coverage.

How do you run a catch-can test?

You lay catch cups in a grid across the zone, run the zone for a set time, measure what each cup caught, and compute the uniformity and the precipitation rate from the results. The cups are identical straight-sided containers, and they sit on the ground inside the wetted area, spaced evenly on a grid rather than aimed at the wet or dry spots you already know about. The Irrigation Association audit method commonly uses around 24 cups for a zone, which makes the lower quarter a clean six cups.

Run the zone long enough to collect a readable amount, often 3 to 10 minutes depending on the head type and your cups. Note the exact run time, because the precipitation rate depends on it. Then measure each cup, either as a depth with a ruler or as a volume you convert to depth using the cup's mouth area. Record every cup. Do not average in your head and toss the slips.

To get DULQ, average all the cups, then average just the lowest 25 percent, and divide the low-quarter average by the overall average. With 24 cups, that is the mean of the lowest six over the mean of all 24. The overall average depth, divided by the run time, gives you the measured precipitation rate for the zone in inches per hour. One test, both numbers.

Run the test when the wind is calm. Wind skews a catch-can test worse than almost anything else, because it pushes the pattern and inflates the gap between the wet and dry cups. If you cannot get a calm window, note the wind, because it explains a bad number that the heads alone do not.

Catch cup

An identical straight-sided container set on the ground to collect applied water during the test

Lower quarter

The driest 25 percent of the cups, which set the run time because they are the starved plants

Scheduling multiplier

1 divided by DULQ, the factor a run time grows by to bring the dry quarter up to its need

How do you calculate precipitation rate?

Precipitation rate, PR, is the depth of water a zone applies per hour, in inches per hour, the same way you would read rainfall. You get it two ways. Measured, it comes straight out of the catch-can test: the average cup depth divided by the run time in hours. Calculated, it comes from the flow and the area, using the 96.25 constant.

The formula is PR equals 96.25 times the zone flow in gallons per minute, divided by the area in square feet. The 96.25 is not arbitrary. One gallon per minute spread over one square foot is 96.25 inches per hour, because a gallon is 0.1337 cubic feet, that depth over a square foot is 1.604 inches per minute, and 60 minutes brings it to 96.25. So the constant is just unit bookkeeping, and it does not change.

The area is the part people get wrong. For a rectangular zone it is length times width. For a single rotor sweeping an arc it is the slice of the circle it covers, and for a full-circle head it is pi times the radius squared. The flow is the total of every head running on the zone, read off the nozzle charts at the actual operating pressure, not the catalog headline. Get the pressure wrong and the GPM is wrong, and the calculated PR drifts away from the measured one.

When the measured PR and the calculated PR disagree by a lot, trust the catch cans and go find out why. The usual answer is pressure, a clogged or mismatched nozzle, or an area estimate that does not match the real wetted footprint.

PR

Precipitation rate, the application depth in inches per hour for the zone

GPM

Gallons per minute, the total flow of all heads on the zone at the real operating pressure

96.25

The constant converting GPM over square feet to inches per hour; one GPM on one square foot is 96.25 in/hr

Field example: a spray zone audited end to end

Take a turf zone with 8 spray heads, each flowing 2.0 GPM on the nozzle chart at 30 psi, watering a rectangle of 800 square feet. Total flow is 16 GPM. Calculated PR is 96.25 times 16 divided by 800, which is 1.93 inches per hour. That is a normal spray rate, fast enough that runoff is a real risk on anything but flat, open soil.

Now run the catch-can test for 5 minutes with 24 cups. The cups average 0.16 inches. Measured PR is 0.16 divided by 5 over 60, or 1.92 inches per hour, which lands right on the calculated number, so the flow and area assumptions check out. The lowest six cups average 0.11 inches. DULQ is 0.11 over 0.16, which is 0.69. That is acceptable for spray turf but leaving room, with a scheduling multiplier of 1 over 0.69, about 1.45.

Say the turf needs 1.2 inches for the week after subtracting rain. The run time is the net need divided by the rate and the uniformity: 1.2 times 60, divided by 1.93 times 0.69, which is about 54 minutes a week. Split across three days, 18 minutes a day. On clay or a slope, that 18 minutes breaks into cycle-and-soak so the water soaks instead of running to the curb.

