Twenty Years Monitoring of Largest Chinese Monsoon Lakes Based on ENVISAT and JASON Altimetry Mission.
Daillet, Sylviane1; Huber, Claire2; Huang, Shifeng3; Cretaux, Jean Francois1; LAI, Xijun4; Chen, Xiaoling5; YESOU, Herve2

Capability of altimetry to be a powerful tool for inland water surfaces survey has already been demonstrated. Therefore it was mostly over large semi stable inland seas or large reservoirs. Within the complex watershed of Yangtze (Chang Jiang) basin, Dongting and Poyang Lakes are considered as hydrological keys components in flood control and reduction for the middle Yangtze basin, being regularly flooded in China. These lakes have specific behavior of monsoon lakes, presenting very important level variations during dry to wet season, the surfaces ratio being about 1 to 5. Levels can vary of more than 10 meters in one month during their infilling. Whereas, during dry season, Dongting and Poyang Lakes can be reduced to a relative narrow river, with a chain of small lakes, water bodies which are disconnected from the main river flow. In this case, the monitoring of water height from space is challenging. All the altimeter data of ENVISAT nominal mission has been processed (from 2002 to 2009) over Dongting and Poyang Lakes. Over Dongting, TOPEX/Poseidon from 1992 to 2002 and JASON2 data from 2008 to 2011 have been analyzed.

After discussion of altimetric solutions accuracy, validity of altimetric series has been checked against optical space observations and gauge station data located both on the Yangtze River and on the main bodies and sub-basins of the lakes. Strong correlation between these two measurements has been demonstrated.

Second key parameters for hydrological application, correspond to the water surface. Within the framework of the ESA MOST DRAGON programs, water surfaces of these lakes are monitored since 2000 based on the exploitation of time series of medium and high resolution satellites. Despite a sampling frequency relatively different for images and water height acquisition i.e. respectively 10 and 35 days, and by the way a rare acquisition of the two parameters on a single date, numerous similarities can be observed between the water height and water extend behaviors. On a first analysis, the two curves present the same peaks and weak values which correspond to the dynamic of monsoon lakes, with low water level and extent during the dry season (November to April) and high water level and extent from June to August (Fig 9 and 11). Knowing that a certain stall might occur during the dry season due to the presence of disconnected lakes, and the maybe too small water bodies to obtain a good altimetry measure, a particular attention was paid on the water level and height minimum values. In fact there is a very good synchronization of the observed behaviors.

Based on exploiting synergistically altimetric series and Earth observation images, the monitoring of the two major fresh water lakes in China over relative long period, 6 and 8 years, was realized. In addition this work present the first exploitation over these lakes of near all altimetric resources (Topex, Envisat, Jason 2) and this over a period twice more than previous studies. Next steps would be firstly to enlarge the monitored area, with the integration of the Yangtze curses, as well as these small lakes bordering, being partially connected with the main river course from Hubei, Jiangxi and Anhui provinces into a water level monitoring system. A second step will be the pursuit of the monitoring based on new altimetric missions, with the first one, Altika, followed by Sentinel 3.

The good knowledge of this zone merging river, lakes and wetlands is an opportunity for realistic simulation of data in the context of SWOT mission preparation.