Assessing ammonium pollution and mitigation measures through a modified watershed non-point source model

Watershed water quality modeling is a valuable tool for managing ammonium (NH₄⁺) pollution. However, simulating NH₄⁺ pollution presents unique challenges due to the inherent instability of NH₄⁺ in natural environment. This study modified the widely-used Soil and Water Assessment Tool (SWAT) model to simulate non-point source (NPS) NH₄⁺ processes, specifically incorporating the simulation of land-to-water NH₄⁺ delivery. The Jiulong River Watershed (JRW) is the study area, a coastal watershed in Southeast China with substantial sewage discharge, livestock farming, and fertilizer application. The results demonstrate that the modified model can effectively simulate the NPS NH₄⁺ processes. It is recommended to use multiple sets of observations to calibrate NH₄⁺ simulation to enhance model reliability. Despite constituting a minor proportion (5.6 %), point source inputs significantly contribute to NH₄⁺ load at watershed outlet (32.4∼51.9 %), while NPS inputs contribute 15.3∼17.3 % of NH₄⁺ loads. NH₄⁺ primarily enters water through surface runoff and lateral flow, with negligible leaching. Average NH₄⁺ land-to-water delivery rate is about 2.35 to 2.90 kg N/ha/a. High delivery rates mainly occur at agricultural areas. Notably, proposed NH₄⁺ mitigation measures, including urban sewage treatment enhancement, livestock manure management improvement, and fertilizer application reduction, demonstrate potential to collectively reduce the NH₄⁺ load at watershed outlet by 1/4 to 1/3 and significantly enhance water quality standard compliance frequency. Insights gained from modeling experience in the JRW offer valuable implications for NH₄⁺ modeling and management in regions with similar climates and significant anthropogenic nitrogen inputs.