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.
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." Encyclopedia of the Modern Middle East and North Africa. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/deserts
"Deserts." Encyclopedia of the Modern Middle East and North Africa. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/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.
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.
"Deserts." Dictionary of American History. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/history/dictionaries-thesauruses-pictures-and-press-releases/deserts
"Deserts." Dictionary of American History. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/history/dictionaries-thesauruses-pictures-and-press-releases/deserts
Desert ecosystems are characterized by an extremely arid, arid, or semi-arid climate, low relative humidity, high air and soil temperatures, strong winds, high solar radiation, low precipitation levels, extended drought periods, soils low in organic matter, low net primary productivity, and a spatially patchy distribution of vegetation and soil resources. In them, water is the predominant controlling factor for most biological processes; precipitation is highly variable and occurs as infrequent and discrete events throughout the year; and precipitation events are highly unpredictable in both space and time. Desert ecosystems may be classified into three groups based on annual precipitation: extremely arid (less than 60 millimeters), arid (60 to 250 millimeters), and semiarid (250 to 500 millimeters). The plant communities of arid lands expand and contract in accordance with alternating wet and dry periods as well as with anthropogenic activities that contribute to desertification (also known as land degradation). While arid ecosystems occur on all continents in both hot and cold environments, this article will not focus on polar deserts.
Distribution of Deserts Worldwide
Earth's major deserts lie within the tropics of Cancer and Capricorn where stable, high atmospheric pressure creates an arid climate at or near latitudes 30°N and 30°S. Deserts are generally located in the interior of large continents. Continental deserts are separated from ocean moisture by large distances or topographic barriers, such as large mountain ranges, which create a rainshadow. Deserts may also be situated on the west coast of large continents adjacent to cold ocean currents, which draw moisture away from the land. Subtropical deserts, such as the Mojave Desert of California, lie within the latitudes of 30°N and 30°S. Cool coastal deserts, including the Peruvian Atacama Desert, occur where cold offshore currents generate high atmospheric pressure and large masses of dry air, which create arid conditions upon their descent. Rainshadow deserts, including the Great Basin Desert in the United States or the Gobi Desert in Mongolia, occur where a topographical barrier such as a mountain range interrupts the flow of moist oceanic air. As moisture-laden air masses travel inland, they are deflected upward on the windward side of a mountain range, lose their moisture, and descend as dry air masses on the leeward side of the mountains. Continental interior deserts, such as the Great Sandy Desert in Australia, occur far from marine moisture.
Plants in the Desert Environment
In order to understand the ways in which plants have adapted to arid lands, it is essential to consider the physical environment. Of all the abiotic constraints imposed on plant activity—high air temperatures, extremely high soil temperatures, high winds, intense solar radiation, and limited moisture—high temperatures and limited water are the two factors that severely limit plant growth. Summer air temperatures in the Sonoran Desert in Arizona may reach 40°C during the day but drop to 15°C at night. Soil temperatures may reach 80°C or higher. High temperatures generally are accompanied by strong winds in coastal deserts, such as the Atacama in South America and the Namib in Africa, as well as in continental deserts, including the Chihuahuan and Sonoran in the United States. As well as producing spectacular dust storms and dust devils (small whirlwinds containing sand or dust), wind also abrades and desiccates desert plants.
Water is the single-most limiting factor to the growth and productivity of desert vegetation. The highly sporadic nature of desert rainfall creates a pulse-reserve system of water and nutrient availability that influences many biological processes, especially plant productivity. In the Chihuahuan Desert of New Mexico, gentle winter rainfall penetrates deep into the soil profile and provides most of the moisture for the growth of perennial shrubs, such as creosote bush and mesquite. In contrast, the high-intensity, brief summer thunderstorms provide minimal water for plant growth because most of the water runs off of the soil surface. Many plant species take advantage of rainfall immediately and grow rapidly following precipitation events, then slow their growth when soils dry and moisture once again becomes limiting.
