Non-point source pollution from agricultural practices is of serious concern within the Boutonne 1,200 km² wide watershed, located in the southwestern France. National and local stakeholders are concerned and willing to get a global assessment of the water resource and a pertinent decision support for targeting appropriate measures on priority areas for restoring the water quality and complying with the European Water Framework Directive. The ETBX team of the National Research Institute of Science and Technology for Environment and Agriculture (Irstea) has developed a method for assessing the impact of agriculture on freshwater quality and aid tools for public decision, whereas the French geological survey (BGRM) of Poitiers specializes in groundwater assessment and monitoring. Thus the “Adour Garonne” basin water agency asked them for a global water assessment, through coupled modeling of the agro-hydrological Soil and Water Assessment Tool (SWAT) –GenLU2 framework and the fully distributed hydro-geological MARTHE model. The first step of the Irstea-BRGM collaboration has been to evaluate the external coupling feasibility in terms of time calculations, temporal and spatial processing scales, and to identify the correct SWAT parameters for entering lixiviated nitrate from the HRUs (calculation units) into the MARTHE meshes (km wide). The second step is to develop a specific interface for internal model coupling so as to simulate on a global SWAT - MARTHE application the hydric and nitrogen balance linked to the agricultural landuse. The ultimate step will be to widen the current modeled area up to the whole Charente watershed (10,000km² wide) and to transfer the application to the local stakeholders.