By Richard Black
Environment correspondent, BBC News website, Nairobi
Rainwater harvesting could prove a cheap, easy solution to Africa's water woes, according to a UN report.
Capture systems help manage water resources more efficiently
Scientists found enough rain falls in some countries to supply six or seven times the current need, and provide security against future droughts.
A pilot project in a Kenyan Maasai community has improved supplies and done away with the daily trek to collect river water.
Currently, 14 out of 53 nations are classified as "water stressed".
This number is forecast to double by 2025.
The UN Environment Programme (Unep) says that a cultural change is needed across the continent.
"The biggest problem is awareness," said Elizabeth Khaka of Unep.
"Many people think of rainwater harvesting as a 'poor person's technology'," she told the BBC News website, "and we have to change that."
Last week, the Kenyan government announced plans to make all new buildings include capacity for rainwater collection and storage.
Using geographical information systems (GIS) technology, scientists from Unep and the World Agroforestry Centre mapped rainfall patterns across nine countries in southern and eastern Africa.
They compared these maps with plots of population density and land use.
"We were very surprised by what we found," said Ms Khaka. "We went over the figures again and again to check them."
Kenya, with a population of about 40 million people, could collect enough rain to supply six or seven times that figure, Unep calculates; Ethiopia, often regarded as a dry country, could collect enough for half a billion people.
"In the popular mind, Africa is seen as a dry continent," said Dennis Garrity, director general of the World Agroforestry Centre. "But overall, it actually has more water resources per capita than Europe.
"However, much of Africa's rain comes in bursts, and is rapidly swept away or is never collected. The time has come to realise the great potential for greatly enhancing drinking water supplies and smallholder agricultural production by harvesting more of the rain when and where it falls."
Some forecasts of future climate predict that extreme droughts will become more common across the continent, so rainwater harvesting could be a cheap and effective way to "climate-proof" some communities, Unep believes.
The technique has been tried in a pilot project in Kisamese, about 30 minutes' drive from Nairobi.
Here, collection and storage facilities including containers and mini-reservoirs, or "earth pans", have been installed. Trenches help water soak into the soil in small kitchen gardens.
"Before this, we had to walk 10km to the river each day to get water," said Agnes Liorket, a leader of the Kisamese community.
In some regions, women spend a third of their calories collecting water
"Now we don't have to do that, and the water is better quality, a lot cleaner," she told the BBC News website.
In some regions of Africa, women spend a third of their calories collecting water.
Not all rain can be collected. Unep estimates at least a third needs to go into lakes, streams and rivers, for use by people downstream and by nature's consumers.
And rainfall patterns vary across the continent. Parts of the West African coast will have six months of daily rain followed by six months with none; other regions are more mixed.
"Even where you have extreme patterns, rainwater harvesting can be very useful," said Elizabeth Khaka.
"You have rain for six months; even if you only collect enough for the next three, that's still better than none."