page_title: Desert
section_title: Water
hierarchical_section_title: Desert / Physical geography / Water
caption_attribution_description: English: The above panorama shows a view of the Atacama Desert as seen from the ESO Paranal Observatory, home to the Very Large Telescope. To the right of the image, one can see the road leading to the summit of Cerro Armazones, a site located in Chile, and a possible home for the future European Extremely Large Telescope (E-ELT). The E-ELT programme office has studied half a dozen potential sites for the future E-ELT observatory, which, with its 40-metre-class diameter, will be the world’s biggest eye on the sky. Various aspects need to be considered in the site selection process. On the E-ELT Site Selection Advisory Committee’s final short list for the recommended site, Armazones is also the committee’s preferred site, because it has the best balance of sky quality across all aspects and can be operated in an integrated fashion with the existing Paranal Observatory.
context_page_description: A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one-third of the land surface of the world is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.
  Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks, which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter, and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms.
context_section_description: One of the driest places on Earth is the Atacama Desert. It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of Antofagasta is just 1 mm (0.039 in) per year. Some weather stations in the Atacama have never received rain. Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. It is so arid that mountains that reach as high as 6,885 m (22,589 ft) are completely free of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 m (14,400 ft) and is continuous above 5,600 m (18,400 ft). Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific.
  When rain falls in deserts, as it occasionally does, it is often with great violence. The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. These can experience flash floods, becoming raging torrents with surprising rapidity after a storm that may be many kilometers away. Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders. The River Nile, the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby. There may also be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes. Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water. The Great Man-Made River is a scheme launched by Libya's Muammar Gadaffi to tap this aquifer and supply water to coastal cities. Kharga Oasis in Egypt is 150 km (93 mi) long and is the largest oasis in the Libyan Desert. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below.
  Lakes may form in basins where there is sufficient precipitation or meltwater from glaciers above. They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas. When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa. The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter. These include the Great Salt Lake, Utah Lake, Sevier Lake and many dry lake beds. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle.
caption: Atacama, the world's driest non-polar desert, part of the Arid Diagonal of South America.
exif: {"Image ImageDescription": "The above panorama shows a view of the Atacama Desert as seen from the ESO Paranal Observatory, home to the Very Large Telescope. To the right of the image, one can see the road leading to the summit of Cerro Armazones, a site located in Chile, and a possible home for the future European Extremely Large Telescope (E-ELT). The E-ELT programme office has studied half a dozen potential sites for the future E-ELT observatory, which, with its 40-metre-class diameter, will be the world\u2019s biggest eye on the sky. Various aspects need to be considered in the site selection process. On the E-ELT Site Selection Advisory Committee\u2019s final short list for the recommended site, Armazones is also the committee\u2019s preferred site, because it has the best balance of sky quality across all aspects and can be operated in an integrated fashion with the existing Paranal Observatory.", "Image Artist": "", "Image Copyright": ""}