Research On Modelling Agricultural Diffuse Source Pollution Load Of Lake Erhai Basin

Lake Erhai located in Dali City of Dali Bai Autonomous Region, is the second largest plateau freshwater lake in Yunnan Province. With the basin's population growth and economic development, water quality is declining and Lake Erhai has become an initial eutrophic lake. The garlic planting, cow raising, and other rural agricultural non-point source pollution from Dali City and Eryuan Country, have become the main reasons for the eutrophication of Lake Erhai and the main problems of Lake Erhai basin area as well.The research was sponsored by the National Water Key Project in the Eleventh Five-Year Plan. Based on the application SWAT model authorized by Agricultural Research Center of the United States Department of Agriculture, the research modeled Lake Erhai basin's agricultural diffuse source pollution and estimated the pollution load quantitively.Through data preparation, Lake Erhai basin's databases were constructed, including digital elevation, drainage networks, land use, soil type spatial databases and meteorological data, land cover, soil characteristics, agricultural management property databases. A digital platform was established for quantitative study on Lake Erhai basin agricultural diffuse source pollution load.In modelling, Lake Erhai basin was divided into 34 subbasins and 189 hydrological response units. Through calibration and validation of Lake Erhai basin's monthly runoff from 2004 to 2005 and monthly total nitrogen and phosphorus load from 2006 to 2007 in both north and south outlests, coefficient of determination R2 and Nash-Sutcliffe efficiency Ens were both above 0.5. It proved that the model simulation in Lake Erhai basin was satisfied with the applicability and accuracy.In simulation of agricultural diffuse source pollution in 2006 in Lake Erhai basin, the load was estimated as TN 2374 t/year and TP 322 t/year, while the intensity was estimated as TN 1.26 t/km/year and TP 0.17 t/km2/year. Northern area was the main source of pollution, accounting for over 50% of total pollution load, while the pollution intensity of the northern area was also the highest in the whole basin. Analysis of the spatial distribution of pollution showed that the pollution load of Sanying Town reached a maximum of TN 559 t/year and TP 75 t/year, followed by the Fengyi Town. The highest pollution intensity was generated from Jiangwei Town, reaching TN 1.77 t/km2/year and TP 0.25 t/km2/year, followed by the Haidong Town and Sanying Town. Analysis of season ditrubution of pollution showed that the largest amount, nearly 50% of the year's TN and TP was generated in summer. Autumn and spring followed. Winter generated the least. Through the analysis of pollution into Lake Erhai, the total pollution load was TN 903 t/year and TP 28 t/year, accounting for 38% and 9% of the total generated pollution load respectively. Northern runoffs were the main channels for 79% of TN and 62% of TP transported into the Lake Erhai. Among the 14 simulated surface runoffs, the northern three rivers made the largest contribution, amounting to TN 711 t/year and TP 17 t/year transported into Lake Erhai.