Estimating actual areal evapotranspiration from potential evapotranspiration using physical models based on complementary relationships and meteorological data

The potential evapotranspiration Ep, which is controlled by the atmospheric conditions and the saturation vapour pressure at the actual surface temperature, represents an upper limit to evapotranspiration from a wet soil-plant surface. The actual evapotranspiration E is the amount of water that evapotranspiration to the atmosphere under given energy and climatic conditions. According to Bouchet, dE+dEp=0. The author has verified the relationship using the measured data of pan evaporation. Both Morton and Brutsaert and Stricker applied the Bouchet's complementary relationship as the basis of their models and used Penman's model for Ep. The author has used both the models to estimate E in the USA, South America and India from meteorological data such as precipitation, temperature, vapour pressure, wind velocity, and solar radiation. This method enables E to be estimated anywhere in the world, especially where the supply of water is less abundant.