Potential Evaporation – A Matter of Definition

The aim of this paper is to discuss the concept of potential evaporation and its use in runoff models. The potential evaporation for forest is defined on basis of estimated minimum canopy resistances for a well-watered spruce forest. The difference between the Penman open water evaporation, commonly used as “potential” evaporation, and a more realistic estimate of the potential evaporation from a dry forest showed a large scatter and a systematic seasonal deviation. Part of the differences were explained by differences in vapour pressure deficit. It was also shown that the evaporation rate of a completely wet forest was typically four times higher than the rate predicted by the Penman equation. The conclusion was that Penman open water evaporation did not give a good representation of forest conditions.