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» Lake Chad, West Africa
 

Lake Chad was once the sixth largest lake in the world, but prolonged drought and increased water use have shrunk the lake dramatically. It now spans less than a tenth of the area it covered in the 1960s. Back then, the lake covered about 25,000 square kilometers, an area the size of the U.S. state of Vermont. Now it’s smaller than Rhode Island.

The fluctuations in lake water levels have stabilized in recent years, but it is still a dynamic environment. Social conflicts and insecurity in the region make it difficult to monitor water levels with ground-based measurements. Satellite imaging has therefore been crucial to monitoring the changes to the lake, especially with the infrared bands on Landsat sensors, which make it easier to distinguish water and vegetation.

These Landsat images show the overall transition of Lake Chad from open water to wetland. The desert appears tan, wetlands are green, and open water is blue.

The black and white 1963 image is from the film-based Argon reconnaissance satellite program, declassified in the 1990s.

Map of the featured area.

 

Lake Chad’s lakebed is flat and shallow, so small changes in water level greatly affect its surface water area. During an average rainy season, the water level in the lake is typically 5–8 meters deep, although it has gone through many wet and dry climatic regimes for thousands of years when water levels were much higher or lower than normal. Therefore, it may be more accurate to think of Lake Chad as a deep wetland than a lake.

In fact, Lake Chad can now be divided into three distinct regions: the southern pool, the northern pool, and the Great Barrier. The southern pool is fed by the Chari River, which provides around 90% of Lake Chad’s water. The northern pool is irregularly flooded and contains a series of sand dune islands. The Komadougou-Yobe River, which has become a seasonal river, enters the northern pool but contributes to the lake only in rainy seasons. A ridge referred to as the Great Barrier separates the two pools.

Water level fluctuations are not new to Lake Chad. In the last 1,000 years, according to fossil evidence, the lake probably dried out a half-dozen times. A chart shows levels since the 1870s, from actual measurements and from estimates based on Nile River discharge.

This image series shows the location of the Chari River delta on the southern end of Lake Chad, along with increasing amounts of wetland.

While the overall trend in this time series is a shrinking lake, the northern portion has some water in the later images, but it varies widely. Water levels there remained below the 1900–2010 mean and far below what it was in the 1960s.

The region has a short rainy season, and changes in rainfall amounts can greatly affect the water supply. About 90% of the rain falls from June to September, but the lake doesn’t rise until November. This is the natural lag time of water flowing through the Chari River system.

In this series of Landsat images from 2017–2018, the water rises through February and then tapers off slowly for several months. So the rise in water levels actually occurs during the dry season.

Water displays as dark blue, and wetlands are green. The brighter speckles are sand dune islands. Landsat will continue to provide information about Lake Chad’s changes, its infrared imaging capability accurately delineating land, water, and wetland.

References

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