| On the basis of previous Taihu Basin rainfall-runoff model, a mended rainfall-runoff model is used to solve the contradiction between the situation that water volume is too much when all paddy fields are irrigated to suitable water depth and the situation that water depths in the subareas are different with the result that irrigation times are not same. The west of Zhangjiagang city was divided into many subbasins with GIS, DEM, and the nonpoint source pollution load of these subbasins is calculated .The model calculation result is benefit to reduce the nonpoint source pollution.There is no point source pollution around the water resource protected area, so water pollution in the rivers come from nonpoint source pollution. The regressive relation between precipitation and CODMn of Xingan water quality station in the 2002 is researched. And the relation among water quality indexes is built with Cluster Analysis branch of Multivariate Statistical Analysis.In order to improve the water environment of Zhangjiagang City, the nonpoint source pollution must be prevented, in this thesis, Best Management Practices (BMP) is introduced. BMP as a practice or combination of practices are effective, practicable means of preventing or reducing the amount of pollution generated by nonpoint source pollution. According to actual circumstance of Zhangjiagang, Wet Detention Pond and Grassed/Vegetated Swale are built to prevented the nonpoint source pollution.
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