Second only to moisture, the availability of soil nutrients, primarily nitrogen and phosphorus, limits plant productivity in deserts. Nitrogen is the key limiting nutrient in North American deserts, phosphorus is most limiting in Australian deserts, while nitrogen, phosphorus, and potassium are limiting in sand dune communities in Africa's Namib Desert. Soil nutrients and organic matter tend to be concentrated in the upper 2 to 5 centimeters of soil with the greatest amounts underneath the canopies of individual desert shrubs in "islands of fertility." These resource islands harbor greater concentrations of water, soil nutrients, and microorganisms than adjacent soils.
Certain plant species, such as creosote bush, are often referred to as nurse plants. Nurse plants effectively reduce high-incident solar radiation and high temperatures under their canopies and create ideal sites for seed germination and seedling growth. The concentration of limiting resources in islands of fertility or under nurse plants generates a spatially patchy distribution of vegetation across the desert. Competition for water maintains this spacing of plants. While this phenomenon has been most studied in U.S. deserts, it occurs in arid lands worldwide.
Hot deserts exhibit generally similar soil types. Immature and alkaline with weakly developed soil horizons , desert soils are dry most of the year, and poor in soil organic matter, nitrogen, and phosphorus, but are rich in inorganic ions, carbonate, and gypsum. The main soil orders of hot deserts are Entisols, soils without well-defined layers that are formed from recently exposed rock, and Aridisols. Aridisols, exclusive to arid regions, contain two dominant suborders: Orthids and Argids. Orthids are young calcareous and gypsipherous soils with a caliche (or calcium carbonate hardpan) within 1 meter of the soil surface. The thickness of the caliche layer has been correlated with the size of creosote bush shrubs in Arizona's Sonoran Desert: the thicker the layer, the smaller the shrubs. Argids are older soils and lack the carbonate hardpan layer, but are clay-rich and may be good agricultural soils when water is available.
Plant Adaptations to the Desert Environment
Desert plant species show various physical, physiological, and pheno-logical (timing of growth and reproduction) characters that enable them to survive and grow in arid, nutrient-limited environments. Some plants, such as summer and winter desert ephemerals , restrict all growth and flowering to periods when water is available. They are able to withstand droughts and high water stress because their underground rhizomes or bulbs remain dormant during the dry season. In extreme droughts, desert ephemerals may remain completely dormant, eliminate reproduction, or limit growth to the vegetative phase. Other species, such as the California poppy and other desert annuals, complete their entire life cycle during the rainy season. Their long-lived seeds germinate only under suitable environmental conditions. As a result, they respond to the pulse-reserve system of resource availability, showing high rates of primary production in favorable years and minimal, or no production, in drought years. Ephemerals and annuals, while showy, produce minimal biomass .
In deserts worldwide, perennial shrubs and subshrubs, such as the creosote bush and jojoba, produce most of the desert plant biomass. These species limit water loss and reduce heat loads at the leaf surface by limiting the surface area to many small single, dissected, or compound leaves covered with a waxy cuticle or leaf hairs. Most shrubs have canopies with a compact globe or inverted cone shape. This morphology allows water to funnel directly to the plant roots and reduces the amount of surface area that is exposed to sunlight. Perennials have a large root-to-shoot ratio, and most roots are distributed in the soil in one of two ways. The roots may be confined to the upper meter of the soil profile and fan out horizontally from the base of the shrub, enabling shrubs access to even the slightest rainfall. Alternatively, the roots may extend deep into the soil profile—up to 12 meters with mesquite—and allow plants to obtain water that is stored at these depths. As with other desert plants, perennials may also limit or suppress flowering and fruiting in years of extreme drought.
Perennials are able to remain metabolically active at very low soil- and plant-water potentials, high internal water deficits, and high temperatures. They have sensitive regulation of leaf stomata as a function of internal and external conditions, including water stress, temperature, atmospheric humidity, and light intensity. Most shrub species acquire carbon throughout the C3 photosynthetic pathway, despite the fact that the alternative C4 pathway is thought to increase the amount of carbon gain per unit of water used (water-use efficiency [WUE]). The only desert perennials that have the C4 pathway are the halophytic (salt-tolerant) species, such as tamarisk, short-lived summer active perennials, and most grasses.
