Four decades of winter wetland changes in Poyang Lake based on Landsat observations between 1973 and 2013

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Poyang Lake, the largest freshwater lake of China, is well known for its ecological importance as a dynamic wetland system. However, due to the significant seasonality of the lake's inundation area, no systematic study has assessed the wetland changes over the past few decades. We addressed this challenge by using four decades of Landsat observations ranging from 1973 to 2013. The images were acquired in the same season to ensure similar phenological and hydrological conditions during each year. Extensive training and validation samples were collected from high-resolution Quickbird imagery to develop a Support Vector Machines (SVM) method for wetland classification of Poyang Lake. To obtain consistent results from different Landsat instruments, an empirical line correction approach was introduced to adjust the sensor-associated differences in band configurations and spectral responses. Significant changes in the major wetland cover types in Poyang Lake were revealed from long-term classification maps. The vegetation coverage of Poyang wetland showed a statistically significant increasing trend during the overall period (15.9km2year−1), and the vegetation tended to spread into the lake center in the Nanjishan Wetland National Nature Reserve (NWNNR) in recent years. At the same time, out-of-phase variability was observed for the mudflats since 1984, with a significant shrinking trend of −12.1km2year−1 (p<0.05). Although sand coverage experienced a rapid decrease from 1973 to 1990 (from 544.3km2 to 62.9km2), it remained at a relative stable low level (<100km2) in the following period. The two national reserves in Poyang Lake shared change patterns similar to those of the entire lake. Although ~70% of the long-term changes in the wetland vegetation area appeared to be explained by local temperature, there was rapid increase after 2002 possibly could be due to the impoundment of the Three Gorges Dam (TGD) in 2003. The large sand area in 1973 was potentially linked to previous human activities in China between the 1950s and the 1970s. The method used in this study could be easily extended to other places in the world to assess the decadal wetland changes, and the information provided in this work is critical for future restoration efforts of the Poyang Lake wetland ecosystem.