Radar altimetry (RA) has been experiencing numerous inovations in terms of acquisition mode (from Low Resolution Mode – LRM – to Synthetic Aperture Radar – SAR –, and even Interferometry SAR – InSAR) and data processing over all types of Earth surfaces (Abdalla et al., 2021). Owing to their availability over almost 30 years, since the launch of Topex/Poseidon and ERS-1 missions in and 1991 and 1992, respectively, the two first missions to provide high accuracy measurements (Stammer and Cazenave, 2017), RA data are increasingly used, over land, to globally monitor the water levels of rivers, lakes and reserveroirs, wetlands and floodplains (Birkett et al., 2011; Crétaux et al., 2017). To ensure the confidence in the reliability of RA-based water levels, it is necessary to determine their quality through comparisons against in-situ measurements. In these operations are routinely performed over the ocean at different calibrration/validation sites (e.g., Bonnefond et al., 2011; Mertikas et al., 2018), few of these facilities are available over land, except at Lake Issykkul, in Kyrgyzstan (Central Asia) (Crétaux et al., 2009). Sentinel-3A and 3B are two of the most recently launched RA missions. They operate in SAR and use the Open-Loop (OL) or Digital Elevation Model (DEM) tracking mode designed to reduce the loss of tracking over hilly areas (Biancamaria et al., 2017; Taburet et al., 2020). Very few studies evaluated the accuracy of both Sentinel-3A and 3B over inland water bodies, and mostly over lakes (Frappart et al., 2021; Kittel et al., 2021; Shu et al., 2021).

Owing the availability of a large number of in-situ measurements, the IND has often been chosen as a study area for validating RA-based water levels (Frappart et al., 2015; Goita and Diepkile, 2012; Normandin et al., 2018). In this study, RA-based water levels defined at the cross-sections of Sentinel-3A and 3B ground-tracks and river and floodplains in the IND were compared to in-situ water stages.

Time-series of water level were produced at the cross-section between an altimeter ground-track and a water body using the the Altimetry Time-Series (AlTiS) software (Frappart et al., 2021). From the Geophysical Data Records (GDR) which contain the along-track altimetry data, AlTiS computed the altimeter height (h) as described in (Crétaux et al., 2017) and Eq. (1): 1h=H-R-ΔRion-ΔRdry-ΔRwet-ΔRsol-ΔRpol-N where H is the altitude of the satellite on its orbit, R the range or distance between the satellite and the surface, ΔRion, ΔRdry, ΔRwet, ΔRsol, ΔRpol, are the corrections to apply to the range to account for the delay introduced by the ionosphere, the troposphere (dry and wet components), and the effects of the solid Earth and pole tides, and N is the geoid model. Following Frappart et al., 2006), the ranges derived from Offset of Center of Gravity (OCOG) (Wingham et al., 1986) were used as they were found to provide more accurate water levels.

Then, the valid height values were manually selected through AlTiS Graphical User Interface (GUI). Once the valid data are selected, the time-series of water levels is generated computing the median of the heights for each altimetry cycle (i.e., every 27 d for Sentinel-3A and B).

The altimetry-based time-series of water levels generated over the rivers in the Inner Niger Delta were compared to in-situ water stages using the Root-Mean-Square Error (RMSE), Relative (ratio of the RMSE to the mean annual amplitude of the water level at the station multiplied by 100) RMSE (RRMSE), and the Pearson correlation coefficient (r).

Several tenths of RA-based time-series of water levels were generated over the IND (rivers and floodplains) using Sentinel-3A and 3B data: 28/24 on the Sentinel-3A/3B RA ground-tracks (Fig. 1). They offer a dense network of RA Virtual Stations (VS) were time-series of water levels are estimated. Two examples of comparisons of in-situ and RA-based time-series of water level are presented in Fig. 2. The first one to a cross-section of Sentinel-3A and 3B located on the Bani River, a south-west tributary to the Niger River, at (4.74∘ W, 13.51∘ N), the second one one to a cross-section of Sentinel-3A and 3B located on the Niger River at (4.27∘ W, 15.46∘ N), on the nort central part of the IND. Comparisons were made with gauge records of Beneny Kegny (4.917∘ W, 13.383∘ N) and Akka (4.233∘ W, 15.400∘ N) stations, respectively. The distance to the Sentinel-3 SV is 28 and 8 km, respectively.

RA-based water levels exhibit similar temporal variations as the in-situ gauge stations, with a well-marked seasonal cycle that can reach 6 m (Fig. 2). RMSE of 0.35 m/0.30 m, and r of 0.99/0.99 were found at Beneny Kegny Akka for Sentinel-3A and 3 B, respectively, and RMSE of 0.45 m and r of 0.98 at Akka for Sentinel-3A. As too few in-situ measurements were collected or made available for the Akka gauge station during the period of operation of Sentinel-3B, we were unable to estimate RMSE, RRSME and r for Sentinel-3B. Nevertheless, as RA-based water levels from Sentinel-3A and Sentinel-3B exxhibit very similar temporal variations and taking into account the good agreement between in-situ and Sentinel-3A-based water levels, similar perfomances can be expected from Sentinel-3B based water stages. This latter example brings into focus the strong interest of radar altimetry to provide a continuous monitoring of water levels in case in-situ gauge stations stopped operating for a reason or another.

The same evaluation parameters (distance between the VS and the in-situ station, RMSE, RRMSE and R) were estimated for 12 Sentinel-3A VS and 12 Sentinel-3B VS. They are presented on Fig. 3, except the RRMSE not shown here. Comparisons were made on distances between the in-situ station and the VS ranging from 7 to 126 km (Fig. 3a). The RMSE range from 0.15 to 1.39 m, with 13 values over 0.5 m (Fig. 3b). If these values can be considered quite high, the corresponding RRMSE are all lower or equal 20 %, and 19 RRMSE values out of 24 are below 15 %. Correlations are all higher or equal to 0.79 (Fig. 3c). Twenty one out of 24 r values are above 0.9, and even, 16 of them are higher than 0.95, confirming the good accuracy of Sentinel-3 based water stages reported in previous studies over rivers (e.g., Bogning et al., 2018; Kittel et al., 2021; Normandin et al., 2018). The high RMSE values can be accounted for the distances between the in-situ gauges and the SV. If the temporal variations are likely to exhibit a similar pattern over distances of several tenths of kilometers, the seasonal amplitude can be affected by changes in depth and width of the river along of the river network. As a consequence, the correlation can still present high values when the RMSE increases. This situation is what is observed here: high correlation values were estimated, slightly decreasing as the distance increases, but RMSE generally increases with the distance (e.g., RMSE of 0.93 and 1.39 m were obtained fro distances of 126 and 93 km, respectively, for corresponding r of 0.79 and 0.91).

An extensive assessment of the quality of the data acquired by Sentinel-3A and 3B was achieved over the IND over the period 2016–2020. Fifty two VS were defined over the rivers and the floodplains using the newly developed AlTiS software, and 24 comparisons against in-situ water levels were made. An overall very good agreement with r values ranging from 0.79 to 1, but generally over 0.90 or 0.95, and RRMSE lower than 20 %, and, most of the times, lower tha 15 %. The high RMSE obtained for some VS (RMSE >0.50 m) are, most of the time, due to the long distance, above 50 km, between the in-situ gauge stations and the VS. Owing to the density of the in-situ gauge stations over the IND and the rapid availability of the water stages, The Inner Niger Delta is a good site for the validation of the recently launched (e.g., Sentinel-6/Jason-CS in November 2020) or to be launched missions (e.g., Surface Water and Ocean Topography – SWOT in fall 2022).