Although it is now the largest desert on Earth, during
the last ice age the Sahara was a savannah with a climate
similar to that of present-day Kenya and Tanzania. The annual
rainfall was much greater than it is now, creating many rivers
and lakes that are now hidden under shifting sands or exposed
as barren salt flats. Over several hundred thousand years
the rains also filled a series of vast underground aquifers.
Modern African nations are now mining this fossil water
to support irrigated farming projects.
The above pair of images shows a small settlement just north of the
border between Egypt and Sudan. The dark circleseach about a
kilometer acrossindicate central-pivot irrigation. A well
drilled in the center of each circle supplies water to a rotating
series of sprinklers. From October 31, 1999 to December 23, 2001 the
number of irrigated fields increased dramatically.
Rainfall in this area of the Sahara is only a few centimeters a
year, so the aquifers will take thousands of years (or longer) to
recharge, making the water a non-renewable resource. Although no
one knows how much water is beneath the Sahara, hydrologists
estimate that it will only be economical to pump water for fifty
years or so. On the other hand, alternative technologies for providing
fresh water in this arid regionprimarily desalinizationare too
expensive for widespread use. Sudan, Libya, Chad, Tunisia, Morocco
and Algeria are some of the other Saharan nations irrigating with
fossil water, but the practice is not limited to Africa. In the southern plains
of the United States, the Olgallala aquifer is being drained faster
than it can be replenished.
These true-color images were acquired by the Enhanced Thematic Mapper
plus (ETM+) aboard NASAs Landsat 7 satellite. The Landsat satellites
enable scientists to monitor land use and land cover change dating back to
1972. Landsat 7 is designed to last until at least 2004, and follow-on
missions are currently being planned.
Images and animation by Robert Simmon, based on data aqcuired by the
Landsat 7 Science Team Landsat data is archived
and distributed by the USGS EROS Data Center