Free Crop Water Requirement calculator with clear step-by-step results.
Applies the standard FAO approach: reference evapotranspiration multiplied by a crop coefficient gives actual crop water use, effective rainfall is subtracted to get the net requirement, and dividing by system efficiency gives the gross depth you must actually pump. One millimetre over a hectare is ten cubic metres.
Crop water use
ETc = ET0 x Kc x days
Gross irrigation
Gross mm = (ETc - effective rainfall) / irrigation efficiency
FAO Irrigation and Drainage Paper 56 tabulates it by crop and growth stage. Values run from about 0.3 at emergence to 1.15-1.2 at full canopy for many field crops.
Only the fraction that infiltrates and stays in the root zone. On heavy soils or steep ground, well under half of a heavy storm may be effective.