Isotopic tracers in precipitation and groundwater as proxies for climate variability and change in the Sahel (North-West Africa)

This study investigates how stable isotopes and tritium in precipitation, groundwater, and surface water record spatial and temporal climate variability across the Sahel. Rainwater shows strong spatial gradients linked to mixed Atlantic and continental moisture inputs. Southern Sahelo-Sudanese and Sudanese zones are characterized by more depleted isotopic signatures, higher rainfall, and stronger monsoon influence, whereas northern Sahelian, Sahelo-Saharan, and Saharan zones show more enriched values due to lower precipitation, higher temperatures, and stronger evaporation. Groundwater shows a broadly meteoric origin across the five climatic zones, but its isotopic composition is strongly influenced by local recharge, evaporation, and interaction with surface water. Rainwater time-series from 1961 to 2019 highlight three climatic phases affecting isotopic composition. Tritium indicates modern recharge, mixing with older groundwater, and, during “Drought period 2”, recharge carrying the bomb-derived atmospheric signal. These tracers reveal climate-driven recharge processes in the Sahel.