Humans to blame for shrinking African lake

MADISON, Wis. – Because of unrelenting human demand for water, Africa’s Lake Chad, once one of the continent’s largest bodies of fresh water, has shriveled to a ghost of a great lake.

In a few decades, Lake Chad has shrunk to a size smaller than Great Salt Lake from a surface area the size of Lake Erie.

“It’s about one-20th of the size it was 35 years ago,” says University of Wisconsin scientist Michael T. Coe, who led the NASA supported study.

“It’s a huge change” from 25,000 square kilometers of surface area in 1963 to 1,350 square kilometers today.

The human need for water, mostly through massive irrigation projects, and the competing demands of the four nations that share the lake, account for almost 30 percent of the observed decrease in lake area since the early 1960s, Coe said.

Bad as that is, the problem is expected to worsen as climate change forces an even heavier reliance on irrigated agriculture.

By merging historical data of climate and water use in a computer model, the Wisconsin team charted the impact of a shifting pattern of climate over the past 40 years and the rapid growth of human consumption of water from Lake Chad and the rivers that flow into it.

The Lake Chad drainage basin, which is similar in size to the Mississippi River basin, is a closed system that depends on monsoon rains to replenish the water.

But the region is extremely sensitive to climate fluctuations and has experienced a significant decline in rainfall since the early 1960s.

The amount of water diverted to nearby fields over the past 40 years has affected the lake’s equilibrium. Add poverty, political instability and national rivalries over a scarce resource to the mix, and a recipe for ecological disaster results, Coe said.

“The Chari River and the lake are the important, life-sustaining systems for this corner of the world,” Coe said. “Irrigation activity is significant, and they now have more capacity than they can use because there is less water.”

Until about 1979, irrigation had a modest impact on the hydrology of the region. But between 1983 and 1994, the amount of water diverted for irrigation quadrupled over water used for the previous 25 years.

In addition to the radically reduced lake surface area, the flow of water from the primary river system that feeds it has decreased by almost 75 percent over the past 40 years.

Because Lake Chad is shallow, it responds rapidly to changes in precipitation and runoff. A shifting climate, with fewer large rainfall events, will place the lake in jeopardy.

“The study illustrates the importance of considering human activities on water resources, even in very large hydrologic systems,” the Wisconsin climate researchers say.

The case of a shrinking Lake Chad demonstrates how human activities exacerbate problems caused by a decline in precipitation.

“The take-home message,” Coe said, “is that humans have a big impact on these systems and now, through the use of computer simulations, we have some predictive abilities for what humans can do to them.”

The future of Lake Chad, Coe says, is gloomy: “It will be a puddle. It will be completely managed. You’ll get crops and drinking water out of it, but you’ll have no ecosystem left to speak of.”

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