| Water quality is becoming an increasing concern in China. The nonpoint source (NPS) pollution is the most important contributor for water quality deterioration in some basin. As a main inflow river of the Taihu Lake, the water quality of Tiaoxi River is very important for local community. With effective control of point source (PS) pollution in the past years, contribution to water pollution from NPS becomes significant in Tiaoxi Watershed. Nonetheless, studies in simulating the migration and transformation of NPS from agricultural activities were limited in the Tiaoxi Watershed. Effective measures have not been put forward to fundamentally solve the problem of water pollution. In order to control agricultural NPS pollution, the contribution of different NPS was analyzed in Tiaoxi Watershed using the methods of mathematical statistics, Geographic Information System (GIS) and watershed model. The key control area of soil erosion and NPS pollution was determined. The effect of BMPs on NPS control in Tiaoxi Watershed was evaluated.The main contents and results were showed as follows:(1) The Soil and Water Assessment Tool (SWAT) was implemented in the East-Tiaoxi Watershed in Zhejiang, China, where NPS pollution was widely concerned. The SWAT was calibrated and validated using daily streamflow and monthly water quality records from 1999 to 2008. The coefficients of determination (R2) were 0.90,0.96,0.89,0.72 in calibration and 0.83,0.80,0.81,0.74 in validation for runoff, sediment, ammonia and total phosphorus, respectively. The simulation results were fine and the model was suitable for the study area.(2) Based on the platform of GIS and universal soil loss equation (USLE), the soil erosion quantities of East-Tiaoxi Watershed were analyzed and evaluated. Results showed that the soil erosion was mainly impacted by hydraulic erosion. The annual average of soil erosion modulus amounted to 2347 t/km2 which fell into the category of mild-degree erosion. As for total region, the soil erosion was stronger in southern and northern parts than that in central part. For different land utilization, the annual average soil erosion intensity was significantly different. The density of soil erosion from high to low was ranked as follows:urban>rural>farmland>forest>orchard. The medium and serious erosion happened in steep slope and farmland (7°～12°), while the intensive and very intensive erosion happened in the farmland with abrupt slope (>12°), urban land and rural land.(3) The indicator system of NPS pollution risk assessment including pollution source statistics, soil erosion intensity and NPS simulation was established. According to the NPS pollution risk evaluation results, the area ratio of extremely sensitive zone, highly sensitive zone, moderately sensitive zone, lowly sensitive zone and slightly sensitive zone in East-Tiaoxi Watershed was 24.3%,38.4%,11.4%,16.8% and 9.1% respectively. Soil erosion was the main source of soil nutrient loss which was affected by rainfall, landuse, topography and vegetation cover. The nutrient chemical form directly impacted nutrient loss.(4) Six management scenarios impacting nutrient loss were applied and analyzed. These scenarios included fertilizer management (e.g.10% decline in fertilizer,20% decline in fertilizer), crop management (e.g. strip cropping, furrow cropping) and landscape change(such as 5% area changed from farmland to forest, and 10% area changed from farmland to forest). The results revealed that strip cropping was more efficient than the other implemented management practices. The NPS pollution removal efficiency was 52.48%,31.2% and 31.09% for sediment, organic nitrogen, and organic phosphorus loadings, and the concentrations of sediment, ammonia and total phosphorus in rivers were decreased by 9.3%,27.2% and 25.3% in the scenario of strip cropping. Landscape change played a positive role in the reduction of pollution load and the improvement of water quality, whereas fertilizer management did not lead to the effective reduction of the NPS pollution in the East-Tiaoxi Watershed. |
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