Eutrophication Risk Assessment of Tributaries in the Three Gorges Reservoir: A Case Study of the Pengxi River

Evaluating the eutrophication risk in reservoir areas is very important for environmental protection. However, the periodic backflow of water in reservoir areas makes evaluating the eutrophication of tributaries in reservoir areas more difficult. In this study, based on the basic principle of the copula function, combined with the water environment simulation results of the Pengxi River, a tributary of the Three Gorges Reservoir, a new perspective for considering the interaction between water environment variables was proposed to evaluate the joint eutrophication risk probability. The results showed that the fluidity of the whole Pengxi River was poor, which was conducive to the deposition and enrichment of pollutants. The total nitrogen (TN) in Hanfeng Lake and the main stream of the Pengxi River was always above the level of moderate eutrophication, while total phosphorus (TP) varied over a large range, from mesotrophic to hyper-eutrophic. In 2021, the joint risk probability of TP and TN was the highest, at 45%, when they were both hyper-eutrophic in Hanfeng Lake. When TP was slightly eutrophic and TN was moderately eutrophic, the joint risk probability was the highest in Pengxi River, at 42%. The relatively clean water of the Yangtze River flowing back into the Pengxi River reduces its eutrophication risk. The results of this study provide a scientific basis for the prevention and control of water eutrophication in the Three Gorges Reservoir.