Irrigation Requirement (ET-based, acre-feet)
Crop ET demand (Kc x ET0 x days), net and gross irrigation depth after rainfall and application efficiency.
Example
You enter
- Crop corn
- Reference ET0 (in/day) 0.25
- Period length (days) 30
- Field area (acres) 80
- Irrigation method (efficiency) 90
- Effective rainfall over the period (in) 1
You get
- Crop ET demand 9
- Net (in) 8
- Gross (in) 8.889
- Total water 59.26
Checked against published worked examples from: FAO / USDA NRCS.
Details, formula, and sources
ScopeCrop ET demand (Kc x ET0 x days), net and gross irrigation depth after rainfall and application efficiency, and total acre-feet / gallons over the field. Per FAO Irrigation and Drainage Paper 56 and the USDA NRCS Irrigation Guide; reference ET0 from your local station.
Formula or tableET_crop = Kc x ET0 x days. Net depth = max(0, ET_crop - effective rainfall). Gross depth = net / application efficiency. Total = gross_in x acres / 12 (acre-feet); gallons = acre-ft x 325,851.
Source and editionFAO Irrigation and Drainage Paper 56 (Crop Evapotranspiration, Allen et al. 1998) by name; USDA NRCS Irrigation Guide by name. Kc values from FAO 56 Table 12.
Free accessFAO 56 free at fao.org; NRCS Irrigation Guide free at nrcs.usda.gov.
What governsEstimate. AHJ and licensed professional govern.
API field namescrop, et_ref_in_per_day, period_days, area_acres, efficiency_pct, rainfall_in, et_crop_in, net_in, gross_in, acre_ft
- Crop coefficient Kc alfalfa 0.95 season average (1.20 within one cutting), corn 1.20, cotton 1.15, wheat 1.15, pasture 0.95, turfgrass 0.95 cool season / 0.85 warm season, vegetables 1.05FAO 56 Table 12 mid-season
- Application efficiency drip 90%, sprinkler 75%, flood 50%NRCS Irrigation Guide
- Acre-foot conversion 1 acre-foot = 325,851 US gallonsphysical fact