Water use by forests, limits and controls.

Based on a review of several studies that have been carried out to determine the water use of forests in relation to other crops in different regions of the world, it is shown that the principal controls on evaporation from forests and shorter crops vary markedly between the temperate and tropical regions and between the wet and dry zones of these regions. Although there are detailed physical and physiological models available that allow calculation of forest water use, these models are not always readily applicable. It is proposed that a knowledge of the limits on evaporation can be used to devise models of varying complexity for estimating water use of forests in different regions and for predicting differences in water use between forests and shorter crops. Limits on evaporation may be related to radiation, advection, tree physiology, soil water, tree size or drop size. Examples are given of the use of models based on the limits concept for solving forest related water resource problems in Malawi and the U.K.