Cacti, common to deserts, show unique adaptations to the desert environment. They have shallow, horizontally extended root systems, an upright, ribbed trunk that reduces the midday heat and solar radiation load and water storage within their trunks. Saguaro cacti, located near Tucson, Arizona, expand and contract like an accordion depending on the moisture conditions. In wet years the cacti are plump and green, but in dry years they are slim and yellow-green in color. Because cacti lack typical broad leaves, the overall green coloring derives from the photosynthetic trunk. Over evolutionary time, cactus "leaves" have been reduced to hairlike spines that reflect solar radiation or spikelike spines that protect the plant from herbivores . Other noncactus species, such as ocotillo and the boojum trees native to Baja California, produce photosynthetically active leaves only in wet years and limit photosynthesis to the stems when drought prevails. Cacti and other succulent species obtain carbon through the crassulacean acid metabolism (CAM) photosynthetic pathway. CAM photosynthesis allows the cacti to open their stomata only at night in order to reduce water loss.
see also Cacti; Desertification; Photosysthesis, Carbon Fixation and.
Anne Fernald Cross
Caldwell, M. M., J. H. Manwaring, and S. L. Durham. "The Microscale Distribution of Neighboring Plant Roots in Fertile Soil Microsites." Functional Ecology 5 (1991): 765-72.
Cross, A. F., and W. H. Schlesinger. "Plant Regulation of Soil Nutrient Distribution in the Northern Chihuahuan Desert." Plant Ecology 145 (1999): 11-25.
Fox, G. A. "Drought and the Evolution of Flowering Time in Desert Annuals." American Journal of Botany 77 (1990): 1508-18.
Le Houérou, H. N. "Climate, Flora and Fauna Changes in the Sahara Over the Past 500 Million Years." Journal of Arid Environments 37 (1997): 619-47.
Mahall, B. E., and R. M. Callaway. "Root Communication Mechanisms and Intra-community Distributions of Two Mojave Desert Shrubs." Ecology 73 (1992): 2145-51.
Martinez-Meza, E., and W. G. Whitford. "Stemflow, Throughfall, and Channelization of Stemflow by Three Chihuahuan Desert Shrubs." Journal of Arid Environments 32 (1996): 271-87.
McAuliffe, J. R. "Markovian Dynamics of Simple and Complex Desert Plant Communities." American Naturalist 131 (1988): 459-90.
Nobel, P. S. Environmental Biology of Agaves and Cacti. Cambridge, UK: Cambridge University Press, 1988.
Schlesinger, W. H., J. F. Reynolds, G. L. Cunningham, L. F. Huenneke, W. M. Jarrell, R. A. Virginia, and W. G. Whitford. "Biological Feedbacks in Global Desertification." Science 247 (1990): 1043-48.
——, J. Raikes, A. E. Hartley, and A. F. Cross. "On the Spatial Pattern of Soil Nutrients in Desert Ecosystems." Ecology 77 (1996): 364-74.
MAJOR DESERTS OF THE WORLD
Great Basin, Sonoran, Mojave, Baja California, Chihuahuan
Patagonian, Puna, Monte, Chaco, Espinal, Peruvian-Chilean, Atacama
Gobi, Takla Makan, Iranian, Thar, Syrian, Arabian, Sinai, Negev
Sahara, Sahel, Somalian, Namib, Karoo, Kalahari, Madagascar
Great Sandy, Gibson, Great Victoria, Arunta, Stuart
"Deserts." Plant Sciences. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/deserts
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See also 142. ENVIRONMENT .
- a religious hermit living alone, often in the desert. —eremitic , adj.
- the systematic study of desert features and phenomena.
- an abnormal fear of dryness and dry places, as deserts.
"Deserts." -Ologies and -Isms. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/deserts
"Deserts." -Ologies and -Isms. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/deserts
"deserts." Oxford Dictionary of Rhymes. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/deserts
"deserts." Oxford Dictionary of Rhymes. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/deserts