Simulation of the water budget and the river flows of the Rhone basin

The paper describes the implementation of a macroscale hydrological model i n the Rhone river basin. The hydrological model is coupled with a soil vege tation - atmosphere transfer scheme in order to resolve the daily cycle of the surface energy balance and the water budget. The water surface routing and the water table evolution are computed in the hydrological model with a daily time step. First, the important database collected in the Rhone basi n on soil, vegetation, hydrological regimes, and atmospheric variables is b riefly described. The coupled model is forced by observed atmospheric quant ities during 1 year. The simulation results are discussed with respect to s tream flows, soil water content, runoff, and surface fluxes. The simulation clearly shows the importance of topography and snow on the hydrological re gime of the Rhone and its tributaries. The simulated spatial variability of evaporation and total runoff are very large within the basin. Small annual evaporation and large runoff are found in the Alps because of the snow pro cesses. On the other hand, the areas experiencing Mediterranean climate con ditions (large annual global radiation, low precipitation) are characterize d by negligible annual runoff. The simulation is used as a reference to tes t aggregation methods accounting for the subgrid variability of surface pro cesses within a large area (128 km by 128 km). It is shown that the aggrega ted surface fluxes, drainage and runoff can be computed with an error lower than 5%, provided that the subgrid variability of precipitation, runoff, a nd vegetation is taken into account. If these subgrid processes are not agg regated, the errors in the simulation of the various terms of the water bal ance may exceed the annual reference by 20%.