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.
In a recent paper in this journal, Bradshaw and colleagues analyse country statistics on flood characteristics, land cover and land cover change, and conclude that deforestation amplifies flood risk and severity in the developing world. The study addresses an important and long-standing question, but we identify important flaws. Principal among these are difficulties in interpreting country statistics and the correlation between population and floods. We review current knowledge, which suggests that the removal of trees does not affect large flood events, although associated landscape changes can under some circumstances. Reanalysis of the data analysed by Bradshaw and colleagues shows that population density alone already explains up to 83% of the variation in reported flood occurrences, considerably more than forest cover or deforestation (o10%). Feasible explanations for this statistical finding -whether spurious or causative -are not difficult to conceive. We, therefore, consider the conclusion of Bradshaw and colleagues to be unsupported. However, their study is a valuable first step to show how these or similar flood data might be used to further explore the relationship between land cover and flooding.
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