Evaluating the effects of different pollution reduction scenarios on the total phosphorus concentration of a mountainous river basin in southwest China using SWAT model: a case study of the Donghe River in Baoshan, Yunnan

Water quality has become a significant concern in many river basins in China due to both point and non-point source pollution. The SWAT model assessed pollution reduction scenarios and their effects on Donghe River basin water quality in southwest China. The calibrated model evaluated existing point and non-point emissions. Three schemes reduced point sources by 30, 60, and 90% and non-point sources by 25, 50, and 75%, respectively. Simulations analyzed annual and monthly total phosphorus (TP) concentrations under the scenarios. Results showed that the scenarios effectively improved water quality, meeting Class IV TP standards annually. However, TP exceeded standards in dry months (January–April, December) under all scenarios. A certain degree of negative correlation (R = −0.52, P = 0.11) between TP and rainfall suggests rainfall that influences TP. Comprehensive measures are needed to achieve standards year-round. In summary, the study found that reducing emissions improved Donghe water quality overall but more work is required to meet standards during dry periods. Rainfall correlates with and may affect TP. The work emphasizes implementing comprehensive approaches for year-round water quality improvements in the basin. HIGHLIGHTS An appropriate SWAT model was established for the Donghe River Basin.; Non-point source pollution in the Donghe River Basin accounts for more than half of the total phosphorus emissions.; Rainfall is a critically important factor influencing the variation in total phosphorus concentration in the Donghe River Basin.; The months of January to April and December each year constitute the most critical period for total phosphorus pollution in the Donghe River Basin.;