Sensitivity of the continental hydrological cycle to the spatial resolution over the Iberian Peninsula

In the broad context of the downscaling methods that are used to study climatic change impacts, the dependence of the surface hydrological processes simulated by the Organising Carbon and Hydrology in Dynamic Ecosystem (ORCHIDEE) land surface model, used in a stand-alone mode, on the spatial scale of the forcings is investigated over the Iberian Peninsula. These prescribed forcings are the outputs of a regional climate model, Pronóstico a Mesoescala (PROMES), with a high spatial resolution (20 km). In the first experiment, the PROMES outputs have been aggregated stepwise to the typical resolution of a general circulation model, and applied to ORCHIDEE, in order to analyze the impacts of the changing resolution on the simulated water balance. Then, subgrid-scale variability (SSV) for the different forcings has been progressively reintroduced. This second experiment is aimed at isolating the crucial elements of SSV that need to be preserved when a disaggregation is being performed. The increase of interception loss when the spatial resolution goes beyond 100 km leads to unrealistic values of the interception loss ratio. In the northern humid region, the reduction of runoff frequency when the forcings are aggregated explains the decrease in runoff production, which can reach half the high-resolution runoff. These impacts drive the adjustment of the other hydrological components. The large increase of interception loss is compensated by a reduced transpiration in a dry climate, which induces a large change in soil moisture content, and by reduced runoff in humid regions. The second experiment underlines the dominant effect of precipitation SSV, and particularly the rainfall frequency, on the correct simulation of the water balance. The significant influence of the thermodynamic variables is also analyzed.