By Roland Pease
BBC science correspondent
Africa could be faced with 25% less water by the end of the century because of global warming, scientists have warned in a new report.
Drought has already caused major problems in Africa
The research, published in the journal Science, shows geographical factors will amplify changes in rainfall patterns resulting from climate change.
Semi-arid areas such as southern Africa would be the most vulnerable.
The authors add that water shortages could provoke conflicts over rivers that cross borders.
Failing rains are already a major cause of hardship in Africa, as witnessed by the current drought in East Africa, and climate scientists are working hard to predict how global warming will change rainfall patterns across the continent.
Managing water resources
The research shows that rainfall has a direct effect on the way water drains into streams and rivers, and therefore on supply.
When rainfall in an area is less than 400mm per year, there is virtually no drainage into rivers and streams. But above this threshold, as rainfall increases from 400mm to 1,000mm per year, drainage also increases.
In effect, this means that in some areas a fall in precipitation can amplify the effect of water loss. For example, in regions that have an average rainfall of 500mm per year, a 10% reduction in rainfall could actually halve the available water.
The wettest and driest regions, such as the Congo basin or Sahara, should not be strongly affected.
But semi-arid regions, such as southern Africa and the Sahel in particular, are extremely vulnerable.
Managing their water resources will become even more critical than it is now, the authors warn, particularly where major rivers cross borders.
But in parts of East Africa, where rainfall is predicted to increase slightly, the extra water could extend existing swamps and the prevalence of the water-borne diseases they foster.
Either way, the study from the University of Cape Town, South Africa, shows adjusting to the effects of climate change could be even harder in Africa than previously realised.