What is a scheduling coefficient?
A scheduling coefficient (SC) compares the average amount of water a sprinkler zone applies with the amount that actually reaches its single driest, contiguous spot — usually the gap between two heads' patterns, or the edge of a zone where coverage thins out. The method traces back to work at the Center for Irrigation Technology (CIT) at California State University, Fresno, which developed and helped popularize scheduling coefficients based on a critical, contiguous area rather than a statistical slice of the readings.
SC matters because a controller can only run a zone for one length of time. If you schedule based on the zone's average catch, the driest spot is under-watered every single cycle. Multiplying the run time by SC brings that dry spot up to target, at the cost of applying more than the target everywhere else.
| Metric | What it compares | Where the "low" group comes from |
|---|---|---|
| Scheduling coefficient (SC) | Overall average catch ÷ driest-area average catch | A specific, contiguous area you identify, such as a head-to-head gap |
| Distribution uniformity, low quarter (DULQ) | Lowest-quarter average catch ÷ overall average catch | The lowest 25% of all readings, wherever they happen to fall |
Because DULQ pools whichever readings happen to be lowest, two scattered weak spots can pull it down even if neither is severe on its own. SC instead asks a narrower, more practical question: how much extra run time does the single worst physical spot need? If you need DULQ rather than SC — for example, to check it against a program's numeric target — use the distribution uniformity calculator instead; it isn't calculated here. Some references also define a related "scheduling multiplier" that adjusts run time from DULQ rather than from a critical area — a different number from the SC this page calculates, even though both aim to fix the same under-watering problem.
DULQ does carry one number worth knowing for context: the EPA WaterSense New Home Specification requires newly installed, WaterSense-labeled irrigation systems to test at a DULQ of 65% or greater. WaterSense's separate best-practices guidance for existing commercial systems recommends that same 65% figure as a target for an audit, but doesn't require it the way the new-home specification does. Neither document sets a required or recommended number for SC itself.
How this calculator works
The calculator averages every reading you enter, separately averages the readings from your driest contiguous area, and divides the two to get SC. If you add your zone's current run time, it multiplies that by SC to get an adjusted run time.
1. Overall average catch
Average catch
Average catch = Sum of all readings ÷ Number of readings
2. Driest-area average catch
Driest-area average
Driest-area average = Sum of driest-area readings ÷ Number of driest-area readings
These readings must come from one contiguous low spot, not the lowest values scattered across the whole grid.
3. Scheduling coefficient
SC
SC = Average catch ÷ Driest-area average catch
SC has no unit — it's a ratio, so any consistent measurement unit works for both inputs.
4. Run-time multiplier and adjusted run time
Adjusted run time
Run-time multiplier = SC
Adjusted run time = Current run time × SC
Multiplying the current run time by SC is what brings the driest spot up to the same depth the average area already receives.
Rounding policy
Average catch values are shown to 2 decimals under 10, 1 decimal from 10 up to 100, and as a whole number at 100 or above, matching the precision of the numbers used in the math. SC and the run-time multiplier are shown to 2 decimals. Adjusted run time is rounded to the nearest minute and shown with "about," since it's built from measured, real-world readings rather than an exact conversion.
When this calculator isn't the right tool
It's built specifically for the scheduling coefficient, based on one contiguous driest area. Use a different tool for:
- Distribution uniformity (DULQ), based on the statistically lowest 25% of all readings rather than a contiguous area. See the distribution uniformity calculator.
- Running the catch-device test itself, including how many devices to use and how long to run them. See the catch-can test calculator.
- Drip and micro-irrigation zones, where emitters rather than spray patterns determine uniformity. See the emission uniformity calculator or the drip irrigation calculator.
- A baseline run time with no uniformity adjustment, such as when you just need minutes from a target depth and precipitation rate. See the sprinkler run time calculator.
Scheduling coefficient calculation example
An auditor sets 12 catch cups across a spray-head zone and runs it for a short test. Readings, in inches: 0.62, 0.58, 0.71, 0.44, 0.68, 0.72, 0.55, 0.63, 0.60, 0.66, 0.50, 0.64. Two cups sitting in the gap between heads catch just 0.44 and 0.50 inch — the driest contiguous area. The zone currently runs 15 minutes per session.
