Observed evidences of shallow groundwater resources threatened by multi-decadal re-greening practices in the Sahel

Abstract. Re-greening initiatives in drylands in response to desertification processes represent a key contributing policy for mitigating climate change impacts, restoring biodiversity, and sustainable land management. However, their long-term impact on surface or groundwater resources are rarely assessed and remains poorly documented at a global scale. This study examines how pioneering (1970s–1990s) re-greening and soil and water control (SWC) efforts have affected both surface water drainage and groundwater dynamics within a transboundary watershed of ~3,000 km² in the central Sahel, West Africa. Using original field data, water table surveys (1960s–2020s) and a series of historical aerial photographs (1950s–1970s) supplemented with high resolution satellite imagery (1980s–2020s), trends in land use, land cover and hydrological changes were reconstructed. First, a marked decrease (1.6 fold) in the drainage network density was delineated in the upstream part of the basin in response to the development of SWC agricultural techniques, that resulted in lower hortonian runoff through gullies to the downstream river valley. Since groundwater recharge was shown to be mostly indirect through the river network, lower runoff entailed lower groundwater recharge. Second, dense reforestation schemes by non-indigenous phreatophyte trees (Azadirachta indica) took place through the main valley to control aeolian erosion processes, inadvertently increasing evapotranspiration discharge fluxes from the initially shallow (< 8 m below ground level) water table. Both of these processes (lower aquifer recharge, higher groundwater uptake by trees) contributed to a long-term plurimetric (3 to 6 m) drop in the piezometric levels, reducing storage or even causing a near dry-up of the alluvial aquifer. Overall, these results highlight a Sahelian trade-off that is relevant for SDG 15 and the flagship African Great Green Wall initiative: while re-greening and SWC practices may improve land conditions, they may also threaten locally fragile groundwater resources.