The impact of forest cover on streamflow has been studied in sub-Mediterranean mountain watersheds using the daily time step conceptual rainfall/runoff model GRHUM. Certain characteristics (pertaining to the soil, vegetation or atmosphere) are used to compute model parameters that describe the behaviour of the various interfaces with respect to water exchange by evapotranspiration. This model enables the simulation of runoff and soil water content evolution. A snow function has been added to the initial version of the model to simulate the hydrological behaviour during the snow melt periods. This model has been applied to the Mont-Lozere experimental watersheds, for which hydro-meteorological data have been available since 1981. A site where a gradual felling of the forest was prescribed between 1987 and 1989 (the Latte watershed) is compared to an unexploited forest site (the Sapine watershed), used as reference. The period covered by the study of the Latte site has been divided into three parts, on the basis of changes in land cover. For both watersheds, the model produced good results over all three periods. It is also possible to identify the role of forests on hydrological behaviour by simulating discharge from the Latte watershed during the felling period using adjusted pre- and post-felling model parameters. This exercise has shown that felling induces a significant increase in high flow rates. A similar approach conducted for the Sapine reference watershed made it possible to evaluate the validity of our model predictions (accurate to within 10% of true values) , the model bias being mostly due to climatic differences between the three simulation periods. Taking into account this model bias, results suggest that felling may increase runoff by 10%.