Abstract:

The dominant method in the field of nonpoint source(NPS)pollution research is NPS simulation model，in which export coefficient model is widely used.But for basins with obvious spatial differences of precipitation and terrain，such as upper reach of the Yangtze River，the model accuracy is limited for the sake of its neglecting of precipitation and terrain impact on NPS pollution.Considering that，an improved export coefficient model is proposed in this article，and the method for determining precipitation and terrain factors was also offered.The model was applied to simulate the total nitrogen load of upper reach of the Yangtze River in 1990 and 2003.It is indicated that average values of relative error for various watersheds reduced from 4502% to 2393% and from 4249% to 1838% respectively by using the improved model.Relative error values for watersheds control by Zhimenda，Luoduxi and Tuojiangdaqiao stations decreased obviously.The relative error values of Zhimenda Station reduced from 20729% to 2484% (1990 year) and from 16325% to 1816% (2003 year) respectively，those of Luoduxi Station decreased from -3372% to -1924% (1990 year)and from -4120% to -2435% (2003 year)respectively，and the relative error value of Tuojiangdaqiao Station reduced from 3247% to -215% (2003 year).It can be concluded that the improved model is feasible and the predicted result is approximately in accordance with the monitoring data.For large scale basin，especially those with obvious spatial differences of precipitation and terrain，the improved model has its theoretical and practical significance.

Key words: export coefficient model, precipitation, terrain, total nitrogen load, accuracy analysis