Continental-Scale Groundwater Response Dynamics to Climate Variability in Africa

Our work has aimed to deal with the long-term groundwater response to understand the natural and human roles at the continental scale, with a time window ranging from 2003 to 2024. A comprehensive mass balance of the terrestrial water storage (TWS) approach was used for the determination of monthly groundwater storage change to attribute its variability to both climatic and human-induced drivers. Our findings show a long-term upward pattern of groundwater storage anomaly (GWSA), demonstrating a dual reality of groundwater response to precipitation, where the flux of the groundwater system (recharge/discharge) is quickly responsive to inter-annual extremes of precipitation, with a response time of 0 months. However, the state (total storage) is slow and integrated with a multi-year response time peaking approximately 29 months after a precipitation event. There is a significant hydro-climatic regime shift that developed after 2018, where groundwater was being replenished to levels never seen before. This surge occurred because of positive precipitation anomalies caused by a prolonged multi-year La Niña phase, and there is a shift within the regime of the hydrogeological system of the continent from negative human contribution at the beginning (2003) to positive human intervention toward the end of the study time range. This is primarily driven by sustained decreasing groundwater withdrawal, mainly for agricultural activities, at a slower rate than the continent's total annual groundwater renewal. Although the continent has recently benefited from a period of intense, climate-driven recharge (post-2019), this has occurred in defiance of a massive and growing human-induced withdrawal signal. This positive continental trend also masks severe concurrent regional droughts, such as the catastrophic 2020-2022 La Niña-induced drought in the Horn of Africa. These findings present the first complete picture of the African groundwater system, underscoring its susceptibility to significant climatic phenomena such as the El Niño-Southern Oscillation (ENSO). It also highlights the critical need to incorporate regional variations and diverse climatic factors in all future water security assessments carried out on water security. Moreover, continental groundwater storage anomaly is a robust proxy for the integrated impact of major climate teleconnections like ENSO. This work measures these opposing forces and finds that the current positive balance of groundwater storage is vulnerable and relies on the proper maintenance of the positive wet climate. This indicates the need for sustainable policy interventions in the groundwater sector based on long-term agricultural water productivity to maintain the value of the most significant African source of freshwater.