Distributed Simulation On Nitrogen Non-Point Source Pollution In The Weihe Watershed

Non-point source pollution is defined to be the runoff transport of dissolved or solid constituents from diffuse sources on the land to streams. In order to analyse the nitrogen non-point source pollution characteristics of hilly area of Weihe River Basin, WeiHe Watershed above Hua-xian Section is selected as a representative hilly watershed for the study.SWAT model developed by USDA Agricultural Research Service (ARS) is used to estimate the non-point source pollution. It is a continuous and distributed model. And it can predict the impact of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions. Maps, weather, properties of soils and land management data were collected for model applying.At first, the map data must be projected to the same coordinate system by GIS software. Weather and other data also need be transformed to meet the need of the model. Then, DEM data of the watershed were used to define the stream network and discretize the subbasins by the SWAT pretreatment tool TOPAZ (Topographic Parameterization). Topographic parameters of the stream network and subbasins were also derived from the DEM. The hydrologic response units (HRUs) the smallest computing unit of SWAT model- were defined by overlaying the land use map and soil map. The soil and land use data are classified for the model and the soil physical properties database file is created. SWAT model is calibrated with observed data for the study area and the important parameters' sensitivity is analyzed.Monthly flow ,sediment and nutrient matters data series of 1987～1990 were used to parameter calibration. According to the coefficient for judging the adaptability of the model, such as Nash-Sutcliffe coefficient and Relative Error, the precision of the SWAT model is fairly good.The calibrated model is used to calculate the loadings of nitrogen non-point source pollutants for a wet year, a normal year and a dry year respectively. The difference of different typical hydrologic years' result is dramatic. The loading of nitrogen non-point source pollution in the wet year is relatively larger and in the dry year smaller since the nitrogen non-point source pollutants are mainly transported in the runoff and the most loading in a year is produced in the flood season. Because SWAT is a distributed model, it is possible to view model output as it varies across the basin and the critical areas and reaches are found in the study area. Each land cover represented in the model yields different results.At the end, the measurements for preventing nitrogen non-point pollution are presented according to the model results and the experiences of other areas.