So research into a vast reservoir of groundwater beneath the continent is meaningful.

Richard Taylor, a hydrologist at University College London, said there is reason to be optimistic.

“What we’re saying here really is that there are quite considerable groundwater resources in Africa that might help to regulate soil moisture and therefore food security,” said Taylor.

The study estimates that there are 0.66 million cubic kilometers of groundwater beneath the continent, an estimated 100 times more than what’s available on the surface.

Maps produced by the British Geological Survey and University College London show the spots where groundwater is most plentiful - in Libya, Algeria, Egypt and Sudan.

The study suggests that countries currently designated as "water scarce" could be sitting on groundwater supplies tens of meters deep.

Currently only 5 percent of African agriculture uses irrigation. Scientists predict the demand for water will grow rapidly with the population.

Inevitably there are fears of over-extraction. But the small-scale nature of much African agriculture could prevent a so-called "tragedy of the commons," Taylor said.

“The actual low productivity aquifers, there’s kind of a silver lining to them being low productivity. It means that an individual largely affects themselves. If they over-pump the aquifer, the geology is such that the impacts of that over-pumping are not translated very far, and they are really only doing themselves injury,” said Taylor.

Aid agencies are warning of an imminent food crisis in parts of Niger and Mali. Could boreholes drilled through to the groundwater solve problems like this? Not necessarily on their own, according to Taylor.

“Part of us putting numbers to it, quantifying the groundwater resources that are available in many locations, is to try to improve the explicit consideration of groundwater as one of many strategies to improve food production and resilience to drought,” he said.

The authors say this is no sure solution - but suggest aquifers could provide a more reliable source of water than the notoriously unpredictable rains.