Crop Evapotranspiration

Abstract Evapotranspiration is a process referring to water transfer from the land to the atmosphere by evaporation, from the soil surface, and by transpiration, through the vegetation. It is a complex process of great relevance for the hydrological balance at different scales. Nevertheless, it is difficult to measure and estimate; hence, almost all evapotranspiration data contains errors. The main biophysical principles of evapotranspiration are described together with the most relevant methods for measurement and estimation. Particular attention is given to crop evapotranspiration used in irrigation scheduling and agricultural water management. The estimation of crop evapotranspiration is presented following the two‐step approach based on crop coefficient and reference evapotranspiration. The concept of reference evapotranspiration is expounded by means of the Penman–Monteith approach that combines surface energy balance and aerodynamic principles of the process. The indicative data of crop coefficient and length of crop growing stages are reported in tables for most relevant agricultural crops and the world growing regions. A detailed description of calculation procedures for the estimation of reference and crop evapotranspiration is presented together with the methods to improve crop coefficient estimate under specific growing conditions. Finally, numerical examples of reference and crop evapotranspiration are given in Appendix 1.