| In the21st century, with the development of economy, water pollution has become a crutial issue in China. With gradual control of point source pollution, nonpoint source pollution has upgraded to a main soure of water pollution. Xitiaoxi watershed is a vital water source in the upstream of Tai Lake watershed, and water quality of Tai Lake is closely linked to that of Xitiao Creek. However, that lacks of systematic and comprehensive studies on nonpoint source pollution of Xitiaoxi watershed is difficult to provide strong and powerful scentifical support for control and management of nonpoint source pollution. In view of this, taking Xitiaoxi watershed in Anji County,Zhejiang Province as a study area, the export coefficient method was used to estimate the export of nonpoint source pollution loadings and water quality index(WQI) was applied to identify water quality. Then based on geographic information system (GIS), SWAT model was utilized to stimulate and evaluate nonpoint source pollution of Xitiaoxi watershed to understand the temporal and spatial distribution of nonpoint source pollution loadings and the law of migration and transformation of that, supposed to provide reference basis for control and management of nonpoint source pollution.The main research results were as follows:(1) The total export of chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH3-N) was2236.12t,1007.22t,78.11t,82.90t. Moreover, it was found that rural sewage, agricultural and forest lands and rural industrial wastewater were main sources of COD, while agricultural and forest lands was the largest contributor to TN. Most of TP was contributed by rural garbage, livestock and poultry and agricultural and forest lands, and NH3-N mostly came from livestock and poultry and rural industrial wastewater. Spatially, the nonpoint source pollution loadings were mainly concentrated in midstream and downstream areas of Xitiaoxi watershed. The exports of nutrients such as COD and TP and NH3-N increased contantly from upstream to downstream areas while TN had similar exports.(2) All tributaries of Xitiaoxi watershed presented obvious temporal variation that water quality gradually improved from January to March (or May) and then declined from March (or May) to July, the worst in July, and then improved gradually again from July to November in Xitiaoxi watershed. However, the overall water quality of Xitiaoxi watershed was good,showing that water quality gradually declined from upstream to dowmstream,the worst in midstream and the water quality of all tributaries declined but Xiaoshu Creek.(3) The annual average export per unit area of sediment,organic nitrogen, nitrate nitrogen, ammonia nitrogen, organic phosphorus and mineral phosphorus of Xitiaoxi watershed in Anji County, Zhejiang Province from2000to2008, respectively, was2508.35t/km2,277.01kg/km2,553.01kg/km2,103.18kg/km2,30.88kg/km2and42.85kg/km2. The rainfall and runoff had a huge impact on sediment of Xitiaoxi watershed. The rainfall and runoff and sediment had a huge effect on nonpoint source pollution loadings of Xitiaoxi watershed, in Anji County, Zhejiang Province, showing from high to low as follows:sediment> runoff> rainfall. The sediment and nonpoint source pollution loadings, which were mostly concentrated on sufficient rainfall period, had the same trend with rainfall and runoff. The exports of sediment and nonpoint source pollution loadings were mainly from the middlestream and downstream of Xitiaoxi watershed in Anji County, Zhejiang Province. The main form of loss of nitrogen loading was soluble, while that of phosphorus loading was particle. Runoff and soil erosion were the main sources of nonpoint source pollution of Xitiaoxi watershed in Anji County, Zhejiang Province. Therefore, when managing and controlling nonpoint source pollution, we should reasonably use chemical fertilizer and strongly take control of pollution sources as well as take positive and effective measures to improve vegetation coverage and reduce runoff and soil erosion. |
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