Recharge in natural and urbanised subhumid dryland basement aquifers in Sub-Saharan Africa

Understanding groundwater recharge processes in dryland and seasonally dry subhumid environments is central to improving water‑resource resilience under increasing climatic variability and land‑use change. This study integrates multi‑year groundwater level observations from crystalline basement aquifers in an unpumped agricultural setting about 30 km north of Harare, Zimbabwe (2018-2025) with recent groundwater‑level data from eight boreholes monitored in the pumped urban area of the city (2022–2025). Together, these high-resolution (30-min frequency) datasets provide insight into both natural and urbanised dryland systems, where recharge is highly sensitive to rainfall variability and human pressures.

Recharge in the agricultural sites was characterised using water‑table fluctuation (WTF) methods, chloride mass balance (CMB), water‑stable isotopes, and dissolved gas residence time tracers. The effect of variation in land use—such as tilled land, land under conservation agriculture, and woodland—on the responses to cumulative rainfall and rainfall events of varying magnitude was studied by integrating daily rainfall data collected at the research site. Recharge was observed for most years across all sites and was controlled by hydrogeological settings, rainfall totals and antecedent conditions, i.e. the groundwater level at the end of the preceding dry season. No measurable recharge occurred at most of the agricultural sites during a year of poor rainfall (380 mm total), highlighting the strong climatic dependency of basement‑aquifer recharge in subhumid drylands. Annual groundwater level variations were mostly limited to 2 to 3 m.

In contrast, the urban groundwater dataset from Harare revealed markedly different recharge behaviour. Groundwater‑level fluctuations were strongly influenced by nearby pumping, producing hydrographs that diverged from the smoother, rainfall‑controlled signals seen in natural settings and showing much stronger annual variation of groundwater levels up to about 15 m. However, pumping did not mask the overall annual recharge pattern, though at some sites, groundwater capture was markedly increased. A subset of the boreholes had similar hydrographs to those in the unpumped, agricultural setting. The variability in drawdown and recovery responses across the eight urban boreholes underscores the need for well‑designed, spatially distributed groundwater‑monitoring networks to evaluate the sustainability of abstraction and to detect changes in recharge availability under increasing urban water demand. In addition, capturing and integrating existing datasets—such as drilling logs, pump tests, and historical abstraction records—can provide valuable baseline information to support groundwater management in Harare.

Taken together, these findings advance understanding of recharge processes in both natural and urban dryland basement aquifers, emphasise the sensitivity of recharge to climatic variability, and highlight the implications for sustainable groundwater management under changing land use and rainfall regimes.