| As the agricultural NPS is the most important factor for water environment deterioration and lake eutrophication in China, more and more scientists and technologists focus on the control countermeasure and pollution mechanism of agricultural non-point source. Chakou watershed is a agriculture region. In recent years, with people’s environmental awareness constantly strengthened, they practice a series of engineering measures to control the loss of the soil, water and nutrients in the watershed, such as returning the farmland to forest project, warping dam engineering, turning hillsides into terraced fields. These projects largely reduced the generation of pollution, but there are also a certain number of loss of sediment and the nutrition from the monitoring data in the outlet of watershed. How to determine and research the quantity of the potential pollution sources in a region constructed with warping dams becomes urgent task of the next step to comprehensively manage the small watershed. Terrace engineering is an important watershed management measures. It has the very high theory value and academic significance through the model to analysis the function of the terrace engineering.AnnAGNPS model is a distributed model for simulating surface runoff, deposit sediment, pollution load coming from watershed. This model has the characteristics of wide application prospect, effective, plentiful parameter data requirements, different places have different value and difficult to achieve. This paper takes the Chakou watershed in loess hilly-gully region as a case. Designed and built the AnnAGNPS models database of Chakou watershed, including climate, land use, soil and topography database. Then, assessed the suitability of AnnAGNPS model in the region constructed with warping dams; Thirdly, identified the critical source area of non-point source pollution; Lastly, analyzed the effect of turning hillsides into terraced fields on the events rainfall runoff, sediment, nitrogen and phosphorus load. The main conclusions are as following:(1) Through a lot of field survey, based on deeply analysis of the meteorology, topography, soil, land use management and other related parameters, this paper established the AnnAGNPS model database with certain accuracy and that database could be suitable for the loess hilly-gully region with middle scale.(2) The main goal of this study was to evaluate the performance of AnnAGNPS in simulating runoff and soil erosion in Chakou watershed. The model was validated using five observed stream flow, sediment, nitrogen and phosphorus load data. The result indicated that runoff volumes predicted by AnnAGNPS compared well with the measured data, The relative error of runoff was from-21.46%to2.26%,(R2=0.9922). However, AnnAGNPS failed to calculate sediment, the relative error of sediment was from-98.28%to24.62%. But it could simulate some storm sediment erosion, nitrogen and phosphorus load and theirs trend. Overall, The simulation results showed that AnnAGNPS had the potential to be used as a valuable management tool in Chakou watershed.(3) Seen from the spatial distribution, non-point source pollution load were uneven distributed in space. There were a big difference in the pollution load with the difference of the way of land use, soil types and operation management. Looked from the probability distribution of sediment loss of per unit area, the sediment erosion coming from the slope cropland, dam land, plain land and garden disturbed by human were larger than that coming from grassland and forest with little human disturbance and the slope cropland was the most serious region of sediment erosion. Nitrogen loss had two main sources, fertilization and soil nitrogen and phosphorus loss mainly come from the soil phosphorus. The most serious area of nitrogen loss was located in both sides of the main channel. The loss distribution of the nitrogen and runoff had the certain similarity. The most phosphorus loss per unit area were farmland planting soybean.(4) With the slope of cropland turning to terrace, the total of rainfall runoff, sediment, nitrogen and phosphorus load decreased, and it had a larger influence on big rainfall events. When the cropland with slope between15°-20°turning to terrace, the decrease of the runoff was maximum and after that the runoff showed a trend of decrease respect to the reduction of the slope of cropland. Effect on sediment loss of per unite area become lower with the reduction of the slope. The degree of reduction of nitrogen and phosphorus load was most obvious at the cropland with the slope>20°turning to terrace. The nitrogen loss of per unit area was at a turning point when15°-20°sloping land turning into terraced fields. The degree of reduction rapidly became lower and after that the influence on nitrogen output per unit area was similar with different slope of cropland turning to terrace. Analysis from the figure of rainfall runoff of the average per unit area, slope cropland turning to terrace had little impact on the most serious water loss areas where these region were not the cropland. The trend of the change of sediment and phosphorus were similar with/to that cropland turning to terrace and reduced the number of the most serious plot, and the change tendency of erosion was from the most serious level directly into minimum level. It also reduced the number of the most serious plot, and changed the other area with serious erosion grade, but the change tendency of erosion was from high level to lower level erosion level. |
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