Groundwater in Africa supports livelihoods and poverty alleviation 1,2 , maintains vital ecosystems, and strongly influences terrestrial water and energy budgets 3. However, hydrologic processes governing groundwater recharge sustaining this resource, and their sensitivity to climatic variability, are poorly constrained 4,5. Here we show, through analysis of multi-decadal groundwater hydrographs across sub-Saharan Africa, how aridity controls the predominant recharge processes whereas local hydrogeology influences the type and sensitivity of precipitation-recharge relationships. Some humid locations show approximately linear precipitation-recharge relationships with small rainfall intensity exceedance thresholds governing recharge; others show surprisingly small variation in recharge across a wide range of annual precipitation. As aridity increases, precipitation thresholds governing initiation of recharge increase, recharge becomes more episodic, and focussed recharge via losses from ephemeral overland flows becomes increasingly dominant. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by largescale climate controls. Intense precipitation, even during lower precipitation years, produces substantial recharge in some dry subtropical locations, challenging the 'high certainty' consensus that drying climatic trends will decrease water resources in such regions 4. The likely resilience of groundwater in many areas revealed by improved understanding of precipitation-recharge
Changes in the intensity of precipitation as a result of global warming are expected to be especially pronounced in the tropics. The impact of changing rainfall intensities on groundwater recharge remains, however, unclear. Analysis of a recently compiled data set of coincidental, daily observations of rainfall and groundwater levels remote from abstraction for four stations in the Upper Nile Basin over the period 1999-2008 shows that the magnitude of observed recharge events is better related to the sum of heavy rainfalls, exceeding a threshold of 10 mm day −1 , than to that of all daily rainfall events. Consequently, projected increases in rainfall intensities as a result of global warming may promote rather than restrict groundwater recharge in similar environments of the tropics. Further monitoring and research are required to test the robustness of these findings, but the evidence presented is consistent with recent modelling highlighting the importance of explicitly considering changing rainfall intensities in the assessment of climate change impacts on groundwater recharge.
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