Four decades of wetland changes in Dongting Lake using Landsat observations during 1978-2018

Dongting Lake wetland serves critical ecosystem function for water regulation and biodiversity protection, yet it is vulnerable to hydrologic cycle variation (caused by anthropogenic activities and climate change). The impoundment of Three Gorges Dam (TGD) has influenced hydrological environment and land cover changes in the wetland. To quantify these influences, long-term land cover variations in the wetland were investigate by applying the Support Vector Machine method to Landsat satellite observations (1978-2018). We also try to explore the driving factors of vegetation changes by combining the classification results with measured hydrological and meteorological data. Some significant results have been revealed as follows: (1) In dry seasons, the most significant changes in Dongting Lake wetland were the vegetation expansion and the water area shrinkage, especially during the pre-TGD period (before 2003). Specifically, the vegetation area increased at a rate of 44.22 km(2)/a and the water decreased a a rate of 37.23 km(2)/a for the period of 1979-2001. But the variation trends of wetland cover types changed after 2003. The vegetation decreased at a rate of 24.36 km(2)/a and water increased a a rate of 20.05 km(2)/a from 2003 to 2017. (2) In we seasons, vegetation (1433.58 +/- 603.48 km(2)) and water area (2153.32 +/- 603.98 km(2)) fluctuated greatly in the pre-TGD period and the variation range narrowed significantly after 2003 (1632.62 +/- 234.39 km(2) of vegetation, 1958.5 +/- 235.04 km(2) of water). (3) Before 2003, the long-term change of vegetation area in dry seasons of the Dongting Lake wetland was strongly linear correlated with inundation area (r = -0.88, p < 0.05), sediment deposition (r = -0.84, p < 0.05) and mean temperature between July and September (r = 0.79, p < 0.05). But these relationships have changed after the operation of the TGD. In all, the variation in hydrological conditions caused by the impoundment of the TGD was likely the main reason for the variation of Dongting Lake wetland in recent decades.