40-Year Volume Changes of West African Lakes Derived from SWOT and Optical Imagery

The thousands of lakes and reservoirs in West Africa are vital water resources for humans and ecosystems. In addition to marked annual and interannual dynamics, some of these lakes have experienced significant variations over the past sixty years related to climate and anthropogenic changes. Since existing studies have addressed the volume evolution of only a few lakes, large-scale volume trends in West Africa remain poorly documented. Here, we use the unprecedented spatial coverage of Surface Water and Ocean Topography (SWOT) swath altimetry, combined with the extensive temporal coverage of Landsat optical image archives, to derive the annual and 40-year volume dynamics of over 1300 West African lakes. Our results show that a few large reservoirs, mostly located in the sub-humid region, control most of the total annual volume dynamics. These large reservoirs have also driven the total lake volume increase observed across the entire study region over the past 40 years. At the individual scale, over the 1984 and 2024 period, 43% of lakes showed insignificant trends, 49% positive trends, mostly related to increased precipitation and reservoir impoundment during the analyzed period, and 8% negative trends. Furthermore, 17% of lakes have decreased in volume over the past two decades, suggesting a potential recent shift in volume for a number of lakes. This work is a significant step toward comprehensively quantifying annual and long-term changes in West African lake volumes. This information can inform hydrological modeling, global change impact assessments, and water management policies.