Résumé
The use of hydrological models for flood forecasting on karstic catchments implies having some information regarding the spatialization of runoff over the basin. Mapping the spatial variability of runoff during flood and recession periods is a challenge, specifically in karstic areas where zones characterized by a high infiltration rate can generate high runoff coefficient during exceptional rainfall events. Karstic areas covering almost 1/3 of France, it is useful to develop a methodology to map runoff karstic zones at a national scale.The aim of our work is to assess on a test basin, the ability of a geomorphological index to localize contributive areas from karstic zones during floods. Our approach uses the index of development and persistence of the river network, IDPR (Gay et al., 2016), as a proxy of infiltration/runoff areas. This index is computed by comparing the topographic thalweg network with the natural hydrographic network. It is available as a 25m spatial resolution grid over France. The test site is the Tarn basin at Millau in south France (2200 km²), characterized by hard-rock areas in the upstream zone (Cévennes) and by karstic areas in the intermediate and downstream ones (Causses). Daily streamflow time series were collected for 13 gauging stations over the past 30 years. Total discharge values (Qt) were separated into baseflow component (Qb) and flood component (Qf) using the BFI method.First results show a good correlation between the Qf/Qt ratio computed with interannual mean data, and the mean and median IDPR of each sub-basin. The IDPR could thus be an interesting approach to map runoff indicators characterizing notably highly contributive karstic zones during flood periods. A sensitivity analysis is also performed on parameters of this relationships, in order to assess the influence of i) information aggregation (catchments surface), and ii) hydrograph separation model. Our results give interesting perspectives on the regionalization of the method.