These images show desertification near the border of Mauritania and Senegal, in westernmost Africa. In the zoomed-in 1972 image you can see a highway leading to the northwest corner of the image; in 1990 you can see that highway surrounded by the bright tones of bare, sandy soil.
As the annotated image indicates, the international border here is the Senegal River, flowing westward and interrupting the arid lands to the north and south. The regional capital Rosso is on the north bank of the river, and the town of Richard Toll ("Richard's Field" in the Wolof language) is on the south bank. Mauritania's National Highway 2 connects the regional capital Rosso to the national capital Nouakchott.
Desertification
When we say land has become desert, what does that mean?
It is not simply the process of an area becoming dry, and it is not simply caused by drought. In fact, desertification is usually defined as a process that happens to land that is already normally arid (dry) or semi-arid. The crucial factor is a decline in the biological productivity of the land-- the amount of vegetation that grows (either naturally or planted by man), as well as the animal life supported by the plants. If desertification continues, eventually the land becomes a desert, with increased wind and water erosion, decreased soil fertility, and decreased water-retention capacity. Plant and animal communities decline in number and diversity, as many species can no longer survive.1
Desertification is often portrayed simplistically as a moving line, with barren sand dunes pushing forward into productive land. But really, desertification is usually a patchy, cancer-like development, spreading outward from pockets of land where the vegetative cover has been harmed or destroyed, so that sandy dry soil begins to drift and vegetation is unable to reestablish itself.2
In western Africa, boreholes (water wells) are often the points of this vegetation disturbance, because they attract livestock which overgraze the land nearby.3 But in this example, the point of disturbance is a paved highway that was built to connect Nouakchott, the national capital of Mauritania, with the regional capital Rosso. Besides vehicle travel on the road itself, this highway has encouraged building, settlement, human travel and animal transport along its corridor. All of these activities consume vegetation-- for grazing, fuel, and building material. The vegetative cover became disturbed, some sandy soil began to drift, and a process of desertification was underway.
Both images are from the dry season which follows the wetter summer growing season. The 1972 image is just on the heels of a dry summer, while the 1990 image follows a rather wet year but is farther into the dry season. In the 1990 image, the gray to the north represents dried-out vegetation, which had grown that wet summer and which is less reflective than bare soil. Note how apparent the highway corridor is. The 1972 image has more bright, exposed sandy soil, with a hint of healthy vegetation (faintly pink tones) in the northwest.
You may also notice a difference in pattern between the puddled- or mottled-looking land in the extreme northwest and the more streaky-looking land just to the east. The prevailing winds over hundreds of years have caused the longitudinal dunes across the whole area, since the soil across this whole area is mostly sandy. But the soils in the extreme northwest have more clays and clayey loams mixed in with the soils. The river bottoms, of course, are very different.4
In the 1990 image you can see many bright red fields. Most of them are sugar cane and rice, and virtually all of them are irrigated. The amount of irrigated land increased greatly between 1972 and 1990, and new irrigation canals are visible. These crops are able to grow in the dry season because they are not dependent on rainfall.
Footnotes
1. The World Bank (The International Bank for Reconstruction and Development), 1985, Desertification in the Sahelian and Sudanian zones of west Africa: Washington, D.C., The World Bank, p. 5.
2. Monique Mainguet, 1994, Desertification; natural background and human mismanagement: Berlin, Springer-Verlag, p. 12.
3. United Nations Environment Programme, 1997, World atlas of desertification, second edition: Edward Arnold, London, p. 74.
4. Remote Sensing Institute, South Dakota State University, 1982, Resource Inventory of Southwestern Mauritania: Brookings, S.D., 334 pp. and 5 maps. Map 3, Soil Map of Southwestern Mauritania.
Charles Toupet, 1977, Atlas de la Republique Islamique de Mauritainie: Paris, Editions Jeune Afrique, 64 pp.
LM1220049007227490 (Landsat 1 MSS, 30 September 1972)
XM8M20504990339 (Landsat 5 MSS, 5 December 1990, from the Maspalomas, Canary Islands, receiving station, via the European Space Agency)
Defense Mapping Agency, 1979 (compiled 1973, revised 1979), Operational Navigation Chart J-1: edition 3, scale 1:1,000,000.
All photographs for this article were taken by Gray Tappan, USGS EROS.
Campbell, Robert Wellman, ed. 1997. "Rosso, Mauritania: 1972, 1990." Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. http://earthshots.usgs.gov. This article was released 14 February 1997.
Other references
Satellite images
Map
Photographs
How to cite this article