Pronounced Seasonal Shifts and Spatially Heterogeneous Drivers of Nutrient Loading in a Monsoon-Controlled Plateau Lake Revealed by LSPC

Nutrient-loading patterns remain poorly characterized in plateau lake watersheds with complex terrain and sparse monitoring. We applied the Loading Simulation Program in C++ (LSPC) to the Yangzonghai Lake watershed to estimate runoff, total nitrogen (TN), and total phosphorus (TP) loads during 2017–2024. Across three monitoring stations, validation Nash–Sutcliffe efficiency values ranged from 0.54 to 0.75 for streamflow, 0.51 to 0.66 for TN, and 0.50 to 0.60 for TP. Approximately 86% of annual inflow occurred during the wet season, whereas the wet-season proportions of nutrient loads were lower, indicating higher nutrient export per unit water volume during the dry season. Total loads and area-specific loading intensities showed contrasting spatial patterns. Southern sub-basins contributed more than 48% of total nutrient loads, while the highest loading intensities occurred in the middle and lower Baiyi River in the north, where domestic sewage was estimated to contribute more than 50%. These findings support differentiated management: agricultural nutrient management and erosion control in the south, and sewage interception and sewer-network improvements in the north. The framework provides spatial information for targeted nutrient management and offers a methodological reference for similar data-limited basins worldwide.