Envisat ASAR Global Monitoring Mode rapid-revisit images employed as part of ESA’s Dragon Programme have charted the hydrological cycle of China’s largest freshwater body, Poyang Lake, whose area fluctuates more than threefold annually. Poyang Lake is situated in Jiangxi Province, around 50 kilometres north of the city of Nanchang. The Lake’s basin is one of the People’s Republic of China’s most important rice-producing regions, but local inhabitants must contend with massive seasonal changes in water level, topped by regular severe floods. Poyang Lake is connected to the Yangtze (Chiang Jiang) River through a narrow channel. In the dry season of a normal year, the area of the Lake shrinks to less than 1000 square kilometres, but by the end of the rainy season its size can grow up to 3500 square kilometres.
Improved understanding of Poyang Lake’s annual dynamic could help with flood mitigation as well as improved ecological characterisation of the surrounding landscape. Accordingly a radar sensor aboard Envisat was used to regularly monitor the Lake during 2004 to 2005. Envisat’s Advanced Synthetic Aperture Radar (ASAR) instrument records the signal reflectivity of the Earth’s land, sea and ice surfaces, and works on even through clouds, rain or local darkness. ASAR functions in various different modes – Global Monitoring Mode (GMM) data being acquired continuously over the land as part of the satellite’s background mission. GMM images have a comparatively low spatial resolution of one kilometre, but with a swath of 400 kilometres they have very wide coverage and a frequent revisit time, useful for tracking dynamic features.