The paper describes the implementation of a macroscale hydrological model in the Rhone river basin. The hydrological model is coupled with a soil-vegetation-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 basin on soil, vegetation, hydrological regimes, and atmospheric variables is briefly described. The coupled model is forced by observed atmospheric quantities during 1 year. The simulation results are discussed with respect to stream flows, soil water content, runoff, and surface fluxes. The simulation clearly shows the importance of topography and snow on the hydrological regime 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 processes. On the other hand, the areas experiencing Mediterranean climate conditions (large annual global radiation, low precipitation) are characterized by negligible annual runoff. The simulation is used as a reference to test aggregation methods accounting for the subgrid variability of surface processes within a large area (128 km by 128 km). It is shown that the aggregated surface fluxes, drainage and runoff can be computed with an error lower than 5%, provided that the subgrid variability of precipitation, runoff, and vegetation is taken into account. If these subgrid processes are not aggregated, the errors in the simulation of the various terms of the water balance may exceed the annual reference by 20%. ¿ 1999 American Geophysical Union