Deserts

DESERTS

Predominant landscape of the Middle East and North Africa.

Stretching from the Atlantic coast in the west to Pakistan in the east, a band of arid land (15° and 30° north latitude) dominates this region. The North African expanse is generally known as the Sahara, although subdivisions within it have individual names indicating the nature of the surface. The terms erg (as in the Great Eastern Erg of Algeria) and serir (as the Serir of Kalanshu in Libya) indicate a region of sand dunes. Where the surface is rocky underfoot the terms used are reg or hamada (for example, the Hamada of Dra south of the Anti-Atlas mountains). Individual areas may also be given the name desert, the Western Desert and the Eastern Desert in Egypt, although they are smaller parts of the whole. On the peninsula of the same name, the Arabian Desert is an extension of the Sahara and is divided into the Rub al-Khali (Empty Quarter, a region of vast sand dunes) and the Nafud and Najd. To the north is the Syrian Desert, and to the east the two deserts of the Iranian plateau are known as the Dasht-e Kavir and the Dasht-e Lut.

The term desert is one in common usage and therefore difficult to define. Most experts prefer to speak of "drylands" or "arid lands" and to define such places through various measures of the availability of water for plant growth (implying that not all deserts are hot.) A common definition of desert, however, is those regions of Earth's surface having fewer than 10 inches (250 mm) of precipitation annually and extreme high temperatures. This classical approach relates such measures to areas with types of vegetation adapted to hot, arid conditions. In areas with much sunshine and small amounts of precipitation and/or natural moisture from the soil, only plants called xerophytes survive—those adapted to such conditions. In certain hyperarid locations, precipitation may be even less and no vegetation of any kind is found.

Desert rainfall is not only sparse but is also extremely variable in time and space as well as in quantity. Such variance means that human occupancy of the desert must depend for survival on reliable springs and rivers for irrigation rather than on precipitation. Traditional pastoral nomadism, located on the desert margins, was adapted to this environment by moving its productive units (i.e., herds and flocks) to where grass and water seasonally occurred. But even nomads ventured into the true desert only for travel as transporters and raiders. The few permanent inhabitants of the deserts were those oasis dwellers dependent upon perennial springs for intensive agriculture and the growing of date palms.

Desert soils are usually of poor quality except for those in the valleys of rivers where alluvial deposits have accumulated. True desert soils—called aridisols —have low biomass, very sparse or no organic acids and gases, few or no bacteria, and are essentially mineral in character. Any rain or sheet flooding and runoff that percolate beneath the surface rapidly evaporate. As a result, soluble salts are precipitated and redeposited, forming a crusty layer on the surface or just beneath it. Repeated leaching and deposition can result in concentrations of sodium chloride (NaCl), white alkali (salt), or similar deposits of sodium carbonate (Na2CO3), black alkali, which poison the soil and make agriculture impossible. Under desert conditions agriculture is extremely difficult, and even the use of irrigation water can cause salinity, through evaporation and the precipitation of the dissolved salts it may carry, which leads to the abandonment of such farmland.

The natural xerophytic vegetation found in deserts has adapted to conditions of high temperatures and scant and irregular amounts of precipitation. Xerophytes often occur as drought-resisting plants with heavy cuticles, which reduce transpiration, or with stomata, which can be closed for the same purpose. Other xerophytes reduce water use by shedding their leaves and remaining leafless during the dry season. Among these plants are the euphorbia and the cacti, the latter originally found only in the Western Hemisphere.

Phreatphytes constitute another class of desert vegetation, which includes palms. These plants have developed long taproots, which reach the water table, allowing them to survive the driest of surface conditions. Other plants evade drought by flowering and seeding only during brief rainy periods.

During the intervening months and years of drought, the seeds remain dormant.

Desert vegetation under such conditions is sparse, and soil-forming conditions (including the creation of humus) are poor. Rainstorms can be intense, although of short duration, and often soil particles are carried away from desert surfaces by sheet flooding. The result of these conditions is erosion—which results in hills lacking deep layers of soil. Their profiles are characteristically steep sided with thick strata forming cliff faces rising vertically from the surrounding plains. Flat-topped mesas and steep buttes dominate the landscape, while valleys are flat bottomed with vertical side slopes. Wind erosion and deposition are also significant factors in desert landscape formation. Crescent-shaped barchan dunes are found where sands are insufficient to completely mantle the underlying surface. Copious sands form "seas," with longitudinal sief dunes and star-shaped rhourd dunes. Such seas, however, are the exceptions and rocky desert surfaces are common.

In desert areas, underground supplies of water assume great importance. Porous and permeable strata deep beneath the surface sometimes contain large quantities of water. Such aquifers may have impervious layers (aquicludes) above and below them that confine the water and keep it from escaping except in limited amounts at oases. Other aquifers occur in unconsolidated alluvial materials in river valleys (Arabic, wadis ). This water is recharged from river seepage and/or rainfall. In the Middle East, most of the major aquifers are non-renewable and contain fossil water, which once used—extracted or mined—will not be replaced. Desert countries, such as Libya and Saudi Arabia, with few or no surface streams have in the last two decades turned to the exploitation of such aquifers as part of their economic development plans. An ambitious agricultural program in Saudi Arabia has used tube wells and central pivot irrigation to produce bumper wheat crops in an otherwise hostile desert environment. Libya is engaged in constructing a "Great Manmade River"—actually a gigantic system of pumps and pipelines—with which to bring water from aquifers beneath the central Sahara to coastal locations, for municipal and agricultural use. In both these cases and others, the critical element is the quantity of water available and whether it will last long enough to justify such expensive projects. Many experts counsel caution in undertaking such attempts to remake, or "green," the desert.

See also Climate; Desalinization; Eastern Desert; Geography; Nafud Desert; Syrian Desert; Water.

Bibliography

Beaumont, Peter. Environmental Management and Development in Drylands. London and New York: Routledge, 1989.

Goudie, Andrew, and Wilkinson, John. The Warm Desert Environment. Cambridge, U.K., and New York: Cambridge University Press, 1977.

Whitehead, Emily E.; Hutchinson, Charles F.; Timmerman, Barbara N.; et al., eds. Arid Lands: Today and Tomorrow: Proceedings of an International Research and Development Conference. Boulder, CO: Westview Press, 1988.

john f. kolars