Crop Evapotranspiration Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026The standard method for estimating crop water needs
Reference evapotranspiration describes water loss from a standardized grass reference surface based on local weather, while a crop coefficient adjusts this baseline figure for a specific crop's actual water use at its current growth stage.
Worked example
For a reference ET of 5 mm/day and a crop coefficient of 1.15 (typical for mid-season canopy development):
ETc = 5 x 1.15 = 5.75 mm/day crop water use
Crop coefficients change throughout the growing season, typically lowest right after planting when canopy cover is minimal, rising to a peak during full canopy development, then declining again as the crop matures and senesces - using a single Kc value across the entire season would significantly misestimate irrigation needs at different growth stages.