Preliminary results of ENVISAT RA-2-derived water levels validation over the Amazon basin

Since the launch of the ENVISAT satellite in 2002, the Radar Altimetry Mission provides systematic observations of the Earth topography. Among the different goals of the ENVISAT Mission, one directly concerns land hydrology: the monitoring of the water levels of lakes, wetlands, and rivers. The ENVISAT Geophysical Data Records products contain, over different type of surfaces, altimeter ranges derived from four specialized algorithms or retrackers. However, none of the retrackers are intended to the processing of the radar echoes over continental waters. A validation study is necessary to assess the performances of the different ENVISAT-derived water levels to monitor inland waters. We have selected four test-zones over the Amazon basin to achieve this validation study. We compare first the performances of these retracking algorithms to deliver reliable water levels for land hydrology. Comparisons with in-situ gauge stations showed that Ice-1 algorithm, based on the Offset Centre of Gravity technique, provides the more accurate water stages. Second, we examine the potentiality to combine water levels derived from different sensors (Topex/Poseidon, ERS-1 and -2, GFO).