- Average catch: the 12 readings sum to 7.33, so 7.33 ÷ 12 ≈ 0.61 in.
- Driest-area average: (0.44 + 0.50) ÷ 2 = 0.47 in.
- SC: 0.61 ÷ 0.47 ≈ 1.30.
- Adjusted run time: 15 min × 1.30 ≈ about 19 minutes.
A larger zone, without a run time
A rotor zone tested with 16 catch cups collects, in milliliters: 120, 115, 130, 90, 125, 128, 118, 122, 108, 132, 95, 126, 119, 121, 100, 124. The two cups at the far end of the pattern, where head-to-head coverage is weakest, catch 90 and 95 mL.
- Average catch: the 16 readings sum to 1,873, so 1,873 ÷ 16 ≈ 117 mL.
- Driest-area average: (90 + 95) ÷ 2 = 92.5 mL.
- SC: 117 ÷ 92.5 ≈ 1.27.
Without a current run time entered, the calculator shows the run-time multiplier (×1.27) but leaves the adjusted run time blank until one is added — it's the same behavior you'll see if you leave that field out yourself.
Using SC to fix run times
Start with a catch-device test, following the catch-can test calculator's guidance on device count, spacing and run time. Once the cans are collected, look at the grid and find the group of adjacent devices with the lowest readings — a real, physical location, not just whichever individual numbers happen to be smallest. That group is your driest-area input; everything you collected is your "all readings" input.
To apply the result, multiply the zone's current run time by SC and reprogram the controller for that zone. A zone running 15 minutes with an SC of 1.30 becomes about 19 minutes; the driest spot gets close to its target depth, and the rest of the zone gets somewhat more than its target. That trade-off is unavoidable with a single run time on an uneven zone — fixing the equipment is the only way to lower SC itself.
Common mistakes
- Using scattered low readings instead of a contiguous area. That's distribution uniformity's job, not SC's; mixing the two methods produces a number that matches neither.
- Testing on a windy day. Wind distorts spray patterns and can manufacture a dry spot that isn't there under calm conditions.
- Applying one zone's SC to a different zone. Head type, spacing and pressure all vary zone to zone, so each one needs its own test.
- Reusing an old SC after repairs. Replacing a clogged nozzle or fixing a tilted head changes the pattern; retest before trusting the old multiplier.
If the adjusted run time causes runoff on clay or a slope, split it into shorter cycles with a break between them using the cycle and soak calculator. And if you're also adjusting run times for the season, layer that on separately with the seasonal adjustment calculator rather than folding it into SC.
What changes your scheduling coefficient
SC responds to whatever makes one physical spot drier than the rest of the zone. These are the usual culprits.
| Factor | Effect on SC | What to check |
|---|---|---|
| Head spacing and layout gaps | Areas where two heads barely overlap catch less water, deepening the driest spot. | Compare spacing with the manufacturer's radius chart; adjust arcs or add heads. |
| Mixed sprinkler types on one zone | Different precipitation rates on the same zone widen the gap between average and driest. | Match nozzles by rate; see the matched precipitation calculator. |
| Wind during the test | Distorts spray patterns and can exaggerate the driest reading. | Retest on a calm day. |
| Low or uneven pressure | Changes throw radius and can create a weak spot at one head. | Check pressure at the head; consider pressure-regulated bodies. |
| Clogged or worn nozzles | Reduces output right at that head, deepening the local dry spot. | Inspect and clean or replace nozzles on a regular schedule. |
| Slope or obstructions | Blocks or redirects spray before it reaches part of the pattern. | Adjust head placement or arc; trim obstructions. |
| Number and placement of catch devices | Too few or poorly placed devices can miss the true driest spot, or mislabel a different one. | Space devices across the whole zone; see the catch-can test calculator. |
Soil plays a role too, though not in the SC formula itself: a driest spot on fast-draining sand may need a shorter cycle even at the same SC as one on slow-draining clay. The soil infiltration rate chart covers how quickly different soils absorb water.
