Abstract:Non-point source (NPS) pollution has become a global environmental issue and a widely discussed form of environmental degradation in recent years. Because of the nature of NPS and the limitations of assessing it through experiments and field measurements, its management is highly dependent on spatial simulation modeling which helps deal with the spatial uncertainty associated with NPS pollution. In the last three decades, several computer simulation models have been developed to provide a better understanding of hydrological systems, sediment transport and associated pollutant loading. Annualized AGricultural Non-Point Source (AnnAGNPS) is a continuous simulation, watershed-scale model intended to be used as a tool to evaluate NPS pollution. In the Three Gorges Reservoir area, non-point source pollution studies are still in their infancy. Although soil erosion has been a major problem in this area, erosion studies are very much limited to using the universal soil loss equation (USLE). The objectives of this study were to prepare a database for the simulation of runoff and non-point source pollution using the AnnAGNPS model, and to assess the applicability and predictive capability of the model as a long-term monitoring tool in Zigui county. The model was calibrated and validated using observed stream flow and sediment load data from the Heigou, Lanlingxi and Shamuxi watersheds in Zigui county. The calibrated model was then used to simulate runoff, sediment yield and nutrient export from the county. Digital elevation models were generated from digitized topographical data. Based on critical source area and minimum source channel length specifications, Zigui county was divided into 1966 cells and 986 channel reaches. Results showed that the model predicted the runoff volume of the three typical watersheds within an acceptable range. This indicates that the SCS curve number method used in the AnnAGNPS model is suitable for runoff volume prediction. Sediment yield predictions for the three typical watersheds were only moderately accurate but the predictions were improved by amending the input parameters for both the RUSLE and HUSLE sub-models. In general, the model performed well in simulating runoff compared to sediment and nutrient loadings. For smaller events, the model generally over-predicted sediment and nitrogen loadings, while under-predicting for larger events. The prediction for total phosphorus showed obvious uncertainties. The model predicted the export from Zigui county in 2006 of 363 mm of runoff, 19.6 t/hm2 of sediment yield, 122 kg/hm2 of total nitrogen, 28 kg/hm2 of total phosphorus, and 581 kg/hm2 of total organic carbon. The simulation results were in accordance with related studies and government statistics. As a watershed management tool, AnnAGNPS can be used to formulate different management strategies for soil and water conservation in hilly parts of the Three Gorges Reservoir area with mixed types of land use and steep slopes.