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Irrigation Water Schedule Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Scheduling methodology from USDA NRCS National Engineering Handbook Part 652 and UC Davis Cooperative Extension irrigation management guides. Last verified 14 June 2026.

ET-based irrigation scheduling applies water to match crop demand, minimising waste and reducing runoff. Enter the crop ET demand for your irrigation interval, the precipitation rate of your system, and your distribution uniformity to calculate the required run time. This method is recommended by USDA NRCS for precision water management.

Sum of daily ET values since last watering
From manufacturer spec or field catch-can test
Well-maintained systems: 0.80 to 0.90
Subtract effective rainfall from ET demand
1.00 in
1.18 in
2.35 hrs
141.18 min

Irrigation scheduling formula (USDA NRCS)

Net demand (in) = ET demand - Effective rainfall
Gross irrigation depth (in) = Net demand / DU
Run time (hr) = Gross depth / Precipitation rate (in/hr)
Run time (min) = Run time (hr) x 60

Dividing by DU (distribution uniformity) ensures the driest areas in the field receive the required net demand. USDA NRCS Part 652, Chapter 1 (Irrigation Guide) specifies this approach for scheduling both sprinkler and drip/microirrigation systems.

Reference ET values by region and season

  • California Central Valley (summer): 0.25 to 0.35 in/day reference ET (CIMIS data).
  • Midwest corn belt (July peak): 0.18 to 0.25 in/day.
  • Pacific Northwest (summer): 0.15 to 0.22 in/day.
  • Florida (summer): 0.20 to 0.28 in/day.
  • Check NOAA climate.gov or your state agricultural weather network for local daily ET data.

Irrigation scheduling calculator: frequently asked questions

What is evapotranspiration (ET) and how is it used in irrigation scheduling?

Evapotranspiration (ET) is the combined water loss from soil evaporation and plant transpiration, expressed in inches per day or per week. It represents the crop's water demand. Irrigation scheduling based on ET replaces only the water lost since the last irrigation, preventing both over- and under-irrigation. NOAA and UC Davis CIMIS publish daily ET data by region.

What is precipitation rate (PR) for an irrigation system?

Precipitation rate is how much water a sprinkler or emitter system applies per hour, measured in inches per hour. PR = (96.25 x GPM) / (row spacing in feet x head spacing in feet) for sprinklers, or calculated from emitter flow rates and area. Manufacturer spec sheets publish PR values for specific nozzles and spacings.

What is distribution uniformity (DU) and why does it affect run time?

Distribution uniformity (DU) is the ratio of the average low-quarter application to the overall average application, expressed as a decimal. A DU of 0.80 means the driest 25% of the area receives 80% of the average. To ensure the driest areas receive the target amount, divide the target depth by DU. Well-designed systems achieve DU of 0.80 to 0.90.

How do I find the ET rate for my location?

NOAA publishes Penman-Monteith reference ET calculations for weather stations across the US through the National Centers for Environmental Information. The California Irrigation Management Information System (CIMIS) and state-based agricultural weather networks publish local ET values. Your state university extension service can direct you to the appropriate local resource.

How often should I irrigate based on ET?

Irrigation frequency depends on soil water-holding capacity and daily ET rate. Sandy soils hold less water and require more frequent irrigation (every 1 to 2 days); clay soils can hold more and may be irrigated every 4 to 7 days. The goal is to refill the root zone before the soil reaches the stress threshold, typically when 50% of plant-available water has been depleted.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.