4.1 Trends and Change Point of Lake Water Area
In Fig.3, the water area from 1977 to 2021 in flood season has a significant downward trend (MK trend test Z=-1.67, passing the 95% test), decrease rate being −9.14km2/a. The largest area was 4662.05 km2 in 1998, and the smallest was 2139.37 km2 in 2006. MK mutation test showed that the change-point year was 2005; after 2005 water area declining rate increased.
In the dry season, the lake water area has a significant downward trend (Mk test Z=-3.09, passing the 99% test) decrease rate being −17.52km2/a. The largest area was 3202.91 km2 in 1997, and the smallest area was775.41 km2 in 2019. MK mutation test showed that the change-point year was 2003. After 2003 water area declining rate increased (Fig 3).