Simulating The Non-Point Pollution Load Of Nirtrogen And Phosphorus In The Heihe Watershed Based On GIS

With the fast development of society and economy, water body pollution is more and more serious. When the point source pollution is under controll to a certain degree, the non-point source pollution becomes the main source of pollution in water body. Heihe River is the main source of domestic water use of Xi'an city, so the condition of water quality influences the development of Xi'an city directly. The study object of this paper is Hehei river watershed; the main contents and achievements are as follows:(1) This paper adopts SCS model to simulate the runoff process of Hehei watershed. It divides the sub-units of watershed by combing administrative region and precipitation station. And it proposed the CN value table suitable to the calculation of runoff producing with Matlab program. The result demonstrated that the simulated runoff process coincides with metrical runoff process, and the relative error of simulated result is–12.9%, so this mode can be used in Heihe watershed.(2) The GIS-USLE model was used to calculate the soil erosion yield. It makes visual analyze and quantitative calculation to the soil erosion yield, using the spatial analysis function of GIS based on ArcView with the information of land usage type, plant cover in soil, precipitation etc. The paper realized the mode`s distributed computing on space, using 15 by 15 meter's grid as calculated units, and mocks the non-point pollution's forming procedure. In addition, the allotment amounts of pollution on each sub-catchment can be directly viewed on lists. It provides a guide rule to control the soil erosion in Hehei and make management decision.(3) The caculation of load of non-point source pollution with ArcView obtains the spatial distributing of the non-point source pollution's load quantum and export of minor watershed's general load. The annual load of dissolved N and P are 402.394t and 21.382t, the annual load of particle N and P are 414.999t and 23.843 t, and the load of TN and TP are 414.999t and 23.843t.(4) By establishing the dependency relation of runoff with TN and TP, it can get the annual ofnon-point source pollution of TP and TN that are 18.031t and 303.180t.It concludes that the caculation results with ArcView are close to the monitoring results.the relative error of TP is 24.37%, and it is 26.94% for TN; in order to furthermore validate the results calculated by ArcView, compared the results with the results former people calculated, the results calculated by ArcView are also suitable.