Numerical simulation of water age and its potential effects on the water quality in Xiangxi Bay of Three Gorges Reservoir

After the impoundment of Three Gorges Reservoir, algal blooms frequently occurred in the tributaries of the reservoir, which has posed a great challenge for the local aquatic environment and ecology. To investigate the main causes and key factors of the algal blooms, the Environmental Fluid Dynamics Code (EFDC) model is applied to the simulation of the three-dimensional hydrodynamics and water quality in the Yangtze River and Xiangxi Bay, and the concept of water age is adopted to quantitatively analyze the effects of hydrodynamics on the water quality by linking water age and phytoplankton levels, which has seldom been explored in previous studies. The model results show good agreement with the field data, and the simulation reproduces the thermal stratification and density current in Xiangxi Bay. The results show that Xiangxi Bay exhibits quite complicated hydrodynamic characteristics, and could be divided into different zones according to the transport process. In the upper reach, the water quality is mainly affected by the upstream inflows, while it is mainly affected by the intrusion flow from the mainstream near the river mouth. Water age and water temperature are found to be the key factors influencing the algal blooms in Xiangxi Bay. Besides, a significant positive correlation has been found between the chlorophyll a (chl-a) concentration in the surface layer and the water age of the dominant water source. This conclusion can be adopted to interpret the longitudinal distribution of chl-a concentration and to explain why Xiangxi Bay is susceptible to algal blooms. The correlation between water age and chl-a concentration is further used to evaluate the effectiveness of water level fluctuation in preventing algal blooms. The analyses and findings in this study could help to better understand the complicated hydrodynamic and water quality processes in Xiangxi Bay following the impoundment, and provide insights into ecological and environmental management of Three Gorges Reservoir.