51. A Modeling Approach for Critical Source Areas Identification and Sources Apportionment for Nitrogen Load in Yuan River Catchment, China

Excessive load of nitrogen from anthropogenic sources is a threat to a healthy aquatic ecosystem. The analysis of nitrogen load, in terms of identifying the critical source areas and apportioning the instream nitrogen to individual sources is supportive to the catchment management. The difficulty of the analysis spatially and seasonally, including the processes of transformation and transportation, has made the Soil and Water Assessment Tool (SWAT), a physically-based hydrological model, a useful tool to quantify the nitrogen load at the catchment scale. This study is focused on the application of the SWAT model at the Yuan River Catchment (YRC), which has been under the threat of extensive nitrogen load in the previous years. The major nitrogen sources identified in the research are urban drainage, as the point source; fertilizer, rural feedlot and rural household, as the diffusive sources. The calibrated hydrological model indicated that the downstream plain region with paddy field and denser rural population released the highest rate of total nitrogen. Fertilizer and waste from feedlots contributes to the 85% of the nitrogen in the stream. The results show the applicability of the SWAT model at the YRC in terms of nitrogen load simulation, and indicate the necessity to tackle the nitrogen from diffusive sources and the critical areas at the YRC.