Preprints
https://doi.org/10.5194/esurf-2021-99
https://doi.org/10.5194/esurf-2021-99

  14 Dec 2021

14 Dec 2021

Review status: this preprint is currently under review for the journal ESurf.

Unraveling the hydrology and sediment balance of an ungauged lake in the Sudano-Sahelian region of West Africa using remote sensing

Silvan Ragettli1, Tabea Donauer1,2, Peter Molnar2, Ron Delnoije3, and Tobias Siegfried1,4 Silvan Ragettli et al.
  • 1hydrosolutions Ltd.,Venusstrasse 29, 8050 Zurich, Switzerland
  • 2ETH Zurich, Institute of Environmental Engineering, Zurich, Switzerland
  • 3Caritas Switzerland, Bamako, Mali
  • 4ETH Zurich, Institute of Science, Technology and Policy, Zurich,Switzerland

Abstract. The presence of ephemeral ponds and perennial lakes in the Sudano-Sahelian region of West Africa is strongly variable in space and time. Yet, they have important ecological functions and societies are reliant on their surface waters for their lives and livelihoods. It is essential to monitor and understand the dynamics of these lakes to assess past, present, and future water resource changes. In this paper, we present an innovative approach to unravel the sediment and water balance of Lac Wégnia, a small ungauged lake in Mali near the capital of Bamako. The approach uses optical remote sensing data to identify the shoreline positions over a period of 22 years (2000–2021) and then attributes water surface heights (WSHs) to each observation using the lake bathymetry. The method represents a significant advancement over previous attempts to remotely monitor lakes in the West African drylands, since it considers not only changes in water depth to explain recent declining trends in lake areas, but also changes in the storage capacity. We recognize silting at the tributaries to the lake, but overall, erosion processes are dominant and threaten the persistence of the lake because of a continuous decrease of the floor level at the outflow. This explains the decreasing trend in WSH even for the wet-season, in spite of positive rainfall patterns.

Silvan Ragettli et al.

Status: open (until 25 Jan 2022)

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  • RC1: 'Comment on esurf-2021-99', Anonymous Referee #1, 17 Jan 2022 reply

Silvan Ragettli et al.

Silvan Ragettli et al.

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Short summary
This paper presents a novel methodology to identify and quantitatively analyze deposition and erosion patterns in ephemeral ponds or in perennial lakes with strong water level fluctuations. We apply this method to unravel the water and sediment balance of Lac Wégnia, a designated RAMSAR site in Mali. The study can be a showcase for monitoring Sahelian lakes using remote sensing data, as it sheds light on the actual drivers of change in Sahelian lakes.