5.2 Influence Mechanism of Lake Water Area Change in Dry Season
In the dry season, temperature and sediment significance correlated with the lake area. Compared to year-to-year images in the early dry season. Images show that the most apparent changes in water area concentrate in the Ganjiang River estuary and Poyang lake estuary (Figure 12). In 1991, 2004, 2006, 2010, 2013, 2017, 2019, and 2021 the water area at the estuary of the Ganjiang River decreased. In 1991, 2006, 2004, 2006, 2010, 2013, 2019, and 2021 the water area at the estuary of Poyang Lake shrank, and the lakebed was visible. Despite the summational sediment of ’five rivers’ has a significant downward trend. Nevertheless, the high sediment of ’five rivers’ during 1980-1990 caused higher sedimentation in the river estuary; the siltation phenomenon is evident primarily at the Ganjiang river estuary. On the other hand, some studies have found that the elevation of the lake estuary decreased significantly, which is related to the factors such as continuous sand mining and current scouring(Guiping et al. 2015; Feng et al. 2011; Ye et al. 2018; Lai, Shankman, et al. 2014; Jing et al. 2017).
MK mutation test confirmed that the change-point year was 2003. The average water area was 2300km2 in the early dry season of the 1990s; after 2003, the average water area decreased to 1500km2. However, sediment and temperature have not changed so dramatically in the same period. Therefore, combined with various characteristics of the water body area in the flood season, the water area has shrunk in advance at the end of the flood season, causing the water area to shrink dramatically in the early dry season. Some researchers had suggested that the Three Gorges Dam (TGD) water impoundment in 2003 significantly exacerbated the water level and runoff decline from late September to November. Thus the Yangtze River’s blocking effect on outflows of Poyang is weakened, and the outflows of Poyang lake through the estuary of the lake more quickly. (Q. Zhang et al. 2014; Lai, Jiang, et al. 2014; Liu et al. 2020) Changes in the Yangtze River runoff impact the interrelationship between the river and Poyang Lake, which disturbs the lake’s hydrological processes.
Given the above discussion, an apparent linkage between the sediment of ’five rivers’ and the water area change in the dry season. Besides, the TGD water impoundment affected the river lake interaction, eventually leading the water of Poyang lake prematurely to flow into the Yangtze River, causing the lake area to decrease earlier at the end of the flood season. Changes in lakebed topography also cause the water to flow more rapidly into the Yangtze river.