Using modelling systems and long time-series climate data, the researchers, from the University of Wisconsin-Madison, found that the new drier climate, combined with higher agricultural demands for water, have caused what was once one of Africa’s largest lakes to shrink. A biosphere model (IBIS) allowed the researchers to simulate the exchange of energy, water and carbon dioxide between vegetation, soil and the atmosphere, and tracked the changes in Lake Chad since 1953. Data from the biosphere model was then inputted into a hydrological model, enabling changes in river discharge, and the amount of water in wetlands and in the lake to be estimated.

“Lake Chad was about 25,000 square kilometres in surface area back in 1963,” said Jonathan A Foley, one of the researchers. Water levels are now reduced to as little as 839 square miles (1,350 sq km).

The researchers calculated that there was a 30% decrease in the lake between 1966 and 1975, with irrigation only accounting for five percent of the decrease. However, agricultural demands increased four-fold between 1983 and 1994 in response to the drier climate, by which time it was accounting for 50% of the additional decrease in the size of the lake, taking into account seasonal variations.

“Climate data has shown a great decrease in rainfall since the early 1960’s largely due to a decrease in the number of large rainfall events,” said Michael T Coe, one of the researchers. Over the last 40 years, the discharge from the Chari/Logone river system, which transports 90% of the runoff generated in the drainage basin, has decreased by almost 75%. “The problem is expected to worsen in the coming years as population and irrigation demands continue to increase,” said Coe.

According to the scientists, regional officials have noticed the dramiatic effect that the shrinking lake is having on its surrounding inhabitants. In the summer of 1998, the president of Chad hosted the 10th Lake Chad summit with leaders from Nigeria, Niger, the Central African Republic, Cameroon and Sudan to discuss how to boost water levels.

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