The study identified areas in the Miyun Reservoir watershed that contribute the most nitrogen pollution under different water conditions. Factors like fertilization, rainfall, and land use affect nitrogen levels. The main sources of pollution are in upper and lower areas with lots of cultivated land and grass. Potential pollution sources are in upper and middle areas with a mix of land types. Low-risk areas are in the middle and left bank with mostly forest land. This information can help improve water protection policies for Beijing.