[Temporal and Spatial Evolution of Non-point Source Pollution Load of Total Nitrogen in Tuojiang River Basin]

Currently, the aquatic environment of the Tuojiang River basin in Sichuan Province is severely polluted by non-point sources of total nitrogen (TN). This study adopts the pollution discharge coefficient method to estimate the TN pollution load of non-point sources in this watershed during 2007-2017. The temporal and spatial distribution and transfer trends of the TN pollution load in the Tuojiang River basin were examined, based on center-of-gravity statistical and spatial analysis technology. This study aimed to provide an accurate theoretical basis for the prevention and early identification of non-point source pollution in the Tuojiang River basin. The results indicate that livestock breeding was the main non-point source of TN pollution and contributed more than 45% to the TN pollution load during 2007-2017. The contribution rate of rural life and domestic waste decreased continually during the study period, whereas that of farmland solid waste and farmland runoff exhibited an increasing trend. The total pollution load of TN exhibited a decreasing trend during 2007-2017. The maximum and minimum TN pollution loads occurred in 2010 and 2017 with values of 5.7×10⁴ t and 4.69×10⁴ t, respectively. Spatial heterogeneity of the pollution load, together with the uneven distribution of rainfall runoff, caused a shift from northwest to southeast in the pollution-load centers of gravity for livestock and poultry breeding, farmland solid waste, and farmland runoff. Southeast of the watershed is the key area for prevention and control of these pollution sources. A shift in the centers of gravity for rural living and household waste pollution, from southeast to northwest, was attributed to agricultural populations transforming to urban populations in the southeastern counties. The maximum transfer range was 66.35 km², and this minimum boundary circle is the key identification area of pollution source pollution load change. Northwest of Tuojiang River basin is the key area in which TN pollution from rural living and rural household waste can be prevented. This research expands the methods for exploring the temporal and spatial evolution of pollution load in the Tuojiang River basin, which is of great significance for improving the aquatic environment and promoting sustainable development of the basin economy.