Many climate scientists already predict that less rain will fall annually in parts of Africa within 50 years due to global warming.
Now new research suggests that even a small decrease in rainfall on the continent could cause a drastic reduction in river water, the lifeblood for rural populations in Africa.
A decrease in water availability could occur across about 25 percent of the continent, according to the new study.
Hardest hit would be areas in northwestern and southern Africa, with some of the most serious effects striking large areas of Botswana and South Africa.
Geologists Maarten de Wit and Jacek Stankiewicz of the University of Cape Town in South Africa conducted the research. Their findings appear in the current edition of the online journal Science Express.
To predict future rainfall, the scientists compared 21 of what they consider to be the best climate change models developed by research teams around the world. On average, the models forecast a 10 to 20 percent drop in rainfall in northwestern and southern Africa by 2070.
The researchers then juxtaposed these rainfall predictions with measurements of Africa's rivers to gauge the future of Africa's water supply.
With a 10 percent drop in rainfall, parts of Botswana (map) would be left with just 23 percent of the surface-water flow it has now, their study showed.
With a 20 percent decrease, Cape Town would be left with just 42 percent of its river water, and "Botswana would completely dry up,'' de Wit said.
In parts of northern Africa, river water levels would drop below 50 percent.
"It's like erasing large sections of the rivers from the map,'' de Wit said of the findings.
Areas in Danger
DeWit and Stankiewicz said they happened upon the results unintentionally while working on a mathematical model to study African river drainage.
The pair developed a measure called drainage density, the total length of a river per unit area, such as square mile.
"If you increase precipitation by [a factor of] two, you would have to add in more riverbeds to get rid of that water. So you would have a higher drainage density,'' de Wit said.
"If you decrease the drainage, you don't need as many riverbeds to get rid of it all."
Equipped with this data, the team then plugged in the climate change models, which all predict various decreases in rainfall.
They found that areas that receive "intermediate" amounts of rain—between 400 and 1000 millimeters (16 and 39 inches) a year—were the most vulnerable to a drop or rise in rainfall.
In those regions the total length of rivers shrinks or expands sharply with rainfall, de Wit explained. About 25 percent of Africa and 75 percent of its countries have some land in this category.
If an area that now receives 600 millimeters (24 inches) of rain per year got just 550 milliliters (22 inches) of rain in the future, the river drainage would drop by up to 25 percent.
A further reduction in rainfall to 450 millimeters (18 inches) would cut river drainage by half.
Less river water would have serious implications not just for people but for the many animal species whose habitats rely on regular water supplies.
"The Cape [of Good Hope in South Africa] is a biodiversity hotspot," de Wit said. "Will it be there in 50 years?''
Less Water, More Conflict
Unlike other continents, Africa does not have a large mountain range that produces snowmelt, rivers, and precipitation, de Wit explained.
"In Africa surface water is extremely important. Most of the people outside urban areas are still reliant on surface water for daily use,'' he said.
If de Wit's estimates are correct, the situation for rural Africans is grave, said Adil Najam, a professor of negotiation and diplomacy at Tufts University in Medford, Massachusetts.
Many people in Africa spend more time and money on acquiring water than nearly any other resource, he said.
If water becomes more scarce or expensive, people will do what they must to obtain it.
"Water is nonnegotiable. If you are poor, you don't stop drinking water,'' Najam said.
Less river water may heighten international conflicts, he added, because many African rivers cross international boundaries.
National Geographic News
March 3, 2006