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 (NH) pollution. However, simulating NH pollution presents unique challenges due to the inherent instability of NH in natural environment. This study modified the widely-used Soil and Water Assessment Tool (SWAT) model to simulate non-point source (NPS) NH processes, specifically incorporating the simulation of land-to-water NH 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 processes. It is recommended to use multiple sets of observations to calibrate NH simulation to enhance model reliability. Despite constituting a minor proportion (5.6 %), point source inputs significantly contribute to NH load at watershed outlet (32.4∼51.9 %), while NPS inputs contribute 15.3∼17.3 % of NH loads. NH primarily enters water through surface runoff and lateral flow, with negligible leaching. Average NH 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 mitigation measures, including urban sewage treatment enhancement, livestock manure management improvement, and fertilizer application reduction, demonstrate potential to collectively reduce the NH 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 modeling and management in regions with similar climates and significant anthropogenic nitrogen inputs.