Un indicateur de vulnérabilité potentielle des eaux de surface aux produits phytosanitaires : évaluation sur le bassin versant de la Leysse (Savoie)

Potential vulnerability of the surface waters of the Leysse watershed (300 km(2)) to pollution by herbicides was assessed by two different conceptual approaches: the comprehensive physically based MIKE SHE-ADM model for simulating the coupled water and solute transfer on one side, a parametric representation of the main involved processes through the VESPP indice which relies on parameters that characterize in a simple manner topographical and climatological features of the site, persistence and transport of the agrochemicals, on the other side. The results show that the spatially distributed hydro chemical model gives a realistic representation of the functioning of the main hydrogeological units of the basin, after its fitting on observed series of streamflows rates and/or piezometry. It also allows to estimate the potential risk that the herbicides remaining at the soil surface would be exported by runoff The use of the VESPP indice calculated at the 1220 grid points of the discretized numerical model leads to the identification of areas of more or less vulnerability. Ranking correlation tests, show that their locations on the watershed are in statistical good agreement with those obtained by MIKE SHE-ADM, the two series being in a satisfying coherence with the physical features of the basin, the land use and the human activities as well. It is concluded that the VESPP indice may constitute a potentially useful tool for managers and policy makers of hydrographic basins, especially for those where data are scarce, even non existent and for which physically based distributed models are generally not well designed to be decision-making tools easily applicable.