Huge reserves of underground water in some of the driest parts of Africa could provide a buffer against the effects of climate change for years to come, scientists have said.
Researchers from the British Geological Survey and University College London have for the first time mapped the aquifers, or groundwater, across the continent and the amount they hold.
"The largest groundwater volumes are found in the large sedimentary aquifers in the North African countries Libya, Algeria, Egypt and Sudan," the scientists said in their paper, published on Friday.
They estimate that reserves of groundwater across the continent are 100 times the amount found on its surface, or 0.66 million cubic kilometres.
Writing in the journal Environmental Research Letters, they cautioned, though, that not all of the reserves are accessible.
Where they are, small-scale extraction by hand pump would be better than large-scale drilling projects, which could quickly deplete the reservoirs and have other unforeseen consequences.
Groundwater is no panacea for Africa's water shortages but it could form an important part of a strategy to cope with an expected sharp increase in demand for water as the continent's population increases.
Even now, some estimates put the number of Africans without access to safe drinking water at more than 300 million and only 5 per cent of arable land is irrigated.
"What's new about what we've identified here is that the [groundwater in] areas to the south of the Sahara and to the north of the Kalahari... are of strategic importance for enabling not only the provision of rural water supply, but also, perhaps, in increasing the irrigation that takes place in sub-Saharan Africa," Richard Taylor, a researcher at the University College London who was involved in the research project, told Al Jazeera.
"Many people have an awareness that the groundwater is there, but the striking thing is that we've quantified it," he said.
"[In the report], we've been very systematic in using evidence that's already been collected and putting it together in a format and in a way that we can now look at the continent as a whole and understand that the groundwater resources really are quite considerable."
The researchers say some of the largest deposits are in the driest areas of Africa in and around the Sahara, but some of them are deep - at 100 to 250 metres below ground level.
"While these groundwater resources are quite vast, it is quite expensive [to extract]," Taylor said.
Phoebe White, a water, sanitation and hygiene specialist for the UK Department for International Development based in Kinshasa, Democratic Republic of Congo, said hand pumps in the DRC cost up to $13,000 apiece and in some areas the aquifers are too deep and other pumps must be used.
In areas of DRC where drilling deep boreholes is required the cost can be around $130,000, although problems of accessibility and infrastructure can push that figure up, according to White.
Mismanagement and neglect
Roger Calow at the Overseas Development Institute, a UK think-tank which was involved in the programme that spawned the research, said the paper shows water shortages in large parts of Africa do not stem from scarcity.
He said that a third of hand pumps across Africa have broken down due to a lack of maintenance.
Researcher Doctor Stephen Foster, a London-based senior adviser for aid group Global Water Partnership and an expert in groundwater issues, noted that projects have so far failed due to cost and logistics problems.
"In northern Nigeria there have been groundwater irrigation projects that have failed because of the rising cost of fuel - a major factor in drilling costs - and distribution difficulties," he said.
"It is not as simple as drilling big bore holes and seeing rice fields spring up everywhere."
But the research could be key to developing sustainable water access for millions of Africans, say the researchers.
"We are providing quantitative evidence here of the dimension of that groundwater resource with the expressed intention that development can be done in a thoughtful and logical manner that in the long term leads to sustainable use of these resources rather than the unsustainable use of groundwater resources that we've observed in places like China, India, and of course historically in places like the United States," Taylor told Al Jazeera.
Focus on management
Aid agencies gave the research a cautious welcome.
"The discovery of substantial water reserves under parts of Africa may well be good news for the continent but it may prove hard to access in the near term and, if not sustainably managed, could have unforeseen impacts," Nick Nuttall, a spokesperson for the United Nations Environment Programme (UNEP) in Nairobi.
Nuttall said over-abstraction exploitation of groundwater in Mexico City, for example, is undermining the foundations of buildings.
He said the focus of efforts to improve water supply should be on better collection and storage.
"The fact is that there is already a tremendous amount of water available for Africa but it is rarely collected".
A study by UNEP and the World Agroforestry Centre found there is enough water falling as rain over Africa to supply the needs of some 9 billion people.
"Ethiopia, where just over a fifth of the population are covered by domestic water supply and an estimated 46 per cent of the population suffer hunger, has a potential rainwater harvest equivalent to the population needs of over 520 million people," Nuttall told the Reuters news agency.