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.

Spatial-temporal characteristics of corrected total phosphorus pollution loads from agricultural non-point sources in Tuojiang River watershed, Sichuan Province of southwestern China.

Traditional method of estimating pollution loads may neglect the internal spatial heterogeneity of socio-economic driving factors, which can result in overestimate and underestimate of pollution loads. In this study, the corrected approach to estimating total phosphorus (TP) pollution load was proposed to explore its future variation to develop effective phosphorus pollution control strategies for water environment management. As the first-class tributary of the Yangtze River, the TP out of limits in the Tuojiang River is serious. Thus, based on the presently related basic datasets related to TP pollution load estimation, we firstly adopted the GM (1,1) model to predict their varied trends from 2021 to 2025. We then used the pollution emission coefficient method to calculate the TP pollution load. Moreover, considering the temporal and spatial heterogeneity of the pollutant generation coefficient, we further introduced population and GDP factors to further modify the pollutant generation coefficient to correct TP pollution load. Finally, we employed the exploratory spatial data analysis (ESDA) method to explore spatial distribution characteristics and spatial autocorrelation of TP pollution load from diverse pollution sources in 2025. The results showed that the total TP pollution load from diverse pollution sources will increase from 12,194.92 t in 2021 to 12,461.26 t in 2025, an increase of 2.18%. More concretely, the TP pollution load from rural domestic sewage, rural domestic waste and livestock, and poultry pollution sources will separately decrease by 94.24 t, 77.9 t, and 86.52 t. However, the TP pollution load from agricultural runoff and agricultural solid wastes pollution sources will increase by 74.52 t and 451.49 t, respectively. The contribution of TP pollution load from diverse pollution sources to total TP pollution load will be as follows: livestock and poultry (63.49%) > agricultural solid wastes (16.72%) > agricultural runoff (12.26%) > rural domestic sewage (4.12%) > rural domestic waste (3.41%). The difference in the spatial distribution of TP pollution load from diverse pollution sources in 2025 will be prominent. TP pollution from rural domestic sewage and rural domestic waste pollution sources is more serious in the Xindu and Longquanyi districts, and that from agricultural runoff and agricultural solid wastes pollution sources is more prominent in the midstream and downstream. TP pollution load from livestock and poultry pollution source is higher in the Renshou, Anyue, Rongxian, Luxian counties, and Jiangyang district. Additionally, TP pollution load from rural domestic sewage, rural domestic waste, agricultural runoff, and agricultural solid wastes pollution sources in 2025 will show a clear spatial correlation, but the spatial correlation of TP pollution load from livestock and poultry pollution source will be weak. The study is effective to eliminate the influence of temporal and spatial variation of pollutants generates coefficients on TP pollution load estimation. The method can reflect the actual condition of pollution loads in watersheds more objectively, which can be applied to estimate other pollution loads of similar watersheds with intensive socio-economic activities. The findings in this study can provide a critical reference for the stakeholders to balance water environment conservation and socio-economic development.