Irrigation Water Requirement Calculator
Units
Growth stage
Summer: 0.15–0.35 in/day; from your state or regional weather network
Rain the root zone actually holds; heavy downpours run off
Water to apply over 7 days
579,285 gal
2.13 in applied · 21.33 acre-inches · 82,755 gal a day
| Crop water use, ETc = Kc 1.2 × ET₀ 0.25 | 0.3 in/day |
|---|---|
| Over 7 days | 2.1 in |
| Minus effective rain | 0.5 in |
| Net irrigation need | 1.6 in |
| Gross, at 75% efficiency | 2.13 in |
FAO-56 crop coefficient method. Kc changes through the season: low after planting, highest at full canopy, falling as the crop matures. Soil water holding capacity decides how often to split this amount into irrigations.
Calculate how much water to apply to a crop or lawn over a period with the FAO-56 crop coefficient method: crop water use from reference evapotranspiration, minus effective rainfall, divided by the efficiency of your irrigation system, in inches, acre-inches and gallons or millimeters and cubic meters.
Formula
Worked Example
Corn at full canopy uses 1.2 × 0.25 = 0.30 in a day, or 2.1 in over the week. Half an inch of rain leaves 1.6 in, and at 75% efficiency you must apply 2.13 in, which over 10 acres is 21.33 acre-inches or about 579,000 gallons.
Frequently Asked Questions
What is reference evapotranspiration (ET₀)?
The water a well-watered reference grass would use in the current weather, combining evaporation and transpiration. State and regional weather networks publish it daily; summer values typically run 0.15–0.35 inches (4–9 mm) a day.
What is a crop coefficient?
The ratio of a crop's water use to the reference grass's at a given growth stage. It is low after planting (often 0.3–0.5), peaks at full canopy (around 1.15–1.2 for corn and many field crops) and falls as the crop matures. FAO Irrigation and Drainage Paper 56 lists values for most crops.
How many gallons are in an acre-inch?
About 27,154 gallons, the water needed to cover one acre an inch deep. An acre-foot is 325,851 gallons. In metric, 1 mm of water over a hectare is 10 cubic meters.