Lu Wen-bin, Hao Shou-ning, Yicheng Fu, Guo Ao-hui, Wang Ping, Tang Peng-cheng, Song Xin-wei
This study addresses the critical knowledge gap in nutrient dynamics within high-altitude fragile ecosystems by investigating the Niyang River, a primary tributary of the Yarlung Zangbo River. A multidimensional diagnostic framework was developed by coupling the Comprehensive Pollution Index (CPI) with a refined Output Coefficient Model (OCM) and stoichiometric correlation analysis, bridging the gap between ambient concentration-based risk profiling and land-use-driven pollution flux calculation. Through systematic monitoring of nine representative sections during contrasting hydrological seasons, the spatio-temporal evolution of Chemical Oxygen Demand (COD), Total Phosphorus (TP), Total Nitrogen (TN), and Ammonia Nitrogen (NH₃-N) was quantified. The integrated modeling approach reveals a distinct spatial purification gradient from the headwaters to the downstream confluence, with the dry season identified as the critical window for pollution management due to reduced dilution capacity. By synthesizing the CPI-based risk ranking with OCM-derived load estimation, this study identifies a synchronous transport-source mechanism: specifically, the strong stoichiometric coupling between organic matter and nutrients (COD-TP and TN-NH₃-N) validates that large-scale pastoral waste and agricultural runoff are not merely parallel but functionally integrated pollution drivers. COD was prioritized as the primary risk indicator within this coupled framework. These findings demonstrate that the synergistic application of concentration-load modeling offers a more robust precision-targeting tool for water quality management in the Third Pole region than traditional single-model assessments, providing a scientific template for ecological preservation in similar alpine basins.