Estimating evaporation

Evaporation is a physical process that must satisfy two basic requirements. There must be an energy supply to provide heat of vaporization; and there must be some transport mechanism for removing the vapor. Combining these concepts it is possible to derive an expression for evaporation rate that depends only on measureable weather elements. By making some simplifying approximations, the weather data needed reduce to (a) duration of bright sunshine, (b) air temperature, (c) air humidity, and (d) wind speed. The quantity obtained is EO, the evaporation rate for a hypothetical open water surface exposed to the measured weather. Formally, and empirically, it is possible to convert EO into ET, the potential transpiration rate from a short green crop cover, completely shading the ground and never short of water. This computation has formed the basis of successful experiments in irrigation control; and, with reasonable empirical modification to allow for conditions when water supply becomes a limiting factor, has been applied with some success in studies of the water balance of catechment areas. Applied to two classical American watershed studies it suggests that some of the Coweeta results may need reconsideration.