[Characteristics and Pollution Source Analysis of Nutrients in Tributary Outlets of Xitiaoxi Watershed]

The input of tributary is the important source of nutrients to the main stream, and it is the key area for water pollution control of watershed. In order to explore the sources of nutrient and seek the effective measures to control the river pollution, the spatial and temporal variations of aquatic parameters and the output of nutrient flux in the tributary outlets of Xitiaoxi watershed were analyzed. The quantitative analysis concerning the contribution of pollution sources from 10 typical tributaries was carried out, using the PMF analytical model. The results showed that the TN and TP concentrations were higher in the middle and lower reaches. In the temporal scale, the concentrations of nutrients in the dry season were higher than those in the wet season. In the dry season, average concentrations of TN and TP were 4.25 mg·L^-1 and 0.11 mg·L^-1, respectively, compared to the corresponding values of 3.15 mg·L^-1 and 0.09 mg·L^-1 in wet season. Because of the differences of surrounding lands, the sources of nitrogen and phosphorus forms were various. The results of PMF(Positive Matrix Factorization) method identified the three nutrient sources as agricultural source, aquaculture source and municipal source. In different seasons and river reaches, the contribution rates of three sources were different. Agricultural source contributed nutrient largely to the tributaries in the upstream reaches, and the aquaculture source was the main contributor in the middle reaches, while the municipal source contributed more nutrient in the wet season than in the dry season. Therefore, to better improve water quality of Xitiaoxi watershed, the surrounding environment of tributary and the characteristics of nutrients in spatial and temporal variations should be considered.