Crop Water Requirements

Abstract

This article summarizes the essential definitions and methodologies for estimating crop water and irrigation requirements. The concept of reference evapotranspiration is assumed relative to a crop canopy such as grass but with constant crop characteristics. The hypotheses on which this approach is based are discussed relative to crop surface and aerodynamic resistances to heat and vapor fluxes. The crop evapotranspiration is defined using crop coefficients applied to the reference evapotranspiration, which reflect the canopy differences between the crop and the reference crop. Both time-averaged and dual-crop coefficients are explained, the first when the coefficients relative to crop transpiration and evaporation from the soil are summed and averaged for the crop-stage periods, the latter when a daily calculation of transpiration and evaporation coefficients is adopted. Corrections for climate and crop density are presented. Finally, essential information on the soil water balance to estimate crop irrigation requirements is provided.