Grundwasser in Afrika stabil – trotz Klimawandel

Immer wieder hört man, dass der Klimawandel die Grundwasser-Ressourcen in Afrika schwinden lässt. Diese Vorstellung ist jedoch falsch, sagt nun eine Studie des University College London (UCL). In humiden Gebieten ist die Grundwasserbildung ziemlich konstant, während sich in arideren Gebieten das Grundwasser episodisch neu bildet, z.B. im Zusammenhang mit Starkregenphasen. Hier die Pressemitteilung des UCL aus dem August 2019:

Groundwater resources in Africa resilient to climate change

Groundwater – a vital source of water for drinking and irrigation across sub-Saharan Africa – is resilient to climate variability and change, according to a new study led by UCL and Cardiff University. A consortium of 32 scientists from across Africa and beyond carried out the research, published in Nature, which shows how groundwater replenishment depends upon heavy rainfalls and flood events, amplified by climate change. The findings are significant given the fact that 1 in 3 people currently lack access to safe water in Sub-Saharan Africa and a dramatic expansion of irrigation is required to feed growing populations.

Groundwater plays a central role in sustaining water supplies and livelihoods in sub-Saharan Africa due to its widespread availability, generally high quality, and intrinsic ability to buffer episodes of drought and increasing climate variability. Professor Richard Taylor (UCL Geography), co-lead on the study, said: “Groundwater offers a potential pathway to sustain increases in freshwater use required to achieve UN Sustainable Development Goals 2 (zero hunger) and 6 (safe water for all).”

“Our study reveals, for the first time, how climate plays a dominant role in controlling the process by which groundwater is restocked. This improved understanding is critical for producing reliable climate change impact projections and adaptation strategies.” The research involved the collation of multi-decadal records of groundwater levels and rainfall to examine how the replenishment of groundwater has responded to variations in climate and geology. The team analysed observations compiled from nine countries across sub-Saharan Africa representing a range of climates from hyper-arid to humid.

The study shows that in humid areas groundwater is replenished primarily by rainfall that directly infiltrates the land surface, whereas in drylands it occurs predominantly by leakage from temporary streams and ponds. Local geology also plays a role in determining the sensitivity of rates of replenishment to changes in climate.

This research, supported by UK research councils (NERC, ESRC, EPSRC), the Department for International Development (DFID) and The Royal Society, also shows that, in drylands, the heavy rainfalls and flood events generating groundwater replenishment are commonly associated with climate variation phenomena such as El Niño and La Niña.

Dr Mark Cuthbert (Cardiff University), co-lead on the study, said: “Previous regional-level assessments of groundwater resources using large-scale models have ignored the contribution of leaking streams and ponds to groundwater supplies, underestimating its renewability in drylands and resilience to climate change.” “Knowledge of the replenishment process can inform strategies to enhance groundwater supplies. In drylands, where groundwater resources are often the only lasting source of freshwater, such strategies can further exploit the predictability of heavy rainfalls and flood events that generate groundwater,” added Dr Cuthbert.  

Hier der Abstract von Cuthbert et al. 2019, erschienen in Nature:

Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa

Groundwater in sub-Saharan Africa supports livelihoods and poverty alleviation1,2, maintains vital ecosystems, and strongly influences terrestrial water and energy budgets3. Yet the hydrological processes that govern groundwater recharge and sustainability—and their sensitivity to climatic variability—are poorly constrained4,5. Given the absence of firm observational constraints, it remains to be seen whether model-based projections of decreased water resources in dry parts of the region4 are justified. Here we show, through analysis of multidecadal groundwater hydrographs across sub-Saharan Africa, that levels of aridity dictate the predominant recharge processes, whereas local hydrogeology influences the type and sensitivity of precipitation–recharge relationships. Recharge in some humid locations varies by as little as five per cent (by coefficient of variation) across a wide range of annual precipitation values. Other regions, by contrast, show roughly linear precipitation–recharge relationships, with precipitation thresholds (of roughly ten millimetres or less per day) governing the initiation of recharge. These thresholds tend to rise as aridity increases, and recharge in drylands is more episodic and increasingly dominated by focused recharge through losses from ephemeral overland flows. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by large-scale climate controls. Intense precipitation, even during years of lower overall precipitation, produces some of the largest years of recharge in some dry subtropical locations. Our results therefore challenge the ‘high certainty’ consensus regarding decreasing water resources4 in such regions of sub-Saharan Africa. The potential resilience of groundwater to climate variability in many areas that is revealed by these precipitation–recharge relationships is essential for informing reliable predictions of climate-change impacts and adaptation strategies.