Benefit of coupling the ISBA land surface model with a TOPMODEL hydrological model version dedicated to Mediterranean flash-floods

The ISBA land surface model and a version of the TOPMODEL hydrological model have been coupled to simulate Mediterranean flash-floods. This coupling makes use of the watershed topography to compute the sub-surface lateral water fluxes and spatial and temporal dynamics of the saturated areas, following the TOPMODEL principles. The ISBA model governs the overall water budget and estimates the runoff supplied to the flow routing model. When applied to six flash-flooding events that occurred recently over South-eastern France the coupled system proved its ability to better forecast both timing and intensity of the flood peaks compared with the ISBA model used alone. To highlight further the differences between the two systems, an idealized framework was set up. The benefit of using ISBA–TOPMODEL instead of the ISBA model alone is clearly shown and leads to more physically consistent soil moisture and discharge simulations. The coupled system is able to simulate soil moisture and discharges that are not only governed by the rainfall, but also by the topography of the watershed.