Potential Evaporation and River Basin Evaporation

Potential evaporation is a measure of the energy available for evaporation, whereas the evaporation from the water, soil, and vegetation surfaces of a region is controlled by the availability of both energy and water. A recent analysis of the regional energy balance has suggested that the potential evaporation is a measure of the extent of regional evaporation that does not occur because of the control imposed on the availability of water by soil moisture and vegetation processes. The suggestion is incorporated into a method for computing regional evaporation from climatological observations. The method is tested by comparing computed potential evaporation—one of its principal components—with observed pan evaporation and by comparing computed regional evaporation—its end product—with river basin evaporation derived from rainfall and runoff records. It is concluded that changes in regional evaporation caused by changes in the availability of water are reflected in the potential evaporation and that the method formulated by the writer may be used as a working hypothesis for computing regional or river basin evaporation from climatological observations.