Impact of Three Gorges Reservoir Operation on Water Level at Jiujiang Station and Poyang Lake in the Yangtze River

The variation in water level at Jiujiang Station (JJS) directly affects flow exchange between the Yangtze River and the Poyang Lake. Quantitative research on the influencing factors of water level changes at JJS is of great importance for water supply and eco-environment protection in the Poyang Lake region. In this study, the Mann-Kendall method was used to test the trend of water level variation, and the impacts of riverbed incision and flow volume changes on water level at JJS were macroscopically analyzed using the observed monthly flow data series from 1981 to 2021. Furthermore, Long Short-Term Memory (LSTM) neural network model was used to simulate the impacts of outflow discharge of Three Gorges Reservoir (TGR) and flow discharge of the interval basin between TGR and JJS on water level at JJS; the partial dependence plot was adopted to analyze the impact of single feature variable variation on the simulation results. The results show that, after the TGR was put into operation in 2003, the water level changes at JJS mainly occurred during the impoundment period, the annual average storage of TGR was decreased 6.9 billion m3, and the annual average runoff volume at JJS was decreased 11.5 billion m3, which resulted in the average water levels at JJS being decreased 1.74 m and 2.11 m in September and October, respectively. The annual average runoff of JJS was increased 4.5 billion m3 with TGR replenishment of 1.8 billion m3 from December to March of the following year. Impacted by riverbed incision, the water levels at JJS were decreased 0.59 m and 0.99 m in September and October and increased 0.63 m from December to March. Every additional 5000 m3/s (1000 m3/s) of TGR outflow discharge could increase 1.0 m (0.16 m) the water level at JJS in September and October (from December to March of the following year).