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Summary: 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. [ABSTRACT FROM AUTHOR]

Innovative coupling between the soil–vegetation–atmosphere transfer (SVAT) model Interactions between Soil, Biosphere, and Atmosphere (ISBA) and the hydrological model TOPMODEL has been specifically designed for flash-flood forecasting in the Mediterranean area. The coupled model described in this study combines the advantages of the two types of model: the accurate representation of water and energy transfer between the soil and the atmosphere within the SVAT column and an explicit representation of the lateral transfer of water over the hydrological catchment unit. Another advantage of this coupling is that the number of parameters to be calibrated is reduced by two, as only two parameters instead of four parameters concern the TOPMODEL formulation used here. The parameters to be calibrated concern only the water transfer. The model was calibrated for the simulation of flash-flood events on the three main watersheds covering the French Cévennes–Vivarais region using a subset of past flash-flood events having occurred since 2000. The complementary subset of flash-flood events was then used to carry out an objective verification of the coupled model after calibration. The evaluation on these six independent past flash-flood events shows satisfactory results. The comparison of the observed and simulated hydrographs demonstrates that no flash-flood peaks are missed. Relevant information for flash-flood forecasting can always be inferred from the simulations, even for those with quite poor results, making the system useful for real-time and operational flash-flood forecasting. [ABSTRACT FROM AUTHOR]