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Monday, October 18, 2010

Drying Lakes

Lake Chad images (1972 and 1987)
 as posted Waterless on the Town Square blog
EarthView is a terrific learning tool, whether viewed from inside or out. During today's program at the Ahern Middle School, a student looking at north-central Africa from outside asked about the body of water surrounded by dry land. What she was noticing was Lake Chad, which was once the sixth-largest lake in the world but had already lost a considerable amount of its surface area by the time EarthView was painted in the 1990s. In that sense it is much like the Aral Sea, which long ago surrendered its status as the world's fourth-largest lake, and which the EarthView artist represented with a similar, broad fringe of dry land.

Both lakes are found in arid or semi-arid lands, and are shared by multiple countries -- Nigeria, Niger, Chad, and Cameroon in the first case and Kazakhstan and Uzbekistan in the latter. Moreover, in the case of the Aral Sea, lands in four additional countries contributed directly to the drainage area of the sea.

Image from the Fire Earth blog
Growing or shrinking lakes represent shifts in the water balance for the watersheds (also known as drainage basins) in which they are found. Lakes can shrink as a result of water use that exceeds rainfall in the basin as a whole. This in turn can affect habitat in and near the lake, the availability of irrigation or drinking water, and even the regional climate.

Both of these shrinking lakes are excellent examples of the use of satellite images to monitor lakes. The  United State Geological Survey  (USGS) includes both Lake Chad and the Aral Sea in its EarthShot series, which documents many other examples of the uses of remote sensing. The Visualizing Earth project at UC-San Diego has a similar Aral Sea page that -- like the EarthShots pages -- allows users to move between satellite and ground-level images.

For decades to come, Lake Chad and the Aral Sea will remain reminders of how seriously humans can damage the natural environments on which we depend. Despite the bad news, some positive steps are being taken: international bodies are working together to protect the wetlands of Lake Chad and a new dam is helping to increase the flow of water to the Aral Sea. People in many countries have much more to do, however, if these lakes are to be fully restored!

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