Conséquences des forêts sur l'écoulement annuel des cours d'eau

Abstract

Runoff results from the difference between rain and evaporation. Hence, knowledge about hydrological consequences of forests requires knowledge on how forests influence both rainfall and evaporation. Forest influences on rainfall mainly depend the scale considered. While it is often accepted that mass clearing of tropical forests on a scale such as that of the Amazon would result in reduction of rainfall, such an effect is probably imperceptible on the scale of local operations, whether clearing or new plantations. On the other hand, forests tend to foster evaporation for two main raisons - first, because of deeper rooting and better use of soil moisture; second, by better advection causing canopy-intercepted water to be evaporated. Different factors, mainly edaphic and climatic, combine in space and time and make for higher evaporation - and therefore less runoff - from forest catchments than from any other type of vegetation. Nevertheless, values are very scattered due the diversity of the combinations of factors behind this runoff reduction.