Scientists say the notoriously dry continent of Africa is sitting on a vast reservoir of groundwater.

They argue that the total volume of water in aquifers underground is 100 times the amount found on the surface.

The team have produced the most detailed map yet of the scale and potential of this hidden resource.

Writing in the journal Environmental Research Letters, they stress that large scale drilling might not be the best way of increasing water supplies.

Across Africa more than 300 million people are said not to have access to safe drinking water.

Demand for water is set to grow markedly in coming decades due to population growth and the need for irrigation to grow crops.

Freshwater rivers and lakes are subject to seasonal floods and droughts that can limit their availability for people and for agriculture. At present only 5% of arable land is irrigated.

Now scientists have for the first time been able to carry out a continent-wide analysis of the water that is hidden under the surface in aquifers. Researchers from the British Geological Survey and University College London (UCL) have mapped in detail the amount and potential yieldof this groundwater resource across the continent.

Helen Bonsor from the BGS is one of the authors of the paper. She says that up until now groundwater was out of sight and out of mind. She hopes the new maps will open people's eyes to the potential.

"Where there's greatest ground water storage is in northern Africa, in the large sedimentary basins, in Libya, Algeria and Chad," she said.

"The amount of storage in those basins is equivalent to 75m thickness of water across that area - it's a huge amount."

According to Helen Bonsor, sometimes the slower means of extraction can be more efficient.

"Much lower storage aquifers are present across much of sub-Saharan Africa," she explained.

"However, our work shows that with careful exploring and construction, there is sufficient groundwater under Africa to support low yielding water supplies for drinking and community irrigation."

The scientists say that there are sufficient reserves to be able to cope with the vagaries of climate change.

"Even in the lowest storage aquifers in semi arid areas with currently very little rainfall, ground water is indicated to have a residence time in the ground of 20 to 70 years." Dr Bonsor said.

"So at present extraction rates for drinking and small scale irrigation for agriculture groundwater will provide and will continue to provide a buffer to climate variability."