Characteristics Of Nitrogen And Phosphorus Loadings Into Receiving Water Body And Migration In Xiangxi River Basin, Three Gorges Reservoir Region

The agricultural non-point source pollution is becoming one of the main factors affected the water eutrophication in the Three Gorges Reservoir Region. Enhancement of the prevention and control of agricultural non-point source pollution is important to protect the water ecological security in this region. As the biggest tributaries of Three Gorges Reservoir Region, Xiangxi River basin belong to typical agroforestry watershed at hilly regions. In recent years, the water eutrophication has been happeneded frequently and the agricultural non-point source pollution threatened the water quality of Three Gorges Reservoir. Currently, agricultural sources of nitrogen(N) and phosphorus(P) production characteristics is unclear, the migration process and critical periods and areas was unidentified, and the amount of N and P load into reservoir was inaccurately. Therefore, taking Xiangxi River basin as the research area in the present study, analyzed the generation characteristics of N and P, the main migration pathway, identified the critical periods and areas and output of N and P loads and assessed the loads into reservoir. The research was based on investigation of agricultural sources, pollution discharge coefficient, section monitor, hydrograph separation and SWAT model method. The results was expected to provide a basis for the agricultural non-point source pollution prevention and control. The main conclusions are as follows:(1) The results of investigation for basic agricultural sources showed that the agricultural areas in Xiangxi River basin were continually increased from 2007 to 2013, increased by 34.6%. The dry land was the main type of the cultivated land and approximately accounted for 50%. The areas of orchard were the highest increase in the 7 years and the increasing rate was up to 34.6%. The pig was main animal on breeding production, which the number of pig increased by 27.0% from 295 thousand in 2007 to 375 thousand in 2013.(2) Characteristics of N and P generation analysis based on discharge coefficient suggested that the risk of agricultural non-point source pollution was increased year by year, and the total N(TN) and total P(TP) was increased by 22.3% and 13.3% respectively from 2007 to 2013. The animal production was the main source of TN discharges, accounted for 81.7% of the total amount, while animal production and crop production were both the main sources of TP discharges, amounted to 52.3% and 41.5% of the total amount, respectively. The region distributed many agricultural sources in the middle and middle-downstream of Xiangxi River were confirmed to the critical areas for high risk basin of non-point source pollution.(3) The results of section monitor and hydrograph separation methods showed that the surface runoff was the main output migration path for N and P, accounted for 82.5% and 87.9% of the annual total amount of N and P, respectively. The dissolved nitrogen(nitrate nitrogen) was the main form of loss of TN in the total runoff, surface runoff and base flow were mainly with, which accounted for 75.2%, 74.1% and 80.3%, respectively. Meanwhile, particulate phosphorus(PP) was main form of TP, which accounted for 80.0%, 81.5% and 69.1%.(4) The results showed that significant relationships were presented between total runoff and rainfall, surface runoff and rainfall from 2011 to 2014(P<0.001). The coefficient of surface runoff was high, and more than 60% of total annual runoff was come from surface runoff. The surface runoff had significantly high positive correlation with the silt content and amount of TP, PP(P<0.001), whereas it had poor correlation with dissolved P and all kinds of N. In the month with the largest surface runoff, it was more easily to occur soil erosion with a lot of silt, N and P in the basin.(5) The watershed section monitoring results indicated that the output of N and P loads from the total runoff, surface runoff and base flow showed obvious seasonal variation, and had significantly relationship with surface runoff. It was a critical period for the loss of N and P from July to September, in which the rainfall was accounted for 62.2% of the total amount per year. The surface runoff was accounted for 64.1% of the total surface runoff per year and 50.2% of the total annual runoff amount. It was affected by rainfall, the output of TN and TP in this period accounted for 61.1% and 79.0% of the annual output load. The output of TN and TP were up to the largest of the total amount in September, in which the output of TN and TP accounted for 35.5% and 50.9% of the annual output loads, respectively.(6) According to requirement of model, the calibration and validation of SWAT model was established by the measured data. The results showed that both the R2(R2≥0.6) and Ens(Ens≥0.5) met accuracy requirements for calibration and modeling, and this model was appropriate for this study area. The N and P contributed coefficient of sub-watershed and watershed and the characteristics of agricultural sources loads into river were simulated by using SWAT model. The attenuation coefficient of TN and TP were 8% and 18% in catchment scale, respectively. The TN and TP from agricultural sources loads into river were accounting for 40.2% and 37.6%, respectively. Therefor, more than half of non-point source loads into river were from other aspects. The critical areas of non-point source pollution for the region that gathered a lot of agricultural nitrogen and phosphorus pollution loads were located in the middle and middle-downstream of Xiangxi River basin, which was consistent with the spatial position of the agricultural critical areas and should be as key regions to enhancd the prevention and control of them.