February 27, 2001

AFRICA'S LAKE CHAD SHRINKS BY 20 TIMES DUE TO IRRIGATION

DEMANDS, CLIMATE CHANGE

In the 1960s, North central Africa's Lake Chad was larger than the state of
Vermont but is now smaller than Rhode Island. NASA-funded researchers using
computer models and climate data now understand why Africa's freshwater Lake
Chad has been disappearing over the last 30 years.

Michael T. Coe and Jonathan A. Foley of the University of Wisconsin-Madison
cite a drier climate and high agricultural demands for water as reasons why
what was once one of Africa's largest freshwater lakes is shrinking. "Lake
Chad was about 25,000 square kilometers in surface area back in 1963," Foley
noted. Now the lake is about one-twentieth the size it was in the mid 1960s.

Their paper titled "Human and Natural Impacts on the Water Resources of the
Lake Chad Basin," is being published today in the American Geophysical
Union's Journal of Geophysical Research. In their paper, Coe and Foley used
an integrated biosphere model (IBIS) with long time-series climate data.
They simulated the exchange of energy, water and carbon dioxide between
vegetation, soil and the atmosphere, and tracked the changes in Lake Chad
since 1953. They input the data from the biosphere model into a hydrological
model and were able to estimate changes in river discharge, the amount of
water in wetlands and in Lake Chad.

Using model and climate data, Coe and Foley calculate that a 30 percent
decrease took place in the lake between 1966 and 1975. Irrigation only
accounted for 5 percent of that decrease, with drier conditions accounting
for the remainder. They noticed that irrigation demands increased four-fold
between 1983 and 1994, accounting for 50 percent of the additional decrease
in the size of the lake.

"NASA Landsat satellite imagery taken of the lake over the last 30 years
really capture the model conclusions and visualize them very well," the
researchers noted.

Lake Chad and the Chari/Logone river system, which transports 90 percent of
the runoff generated in the area basin, are important water resources for
the local population. The lake is 820 feet (250 km) above sea level and is
shared by Chad, Cameroon, Nigeria and Niger. Lake Chad has always undergone
seasonal and inter-annual fluctuations because it is less than 23 feet (7 m)
deep. In recent decades, during wet periods the lake expands up to 10,000
square miles (25,900 square km). The warming climate and increasing
desertification in the surrounding Sahel region have dropped water levels
far below the average dry season level of 4,000 square miles (10,000 square
km) to only 839 square miles (1,350 square km).

The Northern Africa Sahel region has experienced numerous devastating
droughts over the last three decades. "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," Coe said.

Lake Chad's primary source of water comes from the monsoon rains that
typically fall in June, July and August. Meanwhile, the use of water for
irrigation has increased, in response to the drier climate. Over the last 40
years, the discharge from the Chari/Logone river system at the city of
N'Djamena in Chad has decreased by almost 75 percent, drastically reducing
the input into the lake. Between the increase in agricultural water use and
the drier climate, there has been a massive decline in the amount of water
in Lake Chad.

With a drier climate and less rainfall, agricultural areas become more
desperate for water to irrigate their crops, and will continue draining what
is left of Lake Chad. Foley said, "The problem is expected to worsen in the
coming years as population and irrigation demands continue to increase."

Regional officials have noticed the dramatic effect 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.

NASA's Earth Observing System funded the Lake Chad study. The overall goal
of NASA's Earth Observing System is to advance the understanding of the
entire Earth system on a global scale by improving our knowledge of the
components of the system, the interactions between them and how the Earth
system is changing.

Images and additional information can be found at:
http://www.gsfc.nasa.gov/gsfc/earth/environ/lakechad/chad.htm