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Hydrological variables such as surface water extent and soil moisture are continuously being monitored from satellites with good accuracy. However, orbital measurement of river stage and discharge remains a major challenge even today. Although altimeters provide valuable information on water level variations with a precision of few centimetres, they suffer from poor revisit time and limited gauging sites. Microwave radiometers that provide brightness temperature (BT) can be used to fill the gaps between altimeter passes with a revisit time of 1–2 days, although with limited accuracy. This paper presents an approach for estimation of river stage based on passive microwave radiometer using BT of river/measurement target (M) and upstream calibration target (C). We have used altimeter data from Jason-2/3 and BT data from Advanced Microwave Scanning Radiometer (AMSR2) at 36.5 GHz (descending orbit and horizontal polarization) for the period January 2013 to August 2020 to estimate river stage at 12 virtual gauging sites along Indus, Ganga, and Brahmaputra rivers. A new approach for selection of calibration targets is presented in this study which results in higher correlation between altimeter observed river stage and AMSR2 derived C/M ratio when compared to existing methods. Coefficient of determination (R2) for C/M ratio with water level for the period 2013–2017 ranged from 0.45 to 0.89. Model functions were generated between C/M ratio and altimeter observed water level for all the gauging sites. Estimated water levels were validated against altimeter and ground observed values for the period January 2018–August 2020 with RMSE ranging from 0.35 to 1.45 m, which is 7–15% of total stage variability. [ABSTRACT FROM AUTHOR]

Retracking altimeter waveforms over inland water bodies is a challenging task as a wide range of waveform is encountered while the retracking algorithms are available only for a handful of echo shapes. One such waveform shape widely encountered in lakes and reservoirs is the multipeak echo. These echoes are produced when the interacting surface in the altimeter footprint is not homogeneous and a number of different types of surfaces contribute to the resulting waveform. The widely used conventional retrackers, namely the Brown, Beta-5, Ice-2, OCOG, and threshold, can retrack a number of different waveform shapes such as the Brown like waveforms, specular waveforms, and rectangular waveforms but may not perform well for multipeak waveforms. In this article, a technique has been demonstrated to identify the different subwaveforms within a multipeak waveform and identify the subwaveform corresponding to the target at nadir. The subwaveform that is reflected from the nadir surface is identified from apriory information about the surface topography of the area. The subwaveform is then retracked using the 50% threshold to find the correct retracked range and water height. This technique has been tested for nine cycles of SARAL SIGDR data on Ukai reservoir, Gujarat, India, and found to perform much better than the other retrackers, particularly for multipeak waveforms. [ABSTRACT FROM PUBLISHER]