DeWit and Stankiewicz said they happened upon the results unintentionally while working on a mathematical model to study African river drainage.
The team's work involved creating a database of measurements for 1.25 million miles (2 million kilometers) of rivers in Africa taken from topographic maps of individual nations.
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 rainbetween 400 and 1000 millimeters (16 and 39 inches) a yearwere 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.
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