Desert
From Encyclopædia
Deserts are arid land areas where more water is lost through evaporation than is gained from precipitation. Several types of deserts are recognized in addition to the familiar hot, dry desert of rock and sand that is almost barren of
plants. The others include semiarid deserts of scattered trees, scrub, and grasses; coastal deserts, such as the Atacama of Chile; and the deserts on the polar ice cap of the Antarctic and Greenland.Geographers generally categorize deserts as one of two basic types that, together, contain an almost unlimited variety of landscapes, vegetation communities, CLIMATES, and cultural and natural resources. Deserts of the first major group, the result of large-scale climatic patterns, develop in regions of persistent high atmospheric
pressure, where descending AIR MASSES are compressed and heated. Those of the second major type may form behind any large mountain range that creates a
rain shadow effect by preventing moist ocean air from moving inland.DESERT AREAS OF THE WORLDThe deserts of the
world are discontinuously distributed in a pair of parallel belts lying about 25 deg north and south of the equator. Of these, the Northern Hemisphere
Sahara (the
world's largest desert) and the many smaller deserts forming much of the arid portion of the Middle East are HIGH-
pressure REGIONS directly related to global circulation of the
atmosphere. In the Southern Hemisphere, western coastal deserts of South
America (the Atacama), southern
Africa (the Namib and the Kalahari), and the entire continental interior of Western
Australia are arid because they lie under the eastern side of persistent high-
pressure areas.Other Northern Hemisphere deserts are rain-shadow deserts, such as the Mojave of Nevada and eastern California, which lies inland beyond the wall of the Sierra Nevada. In
Central China, the high, cold Gobi Desert lies north of the Himalayas, the
world's highest mountains, which block moist air masses sweeping north from the Indian Ocean. The deserts of eastern and
Central Australia are a product of the
Great Dividing Range and other mountains on the east side of the continent that lie across the path of the prevailing southeasterly trade winds, thus creating rain-shadow deserts in inland areas.Because the individual climatic characteristics of deserts may vary from season to season, techniques have been developed to measure a land area's general aridity (see ARID CLIMATE). Among the measuring techniques commonly used are botanical surveys of species diversity, community pattern, and morphological characteristics, such as water-storing capability. Another
index of aridity is the amount and seasonal distribution of precipitation, especially when correlated with temperature. For instance, in the driest desert areas, mean annual precipitation commonly ranges from a maximum of 430 mm (17 in) to 0 mm, with periods of as much as 30 months when no rain is recorded. In general, throughout desert areas mean annual temperature ranges from 2 deg to 18 deg C (35 deg to 65 deg F) for cold deserts and from 18 deg to 27 deg C (65 deg to 80 deg F) for hot deserts. Another useful
index of desert conditions correlates evaporation and precipitation. In the vicinity of Las Vegas, Nev., a mean annual precipitation of 90 mm (3.5 in) is counterbalanced by a mean annual evaporation of 1830 mm (72 in). Measured by these and other indexes of aridity, over a third of the Earth's land surface exhibits desert characteristics. Of the total land surface area, 4% (5.4 million sq km/2.2 million sq mi) is extremely arid, 15% (20.3 million sq km/7.8 sq mi) is arid, and 14.6% (19.7 million sq km/7.9 million sq mi) is semiarid.WATER SOURCESWith such a large percentage of the Earth's land surface marked by a moisture imbalance, the presence of water, when it is available, is of major importance. Desert water sources are of two major types: surface, including all streams, rivers, and lakes; and subsurface, including all springs and wells. Surface water is commonly temporary, consisting of short-lived streams produced by local, violent thunderstorms (see FLASH FLOOD). In areas where closed basins have no outlet to the sea, shallow temporary lakes may form. Normally, such streams and lakes are not a dependable
water supply.Another form of surface water that is much more critical to human life in desert areas is the permanent river. Since these originate in humid areas outside the boundaries of deserts and then flow across desert land en route to the sea, they are called exotic streams. Typical of such permanent water sources are the Nile in
Egypt and the Tigris-Euphrates in
Iraq. With
