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.