| There are lots of crisscross rivers and lakes in areas of plain river network region. The water in the river flows gently and water environmental carrying capacity is low, and a large number of wastewater discharged directly into the water originated from industrial wastewater, sewage, pesticides and fertilizers in agricultural irrigation with drainage, which causing the whole basin lake polluted seriously. Taihu Lake is the third largest freshwater lake located in areas of plain river network region in China. Fisheries, fruits, tea and some other industries developed rapidly with the agricultural structure adjusted in the Taihu Lake basin. Meanwhile, fertilizers and pesticides applied increased significantly. However, the capacity of small town sewage treatment was limited and the disposal rate was generally low. A lot of urban and rural waste water flowed into the water. It made most rivers polluted seriously. Furthermore, non-point source pollution problems in the small town became more prominent. The pollutant load increased fast, and the proportion of contaminations became much larger.The dissertation chose the Xueyan town as the representative area, which locates in Taihu lake basin in typical areas of plain river network region through comprehensive investigation. With collection of natural geography, social environment, agricultural production and so on, monitor the routine water quality regularly in the study area. It used single-factor analysis and BP artificial neural network method respectively to evaluate pollution situation in the Xueyan town. Adopting the means of combination of qualitative and quantitative analysis, field investigation and laboratory testing to analysis the law of nitrogen, phosphorus, and COD erosion in the watershed of village of Shangjian, Longquan river and Xuyan town in rainfall condition with the rainfall data from April 2009 to March 2010. Research the law of nitrogen, phosphorus, and COD loss along the variation of the Yapu river. Probe into the nitrogen, phosphorus, and COD loss under different land use, moreover, study the nitrogen and phosphorus mechanism deeply in the zone of the watershed of village of Shangjian. Based on the theory of watershed discretization in hydrological model, the typical catchment basin area was selected to study the non-point source pollution in a systematic, comprehensive and in-depth way. AnnAGNPS, Continuous distributed parameter model was used to calculate the pollutant load from non-point pollution. In order to testing the feasibility and application of this model in Xueyan Town, the database of the AnnAGNPS should be established first, and calibrate and validate the AnnAGNPS. Simulated the seasonal and annual nitrogen and phosphorus pollution load in the Xueyan town. Identified the key source zone of nitrogen and phosphorous. Analyzed the model parameter sensitivity, designed the pollution control management measures and quantitatively analyzed the best management practices effectiveness; put forward the corresponding agricultural non-point source pollution control measures. Through these work, the conclusions listed as follows:1.Monitoring the routine water quality regularly in the study area. It used single-factor analysis and BP artificial neural network method respectively to evaluate the degree of the surface water polluted and environmental quality status in the Xueyan town, to identify the key pollution indicators. The results showed that the water quality was very poor in Xueyan town, mainly attributed to class V or worse than class V, especially the total nitrogen pollution which polluted the most seriously,3.25 times more than V class standard. Among the three main rivers, Wujin river, Yapu river and Longquan River, the water quality of Yapu river was better than the other two. During the flood season, from June to September, the water quality was relatively good; during the dry period of November to April relatively poor. The water quality improvement of Yapu river was the most obvious in the flood season; the water quality of Wujin river and Longquan river uppermost deteriorated in the dry period.2.Research on the law of the process of rainfall at one time in the outlet of the watershed of Shangjian Village, Longquan river and Xueyan Town, discussion the characteristics of nitrogen, phosphorus and COD loss under different types of land use within the watershed of Xueyan town, analysis the characteristics of nitrogen and phosphorus loss of Shangjian Village watershed, the research results listed as follows:The concentration of nitrogen in the outlet of the watershed of Shangjian village, Longquan river and Xueyan town had a small change and presented a steady state over time. With the increasing of rainfall intensity, the concentration of nitrogen loss increased. Soluble nitrogen did not change significantly over time and nitrogen concentration in Yapu river had stable changes along the way, while the highest nitrogen output was in the industrial printing factory. On the upper reaches of the small watershed of Shangjian village, nitrogen concentration firstly decreased during the time and then showed a trend of increasing, while in the outlet downstream, total nitrogen concentration increased in the first period and decreased later, nitrogen concentration in the small basin of Shangjian village stably declined along the village. Higher total nitrogen output lied in the vegetable garden in the small basin. The ditch had high output of soluble nitrogen and nitrate nitrogen; the fish ponds had high output of ammonium nitrogen. In Xueyan town, soluble nitrogen was the main component of total nitrogen which mostly was nitrate nitrogen along with a small portion of ammonium nitrogen, especially in the small watershed of Shangjian village, the ammonium nitrogen was lower than 1 mg/L. Overall, the output nitrogen concentration was greater and the pollution was rather severe, exceeding Category V of National Surface Water Quality Standard (NSWQS) (GB 3838-2002)The loss concentration of phosphorus in the outlet of the watershed of Shangjian village, Longquan river and Xueyan town was slow and stable with low rainfall intensity and was larger with the increasing of rainfall intensity, while the concentration of soluble phosphorus loss was flat. The concentration of phosphorus loss along the Yapu river firstly decreased and then increased to a stable situation, and the highest output of phosphorus took palace in the agricultural market in the town. On the upper reaches of the small Basin of Shangjian Village, total phosphorus concentration firstly increased with time and then showed a trend of decreasing while phosphorus concentration increased along the ditch in the Shangjian Village Basin, the output total phosphorus concentration and soluble phosphorus was the highest in the vegetable area. In Xueyan Town, phosphorus consisted of particulate phosphorus mostly and occasionally it may occured that the concentration of soluble phosphorus was slightly greater than particulate phosphorus. In the small basin of Shangjian village the upstream was mainly made up of dissolved phosphorus while the downstream mainly particulate phosphorus and in soluble phosphorus the main state was dissolved reactive phosphorus. Overall, the phosphorus concentration of output was greater and the pollution was rather serious, belong to Categoryâ…£of NSWQS (GB3838-2002)In the outlet of the watershed of Shangjian village, Longquan river and Xueyan town, COD loss concentration took on a sawtooth-like and undulate-like state. On the way along the Yapu river there was a little turn to increase until the Yapu river bridge and COD concentration of output was larger. The COD concentration of industrial printing factory was much larger. Overall, COD concentration of output was not large and did not result in significant organic pollution, belong to Categoryâ…¢of NSWQS (GB3838-2002)3.Through some techniques of the data collection and field survey, indoor test analysis and mathematical statistics, the input parameters of the model, such as hydro-geologic condition, soil physical and chemical properties, and vegetation, are determined. With the application of RS(Remote Sensing Technology), GIS and GPS spatial orientation, combined with information provided by the Xueyan town government, Wujin district, the non-point source pollution spatial database, about the digital elevation model, soil data, data of land use and meteorological data etc, are established in the research area. Based on the synchronous monitoring data of rainfall events, we adjusted the parameters of the model, calibrated and validated its feasibility and applicability in Xueyan town. Simulated the seasonal and annual nitrogen and phosphorus pollution load in the Xueyan town. Identified the key source zone of nitrogen and phosphorous. Analyzed the model parameter sensitivity, designed the pollution control management measures and quantitatively analyzed the best management practices effectiveness; put forward the corresponding non-point source pollution control measures. The results showed that non-point source pollution was closely related with precipitation. Total nitrogen and total phosphorus pollution load peak in the summer, especially in June and July. Total nitrogen and the total phosphorus load accounted for 61.64% and 60.77%, respectively. Nitrogen and phosphorus load output concentrated in steep areas, flat areas were relatively mild. It was closely related with the fertilizer rate, fertilizer application methods, and the surrounding natural vegetation cover environment.
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