irrigation water supplied by these river systems, the two areas became cradles of
civilization. In the New
world, systematic development of the Colorado River, an exotic stream in the southwestern
United States, has led to the creation of extensive agricultural programs in Arizona and California, within the past 40 years.Subsurface water supplies derive from springs and wells. Springs occur where the land surface intersects the water table; they may give rise to small, well-watered areas (oases) that serve as a limited agricultural base for small, local populations. Greater agricultural and urban development of desert areas requires extensive networks of water wells. These may tap groundwater at depths of more than 300 m (1,000 ft). Since, in most desert areas, supplies of groundwater are limited and have taken thousands of years to accumulate, withdrawal of groundwater usually
leads to depletion of the local supply and may sharply limit the extent and duration of desert development. Heavy groundwater withdrawals can also cause widespread subsidence of the land surface, with adverse effects on artificial structures such as canals.DESERT LANDFORMSA barren landscape of rock,
soil, and sand resulting from thousands of years of persistent arid conditions is perhaps the most familiar aspect of deserts. This environment is produced by the same geologic processes of erosion and sedimentation that produce all of the Earth's landforms, but significant differences in magnitude and
frequency of these processes make desert landforms unique. Erosion and
deposition are directly related to rainfall and runoff: rainfall in terms of the
frequency, duration, and intensity of precipitation; runoff in terms of the difference between rainfall applied to the ground surface and the amount of moisture absorbed by the
soil and
plant cover. The initial effect of runoff is cutting, or erosion; then, as the moving
Mass of water loses momentum and volume,
deposition of sediment takes over.The fluvial (running-water) processes of erosion and
deposition that shape desert landscapes differ from those of humid climates mainly in the irregular, unpredictable nature of desert rainfall. Landscapes of arid regions may remain dormant for long periods, during which time neit?er erosion nor
deposition occurs. Then, a single heavy thunderstorm, occurring by chance in terms of time and locality, will produce rapid changes, many of them large, that persist until the next such event.Among the common fluvial erosional forms are dry stream channels (called arroyos in American deserts or wadis in the deserts of
Africa and the Middle East). Often visible in the sandy banks of these shallow channels are lenses of coarse
gravel, produced sporadically by brief periods of intense runoff. Another common erosional form results indirectly from unchanneled runoff, or sheet wash. Runoff of this sort helps to form PEDIMENTS, broad surfaces of low relief that flank many desert mountains and that have been cut across rock or deposits of older sediment.Desert landforms produced by
deposition occur along the margins of mountain fronts. Consisting of great thicknesses of coarse debris weathered from the rock slopes and transported for relatively short distances, these deposits tend to be conical in shape, originating at the mouths of short, steep canyons and spreading in a fan-shaped pattern onto the valley margin (see ALLUVIAL FANS). Where mountain ranges are elongate, fans emerging from adjacent canyons commonly merge to form a broad alluvial apron, or bajada, that may extend for distances of up to 80 km (50 mi). Where mountain ranges enclose basins that have no outlet (as in the basin and range province of North
America),
fine sediment washed from the surrounding fans may be deposited in seasonal lakes. Intermittent lakes such as these develop hard crusts of clay, salt, or both during dry periods. In deserts of the Western Hemisphere, these lakes are called PLAYAS if the surface material is primarily clay and silt, or salars if salt forms the major part of the crust (see
EVAPORITE).The other major processes that shape desert landscapes are those resulting from WIND ACTION. In some regions prevailing winds that occur frequently and with sufficient velocity to move sand may be the dominant force shaping the landscape. This wind action results from a combination of conditions: sparse vegetation; dry, unconsolidated
soils; broad areas with little topographic relief; and favorable location with respect to the global pattern of high and low atmospheric
pressure. Wind-caused (eolian) landforms fall naturally into two categories: erosional (deflation) structures; and depositional features, most commonly SAND DUNES.
