Validation of the altimetry-based water levels from Sentinel-3A and B in the Inner Niger Delta

Adama Telly Diepkilé,Eric Mougin,Flavien Egon,Fabien Blarel,Frédéric Frappart

doi:10.5194/piahs-384-31-2021

Abstract

Abstract. The comprehension of water level fluctuations and the sustainability of the Inner Niger River Delta (IND) is a major concern for the scientific community, but also for the local population. Located in the centre of Mali, the heart of the Sahel, the delta is characterised by a floodable area of more than 32 000 km2 during the rainy season, which contributes very strongly to the vitality of local ecosystem, and is consequently classified as a Ramsar site under the international Convention for Wetlands. In addition, the Delta acts as an environmental and socio-economic development barometer for the entire sub-region. Nowadays, we can observe an increasing fragility of the delta due to climate change, desertification and human activities, and justifies the need for permanent monitoring. The present study is based on the recent successes of radar altimetry, originally designed to monitor the dynamics topography of the ocean, and now very frequently used to retrieve inland water levels, of lakes, rivers, and wetlands. Previous studies evaluated the performances of several radar altimetry missions including Low Resolution Mode (LRM) (Topex-Poseidon, Jason-1/2/3, ERS-2, ENVISAT, and SARAL, and Synthetic Aperture Radar (SAR) Sentinel-3A missions for water level retrievals over 1992–2017. More than 50 times series of water levels were build at the crossing between water bodies and Sentinel-3A and 3B over 2016–2020. Twenty-four comparisons between in-situ and altimetry-based time-series of water levels were achieved over the IND. RMSE generally lower than 0.7 m and r higher than 0.9 were obtained.

Highlights

Radar altimetry (RA) has been experiencing numerous inovations in terms of acquisition mode and data processing over all types of Earth surfaces (Abdalla et al, 2021)
Several tenths of RA-based time-series of water levels were generated over the IND 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

Summary

Introduction

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). 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)

Methods

Results

Conclusion