Unraveling the hydrology of water bodies in the African Sahel Region using continental scale remote sensing

Water resources in the African Sahel Region are under increasing pressure due to climatic changes, population growth and land degradation. Often, societies rely on surface water from lakes and rivers to sustain their lives and livelihoods. It is therefore essential to monitor and understand the dynamics of these water bodies to assess past, present, and future water resource changes.

Here we use satellite imagery and altimetry to determine water level and storage changes in small water bodies across the African Sahel. The method consists of detecting the ever-shifting edge of lakes and rivers in Landsat and Sentinel-2 optical imagery and assigning heights to shoreline points using altimetric data from ICESat-2satellite. This so-called “waterline method” assumes that the water-land boundary can be regarded as a contour line that connects points of equal elevation. We present novel extension of the waterline method which also allows to identify bathymetry changes over time from shoreline position observations. By tracking the temporal changes of surface water contour shapes, we can quantitatively analyse erosion and deposition processes. Past reservoir capacity changes and water storage variations are thus retrieved from optical remote sensing data, which are available over much longer periods of time and at higher revisiting frequenciesthan altimetry data.

The operational implementation of the method offers access to the water levels and storage variations of more than 300 water bodies in 10 Sahelian countries over the period 2000-2021. The identified spatio-temporal trends reveal fascinatingly heterogeneous patterns of drying and wetting across the Sahelian zone. Wet-season water level data reveal increasing trends over the last 20 years from West to East. Dry-season water availability then depends to a large degree on storage capacity.

Finally, we use the method for a detailed attribution analysis to identify drivers of change at Lac Wégnia, a designated RAMSAR site in Mali. The lake is characterized by an alarming decrease of dry-season surface water extent over the last 20 years. We recognize silting at the tributaries to the lake, but overall, erosion processes are dominant and threaten the persistence of the lake because of continuous backward erosion at the outlet of the lake. This explains the decreasing trend in water levels even for the wet-season, in spite of positive rainfall patterns.