Field calculator
Irrigation precipitation rate calculator
The precipitation rate is how fast an irrigation zone puts water on the ground, and it is what sets the run time and whether water soaks in or runs off. The rate in inches per hour equals 96.25 times the total flow in gallons per minute applied to the zone, divided by the zone area in square feet. Enter the combined flow of every head running on the zone and the area they cover. Once you know the rate, the run time follows: minutes equals the target depth in inches divided by the precipitation rate, times sixty. Two cautions make the number useful in the field. The application rate must not exceed the soil's intake rate, or water sheets off and is wasted and erosive, so on slopes and tight clay soils split the watering into several short cycles with soak time between them, which is called cycle and soak. And every head on a single zone should be a matched-precipitation type, so the rate is even across the area rather than flooding some spots while leaving others dry. Treat this as a scheduling estimate, verify it against a catch-can audit, and follow the local watering restrictions.
Result
Irrigation precipitation rate: PR (inches per hour) = 96.25 × total gpm applied to the zone divided by the zone area in square feet. Enter the total flow of all the heads running on the zone and the area they cover. The precipitation rate tells you how fast water is being applied, which sets the run time: run minutes = (target inches of water / PR) × 60. Two cautions make it useful. The rate must not exceed the soil intake rate, or water sheets off and is wasted, so on slopes and tight clay soils split the run into several short cycles with soak time between (cycle and soak). And every head on a zone should be a matched-precipitation type so the rate is even across the area, otherwise some spots flood while others stay dry. Treat this as a scheduling estimate and confirm against the irrigation plan, an audit catch-can test, and the local watering restrictions.
anvilfield.com/calculators/irrigation-precipitation-rate-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Precipitation rate FAQ
What is drip irrigation?
Drip irrigation is low-volume irrigation that delivers water slowly to the root zone through emitters rated in gallons per hour instead of the gallons per minute a spray head throws. It runs at low pressure under mulch, so a well-built zone reaches around 90 percent application efficiency against spray's 50 to 70 percent.
What pressure does drip irrigation run at?
Drip runs at low pressure, commonly 15 to 30 psi, with most point-source and dripline systems best around 25 to 30 psi and thin drip tape lower at 8 to 15 psi. House pressure runs 50 to 80 psi, so a pressure regulator is required to drop it. Confirm the range against the emitter manufacturer.
Do you need a filter for drip irrigation?
Yes. A filter is required on every drip zone because the emitter passages clog on sand, grit, scale, and organics that the water carries. The common landscape range is 150 to 200 mesh, with 150 for typical drippers and 200 for misters and fine sand. It only works if someone cleans it on schedule.
How long do you run drip irrigation?
Drip runs long and infrequent, often 30 to 90 minutes or more every few days, the opposite of spray. The low application rate needs a long run to soak deep, and the days-between interval lets the soil dry. Set the time from the zone's GPH and area or the gallons each plant needs, not the old spray schedule.
What is a pressure-compensating emitter, and when do you need one?
A pressure-compensating emitter holds a constant flow across a range of inlet pressures using an internal diaphragm. You need PC emitters on any slope, where pressure varies top to bottom, and on long runs, where friction drops pressure toward the end. Without them the low or far emitters flood while the high or near ones starve.