Evaporation from unsaturated surfaces: A generalized combination method

The ‘combination method’ for determining evaporation uses both the energy balance and a Dalton-type transport formula. When evaporation is limited by water supply to the evaporating surface and saturation deficit at the surface obtains, a form of the combination method has been used to describe the evaporation in terms of a vapor ‘diffusion resistance’ of the surface, which is related to the surface saturation deficit. We show that this diffusion resistance model is not applicable either to vegetation canopies or to soil, and we have developed a combination formula relating the evaporation from a drying soil to the potential evaporation. This method uses the surface temperature as the only additional measurement to the standard combination formula for potential evaporation, and it compares favorably with the energy balance-Bowen ratio method over a drying soil.