Every number in that schedule came from a measurement. Change the heads, the pressure, or the plant and you re-run it. That is the difference between an audited schedule and a guessed one, and it usually shows up as 20 to 40 percent less water for the same green.

Matched precipitation, and the number-one design error

Every head on a zone must apply water at the same rate. That is the matched-precipitation rule, and breaking it is the most common design mistake in the trade. If a half-circle head and a full-circle head sit on the same zone, the half-circle covers half the area but flows a matched fraction so both lay down the same depth per hour. When the rates are not matched, one part of the zone gets twice the water of another no matter how long you run it, and no schedule can fix it.

This is why nozzles are sold in matched-precipitation sets. A quarter, a half, and a full nozzle from the same family flow in proportion to their arc, so the depth per hour comes out even across the zone. Mix nozzles from different families, or pair a high-flow nozzle with a low-flow one on the same valve, and you have built a permanent dry spot and a permanent wet spot on day one.

The brutal part is that matched precipitation is a design and parts decision, not a schedule decision. You cannot dial it out at the controller. The audit catches it as a split in the catch cans that does not track with the wind or a single bad head, a whole section reading low while another reads high. The fix is at the nozzles, and on a zone someone built wrong, that means re-nozzling to a matched set or splitting the zone.

Spray, rotor, and drip on the same zone

Sprays, rotors, and drip apply water at rates so different that they cannot share a zone, and putting them together is matched precipitation broken in the worst way. A fixed spray head commonly lays down roughly 1.5 to 2.0 inches per hour. A rotor, throwing a stream that sweeps back and forth, commonly applies roughly 0.4 to 0.6 inches per hour, several times slower because the same water covers far more ground. Drip is slower still, measured in gallons per hour per emitter rather than a sweep rate.

Run a spray and a rotor on one valve and the spray dumps three to four times the depth in the same minutes. The spray zone floods while the rotor zone is still thirsty, or the rotor stays brown while the spray drowns. There is no run time that serves both. The nozzle charts give the real rates at your operating pressure, and those charts govern over any rule of thumb.

The audit approach changes by head type. Sprays and rotors get the catch-can test. Drip does not, because the water goes into the soil at the root, not into a cup. You audit drip by checking emitter output against its rated gallons per hour, walking the lines for clogs and blowouts, and confirming the pressure regulator and filter are doing their job. A drip zone fails quietly, one plugged emitter at a time, so the check is per plant, not per pattern.

Why are there dry rings between my heads?

Dry rings come from spacing the heads too far apart, breaking the head-to-head coverage rule. A sprinkler does not water evenly across its radius. It throws the least water close in and the most partway out, so a single head leaves itself short near its own base. The fix designed into every system is overlap: each head must throw all the way to the next head, so the strong part of one pattern covers the weak part of the next. That is head-to-head coverage, sometimes called 100 percent overlap.

Space the heads at their full radius and you get the textbook minimum. In the field, wind eats into the throw, so common practice tightens to roughly 90 percent of the radius, an effective overlap past 100 percent, to hold uniformity when the air moves. Square layouts and triangular layouts change the spacing math, with triangular packing letting heads sit a bit farther apart for the same coverage, but the principle holds: the pattern has to reach the neighbor.

When you walk a zone and see green halos with brown centers, or brown bands exactly halfway between heads, that is a spacing or radius problem, not a schedule problem. Check the nozzle radius against the actual head spacing. A head throwing 12 feet on a 18 foot spacing will never cover, and no run time closes that gap. Either the radius was reduced by low pressure, or the heads were set too far apart to begin with.

Why are my sprinklers misting, or watering in a doughnut?

Both are pressure problems, at opposite ends. Misting and fogging mean the pressure at the head is too high. The nozzle atomizes the stream into a fine spray that the wind carries off the property, so you pay for water that never lands. A doughnut, water close to the head and a dry ring beyond it before it picks up again, means the pressure is too low. The stream has no energy to carry, so it falls short and breaks up.

Each head has a design operating pressure on its chart, and that is the pressure that matters, measured at the head while the zone runs. Spray heads commonly want something near 30 psi, while rotors run higher, often in the 40 to 65 psi range depending on the model and radius. Above the design pressure you mist and waste. Below it you lose radius and break head-to-head, which is how an under-pressure zone fakes a spacing problem.

