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

• SWAT is modified to simulate NPS NH 4 + pollution in the Jiulong River Watershed. • NPS inputs contribute about 15.3 % to 17.3 % of NH 4 + load at watershed outlet. • NH 4 + enters water mainly via surface runoff and lateral flow, with minimal leaching. • Land-to-water NH 4 + delivery varies spatially, with high levels in agricultural areas. • Reducing fertilizer usage by 30 % lowers the NH 4 + load at outlet by 2.5∼4 %. Watershed water quality modeling is a valuable tool for managing ammonium (NH 4 +) pollution. However, simulating NH 4 + pollution presents unique challenges due to the inherent instability of NH 4 + in natural environment. This study modified the widely-used Soil and Water Assessment Tool (SWAT) model to simulate non-point source (NPS) NH 4 + processes, specifically incorporating the simulation of land-to-water NH 4 + 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 NH 4 + processes. It is recommended to use multiple sets of observations to calibrate NH 4 + simulation to enhance model reliability. Despite constituting a minor proportion (5.6 %), point source inputs significantly contribute to NH 4 + load at watershed outlet (32.4∼51.9 %), while NPS inputs contribute 15.3∼17.3 % of NH 4 + loads. NH 4 + primarily enters water through surface runoff and lateral flow, with negligible leaching. Average NH 4 + 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 NH 4 + mitigation measures, including urban sewage treatment enhancement, livestock manure management improvement, and fertilizer application reduction, demonstrate potential to collectively reduce the NH 4 + 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 NH 4 + modeling and management in regions with similar climates and significant anthropogenic nitrogen inputs. [Display omitted] [ABSTRACT FROM AUTHOR]