Change of Water Environmental Capacity in the Xiaojiang River after Impoundment of the Three Gorges Reservoir

The hydrological conditions of the Xiaojiang backwater were greatly changed at the high water level after impoundment of the Three Gorges reservoir.Water velocities were significantly reduced,and the background pollutant concentrations were changed due to inflow from the Yangtze River.Water environment parameters were vastly different compared with those naturally occurring,and the water environmental capacity of the Xiaojiang River was significantly impacted.According to the water functional division and water quality control target,the annual and monthly values of the permanganate index(CODMn),total nitrogen(TN) and total phosphorus(TP) before and after the impoundment were computed based on depth-averaged 2D hydrodynamic and water quality models.The results showed that after the impoundment of the Three Gorges reservoir,the water environmental capacities of CODMn and TP in the Xiaojiang River were reduced by approximately 24.15% and 82.72%,respectively,and that of TN went from unlimited to limited.The reduction in water environmental capacity is one of the major reasons for water eutrophication in the Xiaojiang River.