deserts Deserts are regions characterized by very low annual rainfall (usually less than 300 mm), sparse vegetation, extensive areas of bare, rocky mountains and plateau, and alluvial plains. Sand dunes cover less than a third of desert regions. Many deserts are regions of high temperature (the hot or tropical deserts), but some polar regions (including most of the Antarctic continent) are also classified as deserts.

A generally accepted definition of a desert is a region in which mean annual potential evapotranspiration (Etp) exceeds mean annual precipitation (P) by a factor of two or more. A world map of desert regions (Fig. 1) identifies three main classes of aridity: (1) hyperarid (P/Etp < 0.03); (2) arid (0.03 < P/Etp >  0.20); and (3) semi-arid (0.20 < P/Etp > 0.50). Defined in this way, deserts cover approximately one-third of the Earth's land surface.

Causes of deserts

Desert climates are characterized by low humidity (except in cool, foggy coastal deserts like the Namib), a high daily range of temperatures, and precipitation that is highly variable in time and space. The most extensive deserts lie astride the tropics. Descending, dry stable air masses in the subtropical anticyclonic belts maintain arid conditions throughout the year. The effects of stable air masses are reinforced by large land masses. Long distances to continental interiors restrict the influence of moist oceanic air masses in summer, as in the central Asian and African deserts. In winter, large continental areas develop strong high-pressure cells, reducing the influence of frontal systems. Mountain barriers block rain-bearing winds and create rain-shadow areas in their lee, especially in the Great Basin Desert of North America and in central Asia, where the Himalaya prevent penetration of the south-west monsoon to the Gobi and Takla Makan deserts. Deserts located on the west coast of South America and southern Africa (Atacama, Namib) owe their hyperarid climates to the influence of cold oceanic currents offshore. These reinforce the subsidence-induced stability of the atmosphere by cooling surface air masses and creating a strong temperature inversion.

Desert landforms

Although no landforms or geomorphological processes are unique to deserts, certain characteristics of desert environments have a significant effect on the operation of the major processes of weathering, erosion, transport, and deposition. A sparse vegetation cover with a high percentage of bare ground results in rapid run-off of water when intense rainfall does occur, and enhances the ability of the wind to erode and transport sand and dust (silt- and clay-sized sediment). Sand accumulates in areas of lower wind velocity and transport capacity to form dunefields and sand seas comprised primarily of crescentic, linear, and star dunes, with sand sheets in marginal areas. Dune form is governed by the availability of sand and the variation in wind direction from season to season. Dust storms transport fine-grained material away from desert regions to be deposited in ocean sediments and desert margin soils. Some may reach the polar ice caps.

The excess of evaporation over precipitation gives rise to physical or mechanical, rather than chemical, weathering of rocks, and to upward movement of soil moisture and near surface groundwater. As a result, water-soluble salts (principally sodium chloride, calcium carbonate, and calcium sulphate) accumulate in desert soils, forming calcic and gypsic horizons in the subsoil. Insolation weathering and salt weathering dominate processes of rock breakdown. On a regional scale, lack of water gives rise to internal drainage and thus to playas and salt lakes.

Table 1. Proportions of landform types in deserts (percentage of area covered) (from Cooke et al. 1993)

Deserts in the past

The distribution of desert climates has changed significantly over geological time. Many of the modern areas of aridity originated during late Tertiary times (especially the Mid- to Late Miocene) as the ‘modern’ climate and geography of subtropical regions developed. Aridity in north Africa and Asia intensified with the uplift of the Himalaya and the formation of the Tibetan Plateau, blocking the penetration of the monsoon to central Asia and reinforcing the tropical easterly jet stream that brings dry, stable air masses to the Sahara. Narrowing of the Tethys seaway increased the continentality of northern Africa, culminating in the isolation of the Mediterranean in the Miocene and the first signs of true aridity in the Sahara. The southern African deserts owe their origins to the development of the Antarctic ice sheet and cooling of the Southern Ocean, which led in turn to the formation of the Benguela Current and its upwelling system. Australian deserts developed as the continent ‘drifted’ northward to reach its present latitude in the Miocene. In North America, uplift of the Sierra Nevada and the Transverse Ranges of southern California created a barrier to penetration of moist air masses from the Pacific, giving rise to the Great Basin and Mojave deserts.

