The Sahara Desert covers most of the North African country of Algeria.
Rainfall is scarce, and it comes and goes rapidly, flowing through channels
called wadis. When dry, as they are in this true-color Landsat Thematic
Mapper image from January 1, 1987, the wadis are revealed as pale tracings
across the darker brown and gray shades of the desert landscape. In many
cases, these wadis end in small pools which have no outlet. The water flows
in from the wadis and gradually evaporates, leaving behind salts and other minerals which appear white in this image. At the upper left
of the image, a few pools of bright blue-green water are hanging on despite
the aridity, their greenish color probably due to high salt and mineral
content, or perhaps the growth of algae.
Wadis are prime breeding locations for populations of the desert locust,
which inhabits vast desert areas across northern Africa, Arabia and
southwest Asia. Moisture initiates the hatching of the locusts’ eggs
and also brings a bloom of fresh vegetation, which provides food and a place
for the insects to hide from predators. Normally, the locust prefers a
solitary existence, but when suitable habitat is confined to small areas,
such as narrow wadis, large numbers of locusts may be forced to congregate.
Close physical contact flips a biological switch in the locusts’
metabolism and behavior that makes them more tolerant of each other and
enhances swarming. Locust swarms can migrate hundreds of miles and devour
all vegetation in their path. Monitoring desert habitat for changes in
vegetation using satellite data gives scientists the opportunity to
anticipate where swarms might be forming and to direct ground-based survey
crews with increased efficiency.