Spatial and temporal distribution characteristics of typical pollution loads based on SWAT model across Tuojiang River watershed located in Sichuan Province, Southwest of China

Tuojiang River watershed is an economically developed and densely populated area in Sichuan Province (southwest of China), which is also an important tributary of the Yangtze River. Nitrogen (N) and phosphorus (P) are the main pollutants affecting water quality, but there is still lack of study on the spatial and temporal distribution characteristics of these two pollutants. In this study, the typical non-point source pollution loads in the Tuojiang River watershed are simulated by Soil and Water Assessment Tool (SWAT) model, and the spatial autocorrelation method is used to reveal the spatial and temporal distribution characteristics of the pollution loads from the annual average and water periods. Combined with redundancy analysis (RDA) and geographically weighted regression (GWR) analysis, the main driving factors affecting the typical non-point source pollution loads in the Tuojiang River watershed are discussed from the global and local perspectives. The results show that (1) from different water periods, the pollution loads of total nitrogen (TN) and total phosphorus (TP) in three water periods show obviously different, is the highest in the abundant water period, with 323.4 kg/ha and 47.9 kg/ha, followed by the normal water period, with 95.7 kg/ha and 14.1 kg/ha, and the lowest in the dry water period, with 28.4 kg/ha and 4.2 kg/ha. The annual average value of TN pollution load is higher than that of TP, with 447.5 kg/ha and 66.1 kg/ha, respectively; (2) the TN and TP pollution loads are stable on the whole, and the overall level in the middle reaches is higher. The pollution loads of Shifang City and Mianzhu City are higher in all three water periods. (3) Elevation and slope are two main driving factors affecting the TN and TP pollution loads in the Tuojiang River watershed. Therefore, the visualization and quantification of temporal and spatial distribution characteristics of typical non-point source pollution loads in the Tuojiang River watershed are helpful to provide the basis for scientific prevention and control of pollution in the Tuojiang River watershed and are of great significance to promote the sustainable, coordinated, and healthy development of water environment and economy in the watershed.