Characterization of the Impacts of Underlying Surface Conditions of the Fujiang River Basin on the Non-Point Source Pollution Loads Using the SWAT Model

The objective of this study is to investigate the impacts of underlying surface conditions, i. e., discharged sewage from various sources, runoff paths, and soil background TN and TP contents on the ammonia nitrogen (NH₃), total phosphorus (TP) and permanganate index (I_Mn) loads. The hydrology and transport and transformation processes of the non-point source pollutions at the Fujiang River Basin were simulated with a SWAT model in which the parameters were calibrated by the SWAT-Cup. The global sensitivities of underlying surface condition parameters were evaluated using the Sobol indexes. The Nash-Sutcliffe coefficient, the relative root mean square error, the relative deviation, and the relative total error were used to evaluate the model performance and the results show that the simulated flow rates and NPS loads were accurate. The variation of the NPS pollution loads increased as more pollution discharged into rivers under various underlying surface conditions. The NPS loads were mainly affected by the landscape and the slope length in hills dominated by the natural forests, whereas the NPS loads decreased with the proportion of agricultural land area due to the modulation capacity of the agricultural lands to the runoff. The Sobol sensitivity analysis results demonstrate that flow paths had the most important first-order and total-order effect on the simulated flow and NPS loads. The sensitivity indices of the total phosphorus and the permanganate index were the lowest and the highest, respectively, among three NPS pollutions. Strengthening the control and adjustment ability of the runoff process has the most significant effect on the controlling of non-point source pollution in the Fujiang River Basin.