During the Quaternary period, climatic changes associated with glacial–interglacial cycles at high latitudes resulted in changes in the extent of deserts and the intensity of aridity. The core hyperarid areas, such as the central Sahara and Namib, were relatively unaffected; instead, desert margin areas (the Sahel and Kalahari) were affect most. For example, there is no evidence to suggest that the Namib has experienced any climate wetter than semi-arid at any time during the Quaternary. By contrast, degraded dune systems, now covered by savannah vegetation, occur in sub-humid areas adjacent to the southern Sahara, the Kalahari, and in Australia, and indicate a expansion of arid conditions during glacial periods. In North America, dunes in the semi-arid High Plains were also mobile as little as 1000 years ago in periods of extended drought. Conversely, lake and spring deposits, as well as archaeological and faunal evidence, show that many parts of the Sahara experienced humid conditions in the period 9000–6000 years ago.

In the more distant past, the rock record of ancient aeolian (i.e. wind-blown) sandstone and evaporite deposits is evidence for extensive desert conditions during the Permian (the Rotliegendes Sandstone of the North Sea Basin) and during the late Late Carboniferous, Permian, and Jurassic (e.g. the Weber, Navajo, and Entrada sandstones of western North America). The Old Red Sandstone (Devonian) of Britain is also thought to have been deposited under somewhat arid conditions.

Nicholas Lancaster

Bibliography

Cooke, R. U.,, Goudie, A. S.,, and and Warren, A. (1993) Desert geomorphology. UCL Press, London.
Thomas, D. S. G. (ed.) (1977) Arid zone geomorphology. John Wiley and Sons, Chichester.

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

DESERTS

DESERTS. Definition has been the central problem in the history of the deserts of the United States. The need to ascertain the limits of arability and the difficulty of establishing such boundaries where precipitation fluctuates unpredictably constitute a basic developmental theme for more than half the nation. Archaeological evidences of prehistoric Native American communities indicate that droughts occasioned recurrent disaster to agricultural

societies long ago, as now, in border areas. In 1803 President Thomas Jefferson, seeking congressional support for exploration of the upper Missouri River, summarized existing knowledge of newly purchased Louisiana in describing it as a region of "immense and trackless deserts" but also, at its eastern perimeter, as "one immense prairie"—a land "too rich for the growth of forest trees." The subsequent expedition of Meriwether Lewis and William Clark (1804–1806) marked the official beginning of American efforts to elaborate the description.

Until the 1860s a conception prevailed that the vast province west from the meridian of Council Bluffs, on the Missouri River, to the Rocky Mountains, between thirty-five and forty-nine degrees north latitude, was a "Great American Desert." The explorations of Lewis and Clark, Zebulon Pike, and Stephen Harriman Long, followed by the experiences of traders to Santa Fe, Rocky Mountain fur trappers, immigrants to Oregon and California, soldiers along the Gila Trail, surveyors for transcontinental railroads, and prospectors throughout the West confirmed the appellation.

While commentators agreed that agriculture could have no significant role in the region, they did occasionally recognize that the Great Plains, the mountain parks, and the interior valleys of California and the Northwest afforded excellent pasturage. As livestock industry developed in these areas during the period from 1866 to 1886, redefinition of the limits of aridity evolved. Maj. John Wesley Powell's surveys and, notably, his Report on the Lands of the Arid Region (1878) expressed the new point of view; agriculture, Powell asserted, could be profitably conducted in many parts of the West, but only as an irrigated enterprise and generally as a supplement to stock growing. The collapse of open-range ranching in the mid-1880s emphasized the need for expanded hay and forage production and gave impetus to development of irrigation programs. But Powell's efforts to classify the public lands and the passage of the Carey Desert Land Grant Act of 1894 raised controversy. States east of the 104th meridian were excluded, at the request of their representatives, from the application of the Carey legislation. Farmers during the 1880s had expanded cultivation without irrigation nearly to that meridian in the Dakotas and even beyond it in the central plains. Many were convinced that "rainfall follows the plow." They saw no need to assume the costs and the managerial innovations of supple-mental watering. A new conception of the boundaries of aridity was emerging.

