Analysis of Retrackers’ Performances and Water Level Retrieval over the Ebro River Basin Using Sentinel-3

Abstract

Satellite altimeters have been used to monitor river and reservoir water levels, from which water storage estimates can be derived. Inland water altimetry can, therefore, play an important role in continental water resource management. Traditionally, satellite altimeters were designed to monitor homogeneous surfaces such as oceans or ice sheets, resulting in poor performance over small inland water bodies due to the contribution from land contamination in the returned waveforms. The advent of synthetic aperture radar (SAR) altimetry (with its improved along-track spatial resolution) has enabled the measurement of inland water levels with a better accuracy and an increased spatial resolution. This study aimed to retrieve water levels from Level-1B Sentinel-3 data with focus on the minimization of the land contamination over small- to middle-sized water bodies (130 m to 4.5 km), where continuous clean waveforms rarely exist. Three specialized algorithms or retrackers, together with a new waveform portion selection method, were evaluated to minimize land contamination in the waveforms and to select the nadir return associated with the water body being overflown. The waveform portion selection method, with consideration of the Digital Elevation Model (DEM), was used to fit the multipeak waveforms that arise when overflying the continental water bodies, exploiting a subwaveform-based approach to pick up the one corresponding to the nadir. The performances of the proposed waveform portion selection method with three retrackers, namely, the threshold retracker, Offset Center of Gravity (OCOG) retracker and two-step SAR physical-based retracker, were compared. No significant difference was found in the results of the three retrackers. However, waveform portion selection using DEM information great improved the results. Time series of water levels were retrieved for water bodies in the Ebro River basin (Spain). The results show good agreement with in situ measurements from the Ebro Reservoir (approximately 1.8 km wide) and Ribarroja Reservoir (approximately 400 m wide), with unbiased root-mean-square errors (RMSEs) down to 0.28 m and 0.16 m, respectively, depending on the retracker.

Performance comparison among typical open global DEM datasets in the Fenhe River Basin of China

“…Finally, through accuracy evaluation and performance comparison, the accuracy of the DEM datasets can be improved using different methods (Q. Gao et al, 2019;F. Li et al, 2018;O'Loughlin et al, 2016).…”

Editorial for Special Issue “Advances in Satellite Altimetry and Its Application”

“…The contributions of this special issue are sorted along their application area: the first paper [1] handles method development applicable to various regions of the Earth. Afterward, oceans [2,3], polar regions [4,5], coastal areas [6,7] and inland waters [8] are addressed. The last paper [9] deals with the Moon's topography.…”

“…The last paper [9] deals with the Moon's topography. All relevant satellite radar altimetry measurement techniques are covered: from Low Resolution Mode (LRM) [4][5][6] to Synthetic Aperture Radar (SAR) [1,2,4,5,7,8] and SAR in mode [7] to SWOT [3] as well as laser altimetry [1]. In the following, each paper is briefly summarized.…”

“…Gao et al [8] assessed the performance for Sentinel-3 inland water level products. They focused on the minimization of the land contamination over small-to medium-sized water bodies (down to 130 m).…”

Monitoring Large-Scale Inland Water Dynamics by Fusing Sentinel-1 SAR and Sentinel-3 Altimetry Data and by Analyzing Causal Effects of Snowmelt

“…Thus, to overcome the waveform difference to the standard Brown model [24], a computation-intensive waveform reprocessing (retracking) analysis is usually required for inland waterbodies [4,[25][26][27]. However, the selection of retrackers (ground processing techniques that estimate the range to the point of closest approach on the surface) varies in different studies, and most of them use in-house algorithms or refinements [4,22,28,29], which largely limits the transferability of the method to other regions.…”