| Water and soil resources are most important for human beings survival and development. Soil erosion is a major cause for land resource degradation and the total destruction.The Three Gorges Reservoir (TGR) region is a very severely eroded region in China. The soil erosion and sedimentation of this region have been concerned by government and researchers for years. But the research techniques in this region are traditional and inefficient. The study on tempo-spatial dynamic changes of soil erosion and sediment using physically based distributed model at the large-watershed scale is almost a bank research branch in this region.AnnAGNPS, a continuous-simulation, watershed-scale model, is widely used to evaluate non-point source pollution from agricultural watersheds in a number of countries. AnnAGNPS simulates surface water, sediment, nutrients, and pesticides leaving the cells and their transport through the watershed. It is an expansion of the capabilities of the single event AGNPS. The watershed is subdivided into homogenous land areas (cells) with respect to soil type, land use, and land management. These areas can be of any shape from the original square grid cells of AGNPS to more appropriate hydrologic boundaries that can be generated by terrain-following Geographical Information System (GIS) software. The model enables managers to identify sensitive or critical areas of non point source pollution and to perform and evaluate various"what-if"scenarios in the decision-making process related to watershed management and specific water-quality management.The Daning River Watershed with a contributing drainage area of about 4 181 km2 was selected as the case study. A large number of basic data including digital elevation model (DEM), landuse, soil category and crop management, were collected through the extensive on-site investigation to set up model databe. Based on the calibration and validation of the model, the model was applied to evaluate the tempo-spatial dynamic changes of erosion and sediment, analyze the sediment delivery ration (SDR) of Daning River Watershed and its six tributaries and simulate two best management practices. The main conclusions are drawn as follows:With the aid of 3S (GIS, Remote Sensing, Global Position System) technology, the AnnAGNPS model database was set up in the area by analysis of rainfall characteristics, size of simulating cells, translation of soil relative parameters, characteristic of land use management, and so on.The model was calibrated using the monitoring runoff data at Wuxi hydrological station in 1998. The model was validated on the basis of the monitoring runoff data between 2003 and 2005 at the station and average annual sediment loading data from Dachang hydrological station. The calibration result for runoff is very satisfactory (relative errors -1.09% by year and Nash-Sutcliffe coefficient (NS) 0.97 by month). The regression of the monthly predicted runoff with the observed runoff on the line of equal values is good with a R-square value of 0.97 and a slope of 0.94. The monthly predicted runoff also matches well with the observed runoff in the validation (relative errors < -12% by year and r2> 0.90, NS > 0.80 by month). The model predicted average sediment loading over the eight-year period from 1998 to 2005 is more than 1.8% of the observed data. The predicted runoff and sediment loading compares fairly well with the observed data indicating that the model has an acceptable performance in simulation of runoff and sediment in Daning River Watershed.The tempo-spatial variation of soil erosion and sediment of the watershed is found.The tiny erosion area of the Daning River Watershed has come to 1 996.8 km2, accounting for 47.76% of the total watershed area. The annual average soil erosion rate of the watershed is found to be 1 551.7 t·km-2·a-1, and the rate is 2650.6 t·km-2·a-1 if tiny erosion area is not included. The soil erosion rate and erosion amount vary greatly among different land use types. The sloping lands and grasslands with low coverage cover 22.33% and 5.34% of the total area respectively, however, their erosion amounts accounted for 55.59% and 10.90% of that in the watershed. The study demonstrates that sloping lands and grasslands with low coverage is the main source of soil loss and the key of soil loss control is to utilize them rationally.The sediment budget of the watershed clearly shows that the average annual erosion amount of the Daning River Watershed is 6.488 million tons, 48.97% of the sediment is deposited in the reach networks, 21.05% is deposited in the cells and the rest is the output of the watershed. In the Daning River Watershed, runoff is positively correlated with sediment loading and annual and interannual variations of them vary greatly. The runoff and sediment losses mainly occurred between June and August. More than 64% and 74% of total runoff and sediment loading, respectively, are lost during this period. July is the peak month and more than 27 % and 34% of total runoff and sediment loadings are lost. Sediment loading is related to total annual runoff, especially to runoff distribution and intensity.The characteristics of sediment delivery are studied. Xixi River, one of the biggest tributaries in the Daning River Watershed, has largest runoff loading contributions up to 20.97% of the watershed outlet. But Dongxi River, the second tuiburary, has largest sediment loading contributions up to 26.88% of the watershed outlet. The area of left bank from the watershed main stream from Wuxi to Dachang is also the sediment source. Viewed from three main stream stations, i.e. Wuxi, Dachang and the watershed outlet, the sediment delivery ratio (SDR) is positively correlated with sediment loading and negatively correlated with the watershed area. But the relations are not significant. In the six tributaries, the maximum SDR is Xixi River up to 0.531 and the minimum SDR is Houxi River up to 0.376. The SDR of Daning River Watershed is 0.30, consistent with relevant conferences.Two BMPs have been made quantitative assessment with the AnnAGNPS before and after applied, and this method gives the scientific basis for soil erosion and sediment controlling measures. Reconverting farmland with larger than 25 gradients to forest with medium coverage will reduce 24.5% of the total erosion amount and sediment loading.The study demonstrates that AnnAGNPS model is successfully applied to research and control soil and water loss of Daning River Watershed, Three Gorges Reservoir Region. The model is a research tool and practical means for optimal watershed management and rational landuse resources, and can be applied for other comparable areas of the Three Gorges Reservoir Region.
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