Abstract
Access to fresh water is a key issue for the next decades in the context of global warming. The water level of lakes is a fundamental variable that needs to be monitored for this purpose. The radar altimetry constellation brings a worldwide means to this question. Recent advances in radar altimeter onboard tracking modes have allowed monitoring thousands of lakes and rivers. Now, measurements are widely available with better resolution: it is time to drastically improve the processing. The altimetry waveforms over lakes are difficult to analyze and very different from the ocean ones. We face a large variety of signals due to surface roughness, lake geometry, and environment. The inversion process, named retracking, shall be able to describe all these components. We propose here a retracking based on physical simulations taking as inputs the lake contour and the instrument characteristics. Fitting the simulation on the waveforms gives the water surface height. The algorithm has been tested on the Sentinel-3A and Sentinel-3B time series over Occitan reservoirs (France) and Swiss lakes and compared to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> references. Over small Occitan reservoirs (few ha to few km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), the unbiased root-mean-square error (ub-RMSE) is better than 14 cm. Over the medium-size Swiss lakes, the ub-RMSE is better than 10 cm for most of them.
Highlights
IT was pointed out in the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC), the urgent need for perennial observational infrastructures allowing to monitor the climate system and its natural components
The method proposed in this paper is in the continuation of numerical retracking over ocean proposed by [38] and adapted here to lakes observations
Satellite altimetry has become over the years an ideal technique to measure water levels on land, especially over lakes which are counted by millions
Summary
IT was pointed out in the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC), the urgent need for perennial observational infrastructures allowing to monitor the climate system and its natural components. The SAMOSA+ model still accounts for an open ocean situation and cannot model complex lakes geometry To overcome this limitation, Gao et al [41] have proposed to use SAMOSA+ on limited waveforms range gates, selected using an input DEM, to discard range gates with land contamination. Empirical approaches have been proposed with thresholding methods to detect the leading edge of the waveform, among them is the OCOG algorithm [44] It is the baseline retracker for hydrology in the Sentinel-3 PDGS products. The method proposed in this paper is in the continuation of numerical retracking over ocean proposed by [38] and adapted here to lakes observations This approach consists in simulating altimeter waveforms and using it as model inside the retracking algorithm.
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