Validation and sensitivity of the global hydrologic budget in stand-alone simulations with the ISBA land-surface scheme

Global soil moisture data of high quality and resolution are not available by direct observation, but are useful as boundary and initial conditions in comprehensive climate models. In the framework of the GSWP (Global Soil Wetness Project), the ISBA land-surface scheme of Meteo-France has been forced with meteorological observations and analyses in order to study the feasibility of producing a global soil wetness climatology at a 1 degrees x 1 degrees horizontal resolution. A control experiment has been performed from January 1987 to December 1988, using the ISLSCP Initiative I boundary conditions. The annual mean, the standard deviation and the normalised annual harmonic of the hydrologic fields have been computed from the 1987 monthly results. The global maps which are presented summarise the surface hydrologic budget and its annual cycle. The soil wetness index and snow cover distributions have been compared respectively to the results of the ECMWF reanalysis and to satellite and in situ observations. The simulated runoff has been validated against a river flow climatology, suggesting a possible underestimation over some large river basins. Besides the control run, other simulations have been performed in order to study the sensitivity of the hydrologic budget to changes in the surface parameters, the precipitation forcing and the runoff scheme. Such modifications have a significant impact on the partition of total precipitation into evaporation and runoff. The sensitivity of the results suggests that soil moisture remains one of the most difficult climatological parameters to model and that any computed soil wetness climatology must be considered with great caution.