How are crop water requirements determined for irrigation planning?

Crop water requirement (ETc) = reference evapotranspiration (ETo) * crop coefficient (Kc). ETo calculated from weather data using Penman-Monteith equation (FAO-56 method) based on temperature, humidity, wind, and solar radiation. Kc varies through crop growth stages: initial, development, mid-season, late season. Net irrigation requirement = ETc - effective rainfall - groundwater contribution. Gross requirement accounts for application efficiency and conveyance losses. Irrigation scheduling determines when and how much to irrigate.