Drought in the mid-1870s had driven a vanguard of settlers eastward from the James River Valley, a prairie zone normally receiving more than twenty inches of annual rainfall. Drought in the period 1889–1894 forced thousands back from the plains farther west, where average precipitation ranges between fifteen and twenty inches annually. As normal conditions returned, however, farmers in the first two decades of the twentieth century expanded cultivation across the plains to the foothills of the Rockies—in Montana, Colorado, and New Mexico—and in many areas beyond—Utah, Idaho, the interior valleys of California, and eastern Oregon and Washington. Irrigation supplied water to only a small portion of these lands. Dry farming—a specialized program that, ideally, combines use of crop varieties adapted to drought resistance, cultivation techniques designed to conserve moisture, and management systems that emphasize large-scale operations—provided a new approach to the problem of aridity. The deserts, promoters claimed, could be made to "blossom like the rose."

When severe droughts again returned from 1919 to 1922, and from 1929 to 1936, assessment of the effectiveness of dry farming raised new concern for defining the limits of aridity—an outlook most strongly expressed in the reports of the National Resources Board of the mid-1930s but one that still permeates the writings of agricultural scientists. Long-term precipitation records, with adjustment for seasonality and rate of variability in rainfall, humidity, temperature, and soil conditions, now afford some guidance to the mapping of cultivable areas.

By established criteria a zone of outright desert (less than five inches average annual precipitation) ranges from southeastern California, northward through the western half of Nevada, nearly to the Oregon border. Because cropping without irrigation is impracticable when rainfall averages less than ten inches annually, climatic pockets found in all states west of the 104th meridian—most prevalently in Arizona, central New Mexico, eastern Nevada, Utah, and the lee side of the Cascades in Oregon and Washington—may also be defined as arid. Semiaridity—an average precipitation of from ten to fifteen inches annually—characterizes the western Dakotas, much of Montana, and large sections of eastern New Mexico, Colorado, Wyoming, Idaho, Oregon, and Washington. There dry farming may be successful but only when management programs include allowances for recurrent drought. Throughout much of the semiarid region livestock production predominates, with cropping to afford feed and forage supplementary to native short-grass pasturage. In many areas, however, the possibility of raising wheat of superior milling quality, which commands premium prices, encourages alternative land utilization. The costs of marginal productivity must be carefully weighed.

Eastward, roughly from the Missouri River to the ninety-eighth meridian and curving to the west through the central and southern plains, is a subhumid zone, in which rainfall averages from fifteen to twenty inches annually, an amount sufficient, if well distributed, to permit cultivation without recourse to specialized programs but so closely correlated to the margin of general farming requirements that a deficiency occasions failure. Almost every spring, alarms are raised that some areas of the vast wheat fields extending from the central Dakotas, through western Kansas and Nebraska and eastern Colorado and New Mexico, into the panhandles of Oklahoma and Texas have suffered serious losses. There the problem of defining limits of arability is yet unresolved; the boundaries of America's deserts and arid regions remain uncertain.

BIBLIOGRAPHY

Fite, Gilbert C. The Farmers' Frontier, 1865–1900. New York: Holt, Rinehart and Winston, 1966.

Goetzmann, William H. Exploration and Empire: The Explorer and the Scientist in the Winning of the American West. New York: Knopf, 1966.

Hargreaves, Mary W. M. Dry Farming in the Northern Great Plains, 1900–1925. Lawrence: University Press of Kansas, 1993.

Limerick, Patricia Nelson. Desert Passages: Encounters with the American Deserts. Albuquerque: University of New Mexico, 1985.

Teague, David W. The Southwest in American Literature and Art: The Rise of a Desert Aesthetic. Tucson: University of Arizona Press, 1997.

Mary W. M.Hargreaves/a. r.

See alsoAgriculture ; Death Valley ; Great Plains .

116. Deserts

See also 142. ENVIRONMENT .

eremite

a religious hermit living alone, often in the desert. —eremitic , adj.

eremology

the systematic study of desert features and phenomena.

xerophobia

an abnormal fear of dryness and dry places, as deserts.

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