Thematic maps, statistical data, and field and farm sampling have been merged with digital satellite data to implement simulation models for managing the problems related to agricultural non-point source pollution. This has been done putting the different data sources and techniques into the same Geographical Information System (GIS). The integration of GIS with the environmental impact simulator and economic programming model was utilized for estimating the possible changes in environmental impact caused by the recent Reform in the Common Agriculture Policy (CAP) of the European Union. An economic programming model was built to evaluate land use changes induced by the CAP Reform under different hypotheses of technology adjustment. The entire data set has then been utilized as a geographical data base to run non-point source (NPS) simulation models to estimate the impact on water quality of the agricultural systems located within the study area. Nutrient (nitrogen) concentrations were the environmental indicators chosen to evaluate the impacts on water quality. In this way the system created by the linkage of GIS and simulation model (Spatial Environmental Impact Simulator, SEIS) has become a decision support system, for evaluating the effectiveness of environmental interventions on a regional scale.