Application of the Sustainable Nitrogen Management Program in two small agricultural watersheds (Arquennes – Belgium) and impact evaluation by monitoring of nitrate fluxes at outlets and by numerical modeling. Four water catchments used by the Walloon Society of Water (SWDE) in the rural village of Arquennes (Province of Hainaut, Belgium) have been contaminated by nitrate (concentrations between 60 and 70 mg NO3-·l-1) for several years. The catchment basins (31 ha and 47 ha) are located exclusively in a cultivated area on loamy soils. Since October 2004, these basins have been used as a pilot area for the environmental evaluation of good agricultural practices as regards nitrogen management (adaptation of amounts of mineral nitrogen fertilizers, farm manure management, soil cover after harvest, etc.) thanks to monitoring by farmers. This approach fits into the general pattern of the Sustainable Nitrogen Management Program (PGDA in French), which implements the European Nitrates Directive as part of Walloon legislation. The second objective of the working scheme is to study and to set up a decision-making tool aimed at preventing the contamination of water by agricultural nitrate around water catchments. Two numerical models (SWAT and FEFLOW®) were used together in order to simulate the passage of water and nitrogen from the soil surface to the water catchments outlets. Calibration and validation of models were based, among other things, on soil and subsoil characterization and on tracer testing in both saturated and unsaturated zones. Moreover crop yield, farming practices, water and nitrate fluxes in water catchments, groundwater levels and nitrate concentration in groundwater were monitored for eight years. The results of simulations show that the application of PGDA in Arquennes watersheds would lead to a decrease in nitrate concentration (to almost 50 mg NO3-·l-1) in water catchments in 2012. This expected improvement in water quality has been visible since 2009 in water catchments, with mean nitrate ...