The Dongting Lake is the largest lake in the Middle and Lower Yangtze River and ranks the second largest freshwater lake in China. Seasonal fluctuation of water-level in the Dongting Lake forms stable wetland resources. A large area of shallow marshes and mudflats, which are exposed over water surface during the non-flood season in the winter, provide habitats for several migratory birds. In recent years, the effects of the hydrological processes on wetland ecological system have aroused worldwide attention. The complex nonlinear relationship between the Yangtze River and the Dongting Lake has led significant changes in hydrological processes in the Dongting Lake after the running of the Three Gorges Project.
Data sets of surface area in the Dongting Lake were extracted using MODIS imagery from 2000 to 2010. During interpretation process, NDVI index and NIR band threshold were set to 1000, meaning that NDVI and NIR band values less than 1000 were water wetlands and values higher than 1000 were beach wetlands, including mudflat wetlands, grass wetlands and reed wetlands. Since the water-level of the Chenglingji was proportional to the surface area in the Dongting Lake, the observed water-level data sets were used to validate the rationality of the extracted surface area data sets. The results revealed that correlation coefficient of the extracted surface area and water-level was 0.8058. Mann-Kendall (MK) nonparametric test showed that the Z_s statistics were -2.4986, -3.0867 and -2.5421 in the 0.05 level of significance for the whole year, from June to September (flood season) and from October to May in the next year (non-flood season) during 2000 to 2010, respectively. The trends indicated that runoff in the Dongting Lake was reduced dramatically during the last decade. The average surface water area decreased 15.21% and 32.05% during flood season and non-flood season in 2010 compared with that in 2000.
In the paper, wetlands in the Dongting Lake were divided into water wetlands and beach wetlands. More attention was paid to the changes of beach wetlands area during non-flood season because of its importance for migratory bird. The data sets of area of water wetlands and beach wetlands indicated that the area of water wetlands decreased dramatically while beach wetlands increased correspondingly from 2000 to 2010. Compared with the data in February, October, and December in 2000, 29.98%, 26.76%, and 9.02% water wetlands were changed into beach wetlands in the corresponding month in 2010, respectively. According to the statistics of observed water-level of the Chenglingji that firstly and lastly reached 24m, 25m, 26m, 27m and 28m, water-level fluctuation was investigated. The results showed that water-level fluctuation from 24 m to 26 m led to earlier exposure of beach wetlands and later flood of beach wetlands, which prolonged the exposure time of beach wetlands. The change of water wetlands and beach wetlands promoted the evolution of mudflat wetland to grass wetland in the East Dongting Lake at water-level of 24m to 26m. In conclusion, it is multi-factors, including reduction of runoff from Sankou (Songzi, Ouhe, Taiping) and Sishui (River Xiang, River Zi, River Yuan and River Li) and reduction of rainfall etc., that contributed to the responses of wetlands area to hydrological process of the Dongting Lake. The impoundment of the Three Gorges Project in September to October would further aggravate the trend of water wetlands changing into beach wetlands.