Simulating the impact of watershed management for surface water quality protection: A case study on reducing inorganic nitrogen load at a watershed scale

The management and control of non-point source pollutants (NPSPs) has been an increasing concern throughout the world. Controlling NPSP is critical to achieve the goals for improving surface water quality. This study used the Soil and Water Assessment Tool (SWAT) model to compare the effects of different kinds of watershed management measures on the transport of sediments and nutrients (ammonium nitrogen and nitrate nitrogen) in one of the main tributaries of the Xiangjiang River, the Zhengshui River. Our results confirmed that agricultural areas were the dominant sources of nutrients and sediment in the watershed and that a filter strip can significantly mitigate surface water pollution by intercepting and absorbing nutrients. We hypothesized five management plans to improve water quality for achieving the standard set by the National Twelfth Five-Year Plan (NTFYP). These plans were assessed according to their effects, economic cost efficiency and social impacts. The results showed that all five plans can successfully meet the goal of the NTFYP to reduce pollutant loads at the outlet of the watershed by more than 10%; and the results showed that plan that focused on critical source areas (CSAs) did not show its superiority as previous studies had theoretically suggested when the economic and social impacts were considered. Meanwhile, plans which considered economic factors and cost effectiveness would be more feasible and suitable as their cost efficiency were 10% and 30% higher than that of CSA plans for NO3− and NH4+, respectively, and their social impacts were much lower than other plans. Moreover, regression analysis further revealed the main characteristics of subbasin controlling the treatments efficiency of the inorganic nitrogen. Our study highlights the significance of combining theory with actual practice and would facilitate the selection and implement of more effective and reasonable measures to improve water quality.