Distributed sediment and phosporus transport modeling on a medium sized catchment in central germany

Current research suggests that strategies to control nitrogen as well as sediment and phosphorus loss from nonpoint sources should focus on different runoff components and their spatial and temporal variation within the catchment. Therefore nonpoint source models, especially in humid climatic regions, should consider variable hydrological active source areas. These models should also be able to consider runoff generation by saturated overland flow as well as Horton overland flow. A combination of the hydrologic model WaSiM-ETH and the erosion and P-transport model AGNPS Version 5 was chosen for this study. The models were run in the WaSiM-ETH runoff generation mode (Topmodel and Green a. Ampt approach) and the SCS curve number mode, modified by Lutz (1984), to assess these different runoff calculation procedures. A medium sized catchment in the lower Hessian uplands of Germany was selected for the present investigation. The results show that the combined Topmodel/Green and Ampt runoff approach produce reasonabe results for the simulation of runoff volume for single runoff events at the catchment outlet. The more physically based model approach computed a much more realistic distribution of runoff and P yield producing areas than the SCS curve number method, modified by Lutz (1984). Reasonable results were achieved for sediment yield calculations. Particulate P calculations showed larger deviations between measured and calculated values.