Frequently asked questions
What is a scheduling coefficient?
A scheduling coefficient (SC) compares the average water your sprinklers apply across a zone with the amount reaching one specific, contiguous driest spot in that zone, often the gap between two heads' patterns. Divide the overall average catch by the driest-area average catch to get SC. A perfectly even zone has an SC of 1.0; most real zones are higher, meaning the driest spot gets less than the average.
How is SC different from distribution uniformity (DU)?
Distribution uniformity, using the lower-quarter method (DULQ), averages the lowest 25% of all your catch readings wherever they fall across the zone, then divides that by the overall average. SC instead targets one specific, contiguous area you've identified as the driest, which is often a smaller and more extreme spot than DU's lowest quarter. The two numbers answer related but different questions, so a zone can look fine on one and rough on the other.
How do I find the driest contiguous area in my readings?
Look at your catch-device grid and find the group of adjacent devices with the lowest readings, typically near the edge of a zone or the gap where two heads' throw patterns barely meet. It should be a real physical spot you could point to on the ground, not just whichever individual readings happen to be lowest. If your low readings are scattered across the zone instead of clustered together, distribution uniformity is probably the more useful number.
How many catch devices do I need for a reliable SC?
There's no fixed minimum for SC specifically, but more devices give a more reliable picture of where a zone actually runs dry. A typical audit spaces catch devices in a grid across the zone and runs the system for roughly 8 to 15 minutes, or until there's a readable volume in each one. Use enough devices to cover the zone's different sprinkler types and distances from the valve.
The catch-can test calculator walks through spacing and run time in more detail.
How do I use SC to fix my irrigation schedule?
Multiply your zone's current run time by its SC. If a zone currently runs 15 minutes and its SC is 1.3, run it for about 20 minutes instead so the driest spot gets close to its target depth. The rest of the zone will receive more than its target, which is the trade-off of using one run time on an uneven zone.
Can SC be less than 1?
In theory, no. The driest area should never catch more than the zone's average, so SC should never fall below 1.0. If your result comes out below 1.0, it's a sign the readings entered as the driest area weren't actually the lowest, contiguous spot. Recheck your grid and re-enter the readings from the true driest area.
Does wind or pressure affect SC?
Yes. Wind distorts spray patterns, and pressure outside a sprinkler's normal range changes how far and how evenly it throws water, so a catch-device test done on a windy day or at unusual pressure won't represent how the zone performs normally. Test on a calm day at typical operating pressure, and retest if conditions were unusual.
Can I use this calculator for drip irrigation?
Not directly. SC and catch-device testing are built around water landing on a surface from spray or rotor heads. Drip and micro-irrigation deliver water to specific emitter points rather than spraying it over an area, so emission uniformity is the more relevant measure there.
See the emission uniformity calculator or the drip irrigation calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The formula and both worked examples were verified by hand and traced through the calculator's code.
- SC's definition was checked against how it's distinguished from distribution uniformity (DULQ) in the sources below.
- No pass/fail wording is used for SC itself, since no source below sets a numeric SC threshold; results are shown as plain numbers.
Sources
- Irrigation Today. CIT Celebrates 40 Years — on the Center for Irrigation Technology's role in developing scheduling coefficients based on a critical, contiguous area.
- Swanson, C., Fipps, G., & McAfee, J. (2009). Calculation of Uniformity in Landscape Irrigation Auditing. Texas A&M University / Irrigation Association technical paper — catch-can test procedure and the DULQ formula and rating scale.
- U.S. EPA WaterSense. Guidelines for Irrigation Audits on WaterSense Labeled New Homes (Version 1.1) — the 65% DULQ requirement for labeled new homes.
- U.S. EPA WaterSense at Work. Section 5.3, Irrigation — the 65% DULQ figure as general audit guidance rather than a universal requirement.
- Segars, C. & Fontanier, C. Irrigation Audits, Texas A&M AgriLife Extension / SportsField Management — catch-can and scheduling-multiplier terminology.