Know the difference between static and dynamic pressure. Static is the pressure with no water moving, the number on the gauge when nothing runs. Dynamic, or working, pressure is what you have while the zone flows, and it is always lower because the pipe, valves, and backflow eat some of it. Audit on the dynamic number. A system that reads fine static can starve under flow when too many heads share an undersized line.

The durable fix for high pressure is regulation at the head, pressure-regulating spray bodies or a regulator on the line, set to the nozzle's design pressure. EPA WaterSense labels pressure-regulating bodies for exactly this reason. The fix for low pressure is finding where it went: a partially closed valve, an undersized main, too many heads on the zone, or a leak upstream.

Common coverage problems you find on the walk

Before the catch cans go down, walk the zone running and look. Half the coverage problems on a residential or light-commercial system are visible in two minutes, and they explain the bad cups you are about to collect.

Tilted heads throw their pattern into the air or the ground instead of level, so the radius collapses on one side. Sunken heads spray into the surrounding turf and grade instead of over it, watering a six-inch circle and nothing past it. Heads that grew over with turf or got buried by a mower do the same. Clogged nozzles narrow or split the stream, and a single grain of grit changes the pattern. Mismatched nozzles, someone replacing a broken head with whatever was on the truck, break matched precipitation one head at a time.

Then there is the obstruction nobody drew on the plan. A shrub that has filled in, a new fence, a parked trailer, a sign. Anything in the throw line creates a dry shadow behind it and a wet spot where the deflected water piles up. The classic pairing is a dry spot and a wet spot a few feet apart, and they are the same problem: water that should be on the dry spot is landing on the wet one. Fix the head or the obstruction and both clear at once.

Write down what you find before you fix it. The as-found condition is half the value of the audit, because it shows the owner why the bill was what it was.

How do you set irrigation run times from the audit?

The run time is the plant's water need divided by how fast and how evenly the zone applies water. You take the net depth the plants need for the period, divide by the precipitation rate to get a base time, then divide by DULQ to stretch that time enough that the dry quarter also reaches its need. The scheduling multiplier, 1 over DULQ, is what builds the uniformity loss into the clock instead of pretending it away.

In numbers: run time in minutes equals the net need in inches, times 60, divided by the product of PR and DULQ. A zone needing 1.2 inches a week at a PR of 1.9 inches per hour and a DULQ of 0.69 wants about 54 minutes a week. Split that across the number of allowed watering days to get the per-day time. The more uneven the zone, the longer it runs, which is the math reason poor uniformity costs money every single cycle.

Two refinements make the schedule real instead of theoretical. First, the run time per day is often more than the soil can take before it runs off, so it gets split into cycle-and-soak. Second, the plant need is not constant. It rises in summer and falls in spring and fall, so the schedule gets a seasonal adjust rather than one setting all year. Set it once from the audit, then ride the seasonal percentage, and re-audit when heads or plants change.

Watering frequency follows the soil and roots, not the calendar. Deeper, less frequent watering drives roots down and survives heat better than a daily sprinkle that wets only the top inch. The audit gives you the depth per cycle. The soil tells you how often.

Net need

The depth in inches the plants require for the period after subtracting effective rainfall

Seasonal adjust

A controller percentage that scales every zone up or down as plant demand changes through the year

ET and the water budget

Plant water need comes from evapotranspiration, ET, the water lost from the soil and the plant together. Reference ET, written ETo, is the demand for a standard reference surface under the day's weather, published by many local water authorities and weather networks. You scale it to your actual plants with a crop coefficient, Kc, so the plant need, ETc, is ETo times Kc. Turf, shrubs, and trees carry different coefficients, which is part of why they belong on different zones.

A smart controller automates the budget. Weather-based, or ET, controllers pull local ETo and adjust the run time daily so the schedule tracks real demand instead of a fixed clock, which is the single biggest water saver after fixing coverage. Soil-moisture controllers do the same job from the other direction, holding water until the root zone actually dries. Either way, the audit still has to come first, because the controller can only schedule the precipitation rate and uniformity it is given.

On a basic controller, the manual version of this is the seasonal adjust or water budget percentage. You set the peak-summer run times from the audit at 100 percent, then ride the percentage down through spring, fall, and the shoulder months as ET drops. A schedule that runs the July time in October is throwing roughly half the water on the ground for nothing.

