Climatological Estimates of Evapotranspiration

Interactions between evaporating surfaces and the overpassing air suggest that potential evaporation responds in a complementary way to changes in the availability of water for evapotranspiration from the surrounding area. Supporting evidence is found in an analysis of dish and pan evaporation data from irrigated areas and adjoining deserts. The concept is used as the basis for a model which provides estimates of evapotranspiration from routine climatological observations without the need for assumptions concerning this soil-vegetation system and its effects on the availability of water. The model is calibrated at climatological stations in desert areas — where the monthly precipitation approximates evapotranspiration — and applied without local optimization of coefficients over a wide range of environments. The versatility of this unorthodox concept is demonstrated by comparing model and water budget estimates of evapotranspiration for 120 river basins in Canada, Ireland, Kenya, and the southern United States.