Monitoring of water area in the dry season of East Dongting Lake Wetland from 2022 to 2023 based on Google Earth Engine

Wetlands have functions such as hydrological regulation, water purification, and climate regulation. However, under the dual influence of global climate change and human activities, the frequency and intensity of wetland ecological droughts are increasing, leading to shrinkage of wetland area and decline in ecological service functions, seriously threatening wetland ecological security. The East Dongting Lake Wetland is located in Hunan, China. According to the observation results of the Chenglingji Water Level Station, from August 2, 2022 to June 5, 2023, it entered an abnormal dry period of 305 days, which is the longest dry period since observation records began. Our goal is to understand the changes in water distribution in East Dongting Lake during this period based on satellite images.

We used the Sentinel-1 GRD data provided on Google Earth Engine and developed a water area monitoring module based on the OTSU method, which provided us with water distribution data of the East Dongting Lake wetland during the target period. There are 44 available Sentinel-1 GRD images in these 305 days. The average water area in the dry season calculated from the 44 images is 230.75 square kilometers. In contrast, there were 49 available images in the same period of the previous year, and the average water area was 486.32 square kilometers. The average water area in the same period of the previous year was even larger than the maximum water area of 369.06 square kilometers during the target period. This reflects the continued impact of the severe drought in China's Yangtze River Basin in the second half of 2022 on the East Dongting Lake wetland.

We analyzed the water distribution results on August 20, 2022, when the drought was severe. The results showed that contiguous exposed lake beds were exposed in the north and south of the main lake body of East Dongting Lake, and the main flood channel even experienced drying out. Drought has affected the core area of the East Dongting Lake Wetland, causing the water level to drop rapidly into the dry season, resulting in rapid changes in the spatial distribution of the Dongting Lake Wetland's shoals and water bodies. The advance of the dry season mainly affects the seasonal growth of wetland vegetation, including submersed plants (Echinacea, etc.), floating-leaf plants (Rhombus, etc.), emergent plants (Reeds, Typha, etc.), swampy meadows (Carex etc.) and swamp grasses (Nandi, etc.). The drought has even affected the habitat of elk and has had a complex impact on wetland ecology.