Improved inland water levels from SAR altimetry using novel empirical and physical retrackers

Satellite altimetry has proven a valuable resource of information on river and lake levels where in situ data are sparse or non-existent. In this study several new methods for obtaining stable inland water levels from CryoSat-2 Synthetic Aperture Radar (SAR) altimetry are presented and evaluated. In addition, the possible benefits from combining physical and empirical retrackers are investigated.The retracking methods evaluated in this paper include the physical SAR Altimetry MOde Studies andApplications (SAMOSA3) model, a traditional subwaveform threshold retracker, the proposed MultipleWaveform Persistent Peak (MWaPP) retracker, and a method combining the physical and empiricalretrackers. Using a physical SAR waveform retracker over inland water has not been attempted beforebut shows great promise in this study.The evaluation is performed for two medium-sized lakes (Lake Vänern in Sweden and Lake Okeechobeein Florida), and in the Amazon River in Brazil. Comparing with in situ data shows that using the SAMOSA3retracker generally provides the lowest root-mean-squared-errors (RMSE), closely followed by theMWaPP retracker. For the empirical retrackers, the RMSE values obtained when comparing with in situdata in Lake Vänern and Lake Okeechobee are in the order of 2–5 cm for well-behaved waveforms. Combining the physical and empirical retrackers did not offer significantly improved mean track standarddeviations or RMSEs. Based on these studies, it is suggested that future SAR derived water levels are obtained using the SAMOSA3 retracker whenever information about other physical properties apartfrom range is desired. Otherwise we suggest using the empirical MWaPP retracker described in thispaper, which is both easy to implement, computationally efficient, and gives a height estimate for eventhe most contaminated waveforms.