1 The Impact of Interception Losses on the Water Balance in Forested Mountain Ranges

Abstract: Although it is commonly admitted that forest reduces annual runoff, the amount of the reduction may vary considerably as a function of the soil and climatic conditions. Forest enhances evaporation through two main processes: 1) Deeper root systems use the water stored in soil more efficiently during the summer period. As a result, more water is retained in the soil during the following autumn before the resumption of winter discharge, and annual runoff is reduced. 2) Loss by interception is greater in forested areas than for other types of vegetation cover during the winter months, mainly because of more efficient use of advective energy. Studies in small catchments on Mount Lozère (South of France) have shown that during the winter period, "actual " evapotranspiration (calculated by the water balance method) is higher than "potential " evapotranspiration (estimated using a standard equation). These differences are due to interception losses. During the study period, one small spruce-forested catchment was cut and replanted, while another grassland/heath catchment was left undisturbed. Interception losses for the two basins were compared. The study period (1982-1995) covered the pre-cut (1982-1987), cutting (1987-1989) and postcut/regrowth (1990-1995) periods. Results show that cutting the forest did reduce interception losses. However, the hydrological behaviour of the cut catchment changed back to its pre-cut behaviour relatively quickly and clearly before the new plants had developed enough to be considered as forest cover. Keys-words: Forest hydrology; interception; forest evaporation