5.1 Influence Mechanism of Lake Water Area Change in Flood Season
In the prophase analysis, we verified that in flood season, it has a significant correlation between lake area and flow. Many studies also confirmed that in flood season, the Poyang lake water area is affected by two runoff, one from the ’five rivers’ which belong to the Poyang basin, and another from the Yangtze River. They exchange and interplay at the lake estuary(Hu et al. 2007; Z. Zhang et al. 2015). Accordingly, the interaction between The Yangtze River and Poyang Lake will divide into three stages: 1. The Poyang Lake basin’s dominant period, from December to March next year. The runoff in the Poyang Lake basin increases, and the lake water level rises. The lake runoff will flow into the Yangtze River through the estuary. 2. The river lake interaction period, from April to August. The mainstream of the Yangtze River began to rise, and The water level in Poyang Lake is also high. The high water level of the Yangtze River will have a blocking effect on the outflows from the Poyang basin. The runoff of the Poyang basin cannot flow out, generating the flood stagnating in the lake, resulting in the lake water area expansion. 3. The Yangtze River runoff dominant period, from September to November. Influenced by the Yangtze River upstream, the runoff in the Yangtze River increases, and the water level rises. The runoff of the Yangtze River mainstream will flow into Poyang lake through the estuary
Compared to the images of Poyang Lake in September of each year(Fig.11.a). Poyang Lake maintains a high water area even at the end of the flood season in 1983,2020,2021. Combined with hydrological records found that in that case, the mainstream of the Yangtze river had flood events in September. Especially in 2020, suffered an exceeded history record flood in the Yangtze River Basin, simultaneously the water level at Poyang lake broke the historical extreme since the hydrological record. In Figure.11.b, the average area of September in 2011 and 2019 was only 1255.40 km2, but the hydrological records show that the lowest net outflow was in 2018. It is worth noting that serious drought events occurred simultaneously in the Poyang Lake basin and Yangtze River Basin; both drought events lasted long and heaviest. Affected by drought events, in the Poyang Lake basin, the flow of ’five rivers’ decreased, and the Yangtze River has lowered the water level, eliminating the blocking or reversing flow from the Yangtze River to the Poyang Lake; therefore, the water area advance reduction. It should point out that among these changes in the flood season, the water area closely relates to the changes in the river-lake interaction.