Modelling scenarios of agriculture changes on freshwater uses and water quality at a large watershed scale – the case of the Charente watershed (France)

The Charente River Basin (10 000 km2) is linked to the Pertuis Charentais Sea by a large tidal influence. The agriculture impacts strongly the quantity and the quality of freshwaters and influences the salinity and the quality of coastal waters needed by the oyster culture. In the context of 2 projects dealing with ICZM (EU Spicosa project, Respireau), we focused on the environmental assessment of spatialised scenarios of changes in agriculture. Different methods of clustering and data analysis were used to define a relevant typology of spatial units on the area. Combined with an analysis of agricultural practices based on surveys (irrigation, fertilizers and pesticides), the agricultural activities can be described in a simplified way, to be linked with hydrological models regarding the water quantity and quality. Scenarios are built using surveys, expertise and discussion with stakeholders. Measures for agriculture changes are either considered in the agro-environmental programs or defined by stakeholders. Their effectiveness on the reduction of nutrients and pesticides fluxes is assessed when applied on the whole area or within sub-basins, depending on the farming system and vulnerability. Assessment was carried out using the spatially distributed model SWAT. The effectiveness of each scenario is considered in terms of relative reduction of water uptake for irrigation, nitrates, pesticides, suspended matter compared to the current situation.