Runoff, slope, and cycle-and-soak

Runoff happens when the precipitation rate beats the soil's intake rate. The soil can only soak up water so fast, and once you exceed that, the rest sheets off to the curb, taking fertilizer and topsoil with it. Spray zones at near 2 inches per hour overrun most soils within a few minutes. Add a slope and it runs off sooner, because gravity carries it away before it can soak in.

Clay is the hard case. Dense, fine soils have a slow intake rate, so they accept water far slower than sandy soils and pond almost immediately under a spray head. The fix is cycle-and-soak: split the run time into shorter cycles with a soak period between them, often an hour or more, so each cycle wets the surface and the soak lets it move down before the next cycle. Clay wants shorter cycles and longer soaks. Sandy soil tolerates longer cycles and needs less soak.

The audit tells you the total run time. The soil and slope tell you how to deliver it. A zone needing 18 minutes on flat sand might run as one 18-minute cycle, while the same 18 minutes on a clay slope runs as three 6-minute cycles with an hour between. Same water on the plants, none of it in the gutter. If you see water tracking down the sidewalk during a cycle, the cycle is too long for that soil, full stop.

Auditing drip and the emitter check

Drip does not take a catch-can test, because the water goes into the soil at the root rather than across a surface you can collect. You audit it by output and by inspection. Check that the system pressure sits in the emitter's regulated range, usually low, often around 15 to 30 psi behind a regulator, and confirm the filter is clean, because drip lives or dies on filtration.

Then walk every line. Pull a sample of emitters and measure or feel the flow against the rated gallons per hour. Look for the two failure modes that define drip: clogs that starve a plant, and blowouts or cut tubing that flood one and starve the rest of the line. A drip zone fails one emitter at a time and shows it as a single wilting plant in a healthy bed, so the audit is plant by plant, not pattern by pattern.

Drip earns its keep on uniformity and on beds, where spray would water the mulch and the fence as much as the plant. A maintained drip zone runs a high uniformity that spray cannot match. An unmaintained one is a row of dead plants downstream of the first clog, which is why the emitter check belongs on the same schedule as the catch-can audit, not after the complaint.

Backflow and cross-connection

Every irrigation system tied to a potable supply needs a backflow preventer, because the irrigation side is a cross-connection that can pull pollutants back into the drinking water if the supply pressure drops. Lawn water sits next to fertilizer, pet waste, and standing puddles, and a pressure drop upstream can siphon that back through the heads into the main if nothing stops it. The preventer is the one stop.

The device type depends on the hazard and the local code, from atmospheric and pressure vacuum breakers up to reduced-pressure assemblies, with the assemblies tested on a schedule by a certified tester. The audit is not the place to skip it. Confirm the right device is installed for the application, that it is the assembly the water authority requires, and that its test is current. A failed or missing backflow preventer is a health issue, not a performance one, and it outranks every uniformity number on the page.

The plumbing detail and the testing procedure are their own subject. For the audit, the job is to note the device, its type, and its last test date, and to flag anything missing or out of test to the responsible party.

The audit as the conservation and rebate driver

Most audits get ordered for a reason beyond curiosity, and that reason is usually water and money. Landscape irrigation is the largest discretionary water use on many properties, and a system running a poor uniformity on a guessed schedule can waste a third or more of what it uses. The audit is how you prove the waste and how you claim it back.

Water authorities and utilities run rebate and efficiency programs that pay for audits, weather-based controllers, pressure-regulating bodies, and high-efficiency nozzles, and many require a documented audit to qualify. Under drought restrictions, the audit is also the defense: it shows the property is watering to need, not by habit, which matters when allocations tighten and watering days get capped. The same catch-can numbers that build the schedule also build the rebate paperwork and the compliance record.

EPA WaterSense anchors the efficiency side, labeling controllers and spray bodies that meet its criteria and certifying irrigation professionals through its program. The Irrigation Association trains and certifies the auditors. Naming those on the report is not decoration. It tells the utility the audit was done to a recognized method, which is often what the rebate requires.

