Evaporation and Evapotranspiration Processes

Evapotranspiration (ET) is the sum of vaporization by the combined processes of evaporation (E) and transpiration. Reference crop ET, or reference evapotranspiration (${\mathit{E}}_{\mathit{ref}}$), addresses some of the ambiguities associated with definitions for potential ET and serves as a consistent climatic index for ET. The most common method of estimating E or ET is estimating the energy available for E or ET. The most accurate means of estimating ET is by applying methods that consider all primary factors affecting the ET rate, such as the Penman-Monteith equation. Those factors include weather, which supplies both the energy to evaporate water and the means to transport water vapor from the surface. Most current estimates of E and ET are based on some form of the energy balance equation, or a combination equation containing energy balance and aerodynamic components.