Exploring the contribution of individual agricultural sources to nutrient export at watershed mouth

Agricultural nonpoint source (NPS) pollution strongly results in the accelerated eutrophication of surface water in many important lake basins. Some export coefficient methods are commonly used to estimate NPS pollutant export fluxes, but these methods typically ignore the aquatic retention that nutrient transport through watershed. Soil and Water Assessment Tool (SWAT) has been successfully used to simulate the exportation of nonpoint pollutant at the basin scale and has a comprehensive consideration of the pollutant migration process finally releasing into reservoir area. In order to identify agricultural nonpoint sources and their quantitative contributions to nonpoint source, the export coefficient method and SWAT were both used in this study. Agricultural production statistic data was utilized to estimate the pollutant production from different agricultural sources in each subbasin, and the pollution contribution from each subbasins were separately calculated by the pollution export coefficient method. The SWAT was used to simulate the pollutant migration process from the pollutant production in subbasins to the watershed outlet. Finally, the contributions of pollutant load from different agricultural sources were calculated by the pollution contribution content from each subbasin multiplying their pollution contribution coefficient respectively. We illustrated this method in the Xiangxi river basin of the Three Gorges Reservoir Area, Hubei province, China. Total nitrogen (TN) and total phosphorus (TP) pollution from agricultural source were identified. The quantitative results indicated that TN and TP load releasing into the reservoir area from agricultural source were 1229.5 t a^-1 and 82.4 t a^-1, accounted for 40.2% and 37.6% of the whole contribution content, respectively. Livestock breeding was the main source of TN load, accounted for 81.7% of agricultural TN load in the study area. Livestock breeding and cropland farming were both the main source of TP load, accounted for 52.3% and 41.5% respectively. This research proposed a more comprehensive method to identify the contribution of different agricultural source for nonpoint pollution to the reservoir area or lake basin.