The paper presents a complex environmental engineering tool, which is appropriate to support decision making in watershed management. The PhosFate tool allows planning best management practices (BMPs) in catchments and simulating their possible impacts on immissions. The method has two parts: (a) a simple phosphorus (P) fate model to calculate diffuse P emissions and their surface transport, and (b) an interactive tool to design BMPs in small catchments. The fate model calculates diffuse P emissions via surface pathways. It is a conceptual, distributed parameter and long-term (annual) average model. The model also follows the fate of emitted P from each cell to the catchment outlets and calculates the field and in-stream retention. The fate model performed well in the Zala River catchment as a case study. Finally, an interactive design tool was developed to plan BMPs in the catchments and simulate their possible impacts on diffuse P fluxes. Different management scenarios were worked out and their effects evaluated and compared to each other. The results show that the approach is suitable to test BMP scenarios at small catchment scale.