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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

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The distributions of/i180, salinity, temperature, and nutrients have been used to quantify water sources to the Mackenzie shelf in the Beaufort Sea. Comparison of water mass analyses with satellite imagery confirms that the meteoric (runoff) water is associated with the Mackenzie plume. The seasonally variable surface layer for the shelf is viewed as cycling between a "reverse estuary" in winter, when the polar mixed layer (PML) is formed, and a positive estuary in summer when the shelf waters respond to freshwater inputs (runoff and ice melt). We infer a standing stock of 3.7 m fresh water at the end of summer 1986, of which 30% owes its origin to the melting of sea ice; our data coupled with river flow imply a freshwater flushing time for the Mackenzie shelf at about 150 days. To re-form the PML during winter requires the removal of this seasonal fresh water through the combined processes of flushing and ice formation: once this fresh water has been removed, continued ice growth can produce "new" brine which would be observed as a deeper and saltier PML from the previous year. A simple geochemical model shows that autumn conditions (freshwater accumulation) and the rate of flushing are important controls on the potential of the shelf to produce "new" brine and that winter runoff, were it to distribute evenly across the shelf, is sufficient to inhibit brine production. INTRODUCTION Arctic Ocean studies have recently focused on freshwater budgets [Aagaard and Coachman, 1975; Hanzlick and Aagaard, 1980; Anderson et al., 1983] and particularly on the two main sources, runoff (including Pacific Ocean sources) and ice melt [e.g., Tan and Strain, 1980; Ostlund and Hut, 1984]. The reason for this interest is simple: fresh water plays a key role in maintaining the Arctic halocline and, by inference, controls ice cover and deep water formation, both of which have climatic implications [Carmack, 1989]. The Arctic Ocean can be considered as a large, complex estuary where seasonal runoff impinges on wide shelves synchronously around the basin edge; in fact, the combined inflow (about 3500 km3yr -• [Treshnikov, 1985]) is surpassed in magnitude only by the Amazon. Additional runoff (estimated at 1670 km3yr -1 when referenced to salinity of 34.8 practical salinity units (psu) [Aagaard and Carmack, 1989]) enters indirectly through Bering Strait. Unique to cold region estuaries is a melt-freeze cycle which can cause positive or negative estuarine forcing [Carmack et al., this issue]. The logical place to look for reverse estuarine behavior (surface production of brine) is in the shear zone between landfast ice and the moving pack and in polynyas where water is kept open for long periods in winter. There seasonal processes have been implicated in producing brine drainage and hence maintaining the halocline [Melling and Lewis, 1982; Aagaard et al., 1981]. We expect therefore that the important estuarine processes (both negative and positive) occur over the shelves, are strongly seasonal, and are unevenly distributed within the Arc...

One of the key uncertainties surrounding the impacts of climate change in Africa is the effect on the sustainability of rural water supplies. Many of these water supplies abstract from shallow groundwater (<50 m) and are the sole source of safe drinking water for rural populations. Analysis of existing rainfall and recharge studies suggests that climate change is unlikely to lead to widespread catastrophic failure of improved rural groundwater supplies. These require only 10 mm of recharge annually per year to support a hand pump, which should still be achievable for much of the continent, although up to 90 million people may be affected in marginal groundwater recharge areas (200-500 mm annual rainfall). Lessons learnt from groundwater source behaviour during recent droughts, substantiated by groundwater modelling, indicate that increased demand on dispersed water points, as shallow unimproved sources progressively fail, poses a much greater risk of individual source failure than regional resource depletion. Low yielding sources in poor aquifers are most at risk. Predicted increased rainfall intensity may also increase the risk of contamination of very shallow groundwater. Looking to the future, an increase in major groundwater-based irrigation systems, as food prices rise and surface water becomes more unreliable, may threaten long-term sustainability as competition for groundwater increases. To help prepare for increased climate variability, it is essential to understand the balance between water availability, access to water, and use/demand. In practice, this means increasing access to secure domestic water, understanding and mapping renewable and non-renewable groundwater resources, promoting small-scale irrigation and widening the scope of early warning systems and mapping to include access to water.Key words groundwater; climate; Africa; water supply; drought; agriculture Quel impact aura le changement climatique sur les approvisionnements ruraux en eaux souterraines en Afrique? Résumé Une des incertitudes principales concernant les impacts du changement climatique en Afrique est l'effet sur la durabilité des approvisionnements ruraux en eau. Nombre de ces approvisionnements prélèvent dans des nappes souterraines peu épaisses (<50m) et sont l'unique source sûre d'eau potable pour les populations rurales. Les analyses des études existantes de précipitations et de recharge suggèrent que le changement climatique est peu susceptible de conduire à une défaillance catastrophique généralisée des ressources rurales améliorées en eaux souterraines. Ces dernières ne nécessitent que 10 mm de recharge par an pour satisfaire une pompe manuelle, ce qui devrait être réalisable pour une grande partie du continent, bien que 90 millions de personnes puissent être affectés dans des zones marginales de recharge en eaux souterraines (200-500 mm de pluviosité annuelle). Les leçons apprises sur les comportements des ressources souterraines au cours des sécheresses récentes, renforcées par des modélisations hydrogéologi...