The audit report: as-found versus as-left

The report is the product. It records two states for every zone: as-found, the condition and numbers when you walked up, and as-left, the condition and schedule when you walked away. The gap between them is the value you delivered, and it is what the owner and the utility pay for.

As-found captures the broken heads, the mismatched nozzles, the obstructions, the measured DULQ and PR, and the schedule that was running. As-left captures what you fixed, the new uniformity and rate after the fixes, the new schedule with its cycle-and-soak and seasonal settings, and the backflow status. Photos of the dry spots before and the corrected coverage after make the case better than any number, because the owner can see the brown go away.

This is the part that falls apart on paper. Catch-can slips get lost, the schedule lives in someone's head, and six months later nobody can prove what was set or why. Capturing the audit as it happens, the zone, the cups, the pressure, the photos, and the final schedule in one record, is what turns a good field test into a defensible document. That is the funnel FieldOS is built for: the as-found numbers, the photos, and the as-left schedule recorded on site and kept with the property, so the next tech and the utility both see the same thing.

What to document

Record enough per zone that someone who was not there can reproduce the schedule and defend it. The minimum is the head type and count, the flow, the area, the precipitation rate, the lower-quarter uniformity, the operating pressure, and the run time you set. Add the as-found problems and the photos, and the report stands on its own.

The table below is the per-zone line. Fill one for every zone, not one for the system, because the whole point of the audit is that zones differ and the schedule is per zone.

Common mistakes

• Mixing rotors, sprays, or drip on one zone, so no run time serves the whole zone.
• Spacing heads past head-to-head coverage and trying to fix the dry rings with more run time.
• Auditing on static pressure instead of the dynamic pressure the zone runs at.
• Building the schedule from a guess or last year's clock instead of a measured PR and DULQ.
• Scheduling to the average catch instead of the lower quarter, which starves the dry spots.
• Running long single cycles on clay or slopes and sending the water to the curb instead of cycle-and-soak.
• Leaving the July run time in October because the seasonal adjust was never set.
• Replacing a head with a mismatched nozzle and breaking matched precipitation one head at a time.

Field checklist

Standards and references

The Irrigation Association is the body that defines the landscape audit. Its Certified Landscape Irrigation Auditor program teaches the catch-can method, the lower-quarter distribution uniformity, and the scheduling math used throughout this guide, and an audit done to that method is what utilities recognize. When you name a method on a report, name the IA audit procedure.

ASABE/ICC 802, the Landscape Irrigation Sprinkler and Emitter Standard, sets the test methods and performance requirements for sprinklers, bubblers, and emitters, including lower-quarter uniformity criteria for installed devices. It is a consensus standard with editions across recent years, so confirm the edition a specification or jurisdiction actually references before you cite a specific number from it.

EPA WaterSense runs the federal efficiency program, labeling weather-based controllers and pressure-regulating spray bodies that meet its criteria and certifying irrigation professionals through approved programs. It is the label utilities point rebates at. Above all of it sits the local water authority, whose rules on watering days, allocations, backflow devices, and rebate eligibility govern the actual job. Cite the IA method and the standard that controls the point, and let the local water authority's rules override any general target.

Units and terms

Irrigation numbers cross between depths, flows, and pressures, and the same idea reads differently across a controller, a nozzle chart, and a utility form.

Precipitation rate and plant need are depths in inches per hour or inches per week, the same units as rainfall, so a zone applying 2 inches per hour against a 1.2 inch weekly need is directly comparable. Flow is gallons per minute for sprays and rotors, and gallons per hour per emitter for drip. Pressure is pounds per square inch. Uniformity and the seasonal adjust are dimensionless, read as a fraction or a percent. Keep the depth, the flow, and the pressure straight and the audit math stays honest.

DU / DULQ

Distribution uniformity, and its lower-quarter form, the dry quarter average over the overall average

Precipitation rate (PR)

Application depth in inches per hour, measured by catch can or calculated by the 96.25 formula

ET / ETo / ETc

Evapotranspiration: reference demand ETo scaled by a crop coefficient to the plant need ETc

Matched precipitation

Every head on a zone applying the same depth per hour, the condition for a fixable schedule

Cycle-and-soak

Splitting a run time into shorter cycles with soak breaks so water infiltrates instead of running off

GPM

Gallons per minute, the zone flow at the real operating pressure that feeds the PR calculation

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