By Alex Kirby
BBC News Online environment correspondent
The spectre of famine and reliance on outside help could soon threaten large parts of Africa, scientists believe.
Droughts will mean less food
They think increasing water scarcity may leave much of the continent not only thirsty, but without enough water to grow sufficient food for its needs.
On present trends, they expect one in three of the world's people will be affected by water shortages in 2025.
The annual crop loss across Africa could be as much as the entire grain harvest produced by the US and India.
The scientists, from the Consultative Group on International Agricultural Research (CGIAR), were speaking at the launch of the group's Challenge Programme on Water and Food.
The programme is trying to find ways to improve the management of available water, and will work on technologies to increase crop yields while cutting the amount of water needed.
Food beyond reach
Answers are likely to include higher-yielding crops which are more resistant to drought, and farming methods that combine agriculture with fish-farming.
CGIAR says water scarcity projections for Africa south of the Sahara suggest household water consumption there will by 2025 show the highest proportional increase of any world region.
Hundreds of millions of people will be affected by drought
With "business as usual" policies and investments, the group says, the number of Africans without access to clean water will more than double to 401 million, though at worst the total could be 523 million people.
CGIAR says: "The region will face a 23% shortfall in crop yields because of insufficient water supply, and cereal imports will have to more than triple to 35 million tons in the next 23 years to keep pace with demand.
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"Under these conditions, many poorer African countries will be unable to finance the required imports of food, leading to rising levels of hunger and malnutrition and greater dependence on international financial support or food aid."
Professor Frank Rijsberman of CGIAR said: "If present trends continue, the livelihoods of one-third of the world's population will be affected by water scarcity by 2025. We could be facing annual losses equivalent to the entire grain crops of India and the US combined.
Nature squeezed out
"Agricultural subsidies in North America and Europe determine where food is grown, and policy decisions taken in the World Trade Organisation are possibly the single most dominant factor shaping the global demand for food and consequently the amount of water required to grow that food...
"We have to make this issue is everyone's business because it will affect everyone's future."
Old and young will suffer most
Globally, agriculture consumes about 70% of the world's fresh water, and nearly 90% in developing countries.
CGIAR says this is sharpening competition between farmers' needs and those of the natural world. In the last 50 years, 40% of the world's wetlands have been lost.
Its researchers will be working in nine large river basins in Africa, Asia and Latin America, using them as "living laboratories" to test their findings.
One research project already approved will examine how to improve barley varieties in Ethiopia, and another will work on ways to use floodwaters for breeding fish, to improve people's nutrition and tackle poverty.
This project will include the Indus-Ganges and Mekong basins in Asia, and the Niger in Africa.
A CGIAR team will also analyse India's huge national river-linking project, a $120bn scheme which aims by 2016 to link 37 rivers and channel water from the Ganges-Brahmaputra-Meghna basin to drought-affected parts of western India.