Arid

A region is arid when it is characterized by a severe lack of available water, to the extent of hindering or preventing the growth and development of plant and animal life. Environments subject to arid climates tend to lack vegetation and are called xeric or desertic. Most "arid" climates straddle the Equator; these places include parts of Africa, South America, Central America, and Australia.

1893 Arid regions of the western united states
Arid regions of the Western United States as mapped in 1893

Change over time

The distribution of aridity observed at any one point in time is largely the result of the general circulation of the atmosphere. The latter does change significantly over time through climate change. For example, temperature increase (by 1.5–2.1 percent) across the Nile Basin over the next 30–40 years could change the region from semi-arid to arid, resulting in a significant reduction in agricultural land. In addition, changes in land use can result in greater demands on soil water and induce a higher degree of aridity.

See also

References

  • Griffiths, J. F. (1985) 'Climatology', Chapter 2 in Handbook of Applied Meteorology, Edited by David D. Houghton, John Wiley and Sons, ISBN 0-471-08404-2.
  • Durrenberger, R. W. (1987) 'Arid Climates', article in The Encyclopedia of Climatology, p. 92-101, Edited by J. E. Oliver and R. W. Fairbridge, Van Nostrand Reinhold Company, New York, ISBN 0-87933-009-0.
  • Stadler, S. J (1987) 'Aridity Indexes', article in The Encyclopedia of Climatology, p. 102-107, Edited by J. E. Oliver and R. W. Fairbridge, Van Nostrand Reinhold Company, New York, ISBN 0-87933-009-0.
  • Blue Peace for the Nile Report, 2009, Strategic Foresight Group
Al-Arid

Al-Arid (Arabic: العريض‎) is a Syrian hamlet located in the Mahardah Subdistrict of the Mahardah District in Hama Governorate. According to the Syria Central Bureau of Statistics (CBS), al-Arid had a population of 124 in the 2004 census. Its inhabitants are predominantly Sunni Muslims.

Alluvial fan

Alluvial fans are triangular-shaped deposits of water-transported material, often referred to as alluvium. They are an example of an unconsolidated sedimentary deposit and tend to be larger and more prominent in arid to semi-arid regions. These alluvial fans typically form in elevated or even mountainous regions where there is a rapid change in slope from a high to low gradient. The river or stream carrying the sediment flows at a relatively high velocity due to the high slope angle which is why coarse material is able to remain in the flow. When the slope decreases rapidly into a relatively plain or plateau, the stream loses the energy it needs to move its sediment. Deposition subsequently occurs and the sediment ultimately spreads out, creating an alluvial fan. Three primary zones occur within an alluvial fan which includes the proximal fan, medial fan, and the distal fan.

Arid Forest Research Institute

Arid Forest Research Institute (AFRI) is a research institute situated in Jodhpur, Rajasthan, India. The institute conducts scientific research in forestry in order to provide technologies to increase the vegetative cover and to conserve biodiversity in the hot arid and semi-arid regions of Rajasthan and Gujarat. It operates under the Indian Council of Forestry Research and Education (ICFRE) of the Ministry of Environment and Forests, Government of India.

Atacama Desert

The Atacama Desert (Spanish: Desierto de Atacama) is a desert plateau in South America covering a 1000-km (600-mi) strip of land on the Pacific coast, west of the Andes mountains. The Atacama desert is one of the driest places in the world (the driest being the McMurdo Dry Valleys) , as well as the only true desert to receive less precipitation than the polar deserts. According to estimates, the Atacama Desert occupies 105,000 km2 (41,000 sq mi), or 128,000 km2 (49,000 sq mi) if the barren lower slopes of the Andes are included. Most of the desert is composed of stony terrain, salt lakes (salares), sand, and felsic lava that flows towards the Andes.

The desert owes its extreme aridity to a constant temperature inversion due to the cool north-flowing Humboldt ocean current, and to the presence of the strong Pacific anticyclone. The most arid region of the Atacama desert is situated between two mountain chains (the Andes and the Chilean Coast Range) of sufficient height to prevent moisture advection from either the Pacific or the Atlantic Oceans, a two-sided rain shadow.

Desert

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. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur.

Plants and animals living in the desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and often spines to deter herbivory. Some annual plants germinate, bloom and die in the course of a few weeks after rainfall while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Animals need to keep cool and find enough food and water to survive. Many are nocturnal and stay in the shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy.

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert farming is possible with the aid of irrigation, and the Imperial Valley in California provides an example of how previously barren land can be made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara Desert, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy.

Desert climate

The desert climate (in the Köppen climate classification BWh and BWk), is a climate in which there is an excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert climates hold little moisture and evaporate the little rainfall they receive. Covering 14.2% of earth's land area, hot deserts may be the most common type of climate on earth.Although no part of Earth is known for certain to be absolutely rainless, in the Atacama Desert in northern Chile, the average annual rainfall over a period of 17 years was only 5 mm (0.2 in.). Some locations in the Sahara Desert such as Kufra, Libya record only .86 mm (0.03 inches) of rainfall annually. The official weather station in Death Valley, United States reports only 60 mm (2.3 inches) annually, and in one period between 1931 and 1934 (40 months) only 16 mm (0.64 inches) of rainfall was measured.

There are two variations of a desert climate: a hot desert climate (BWh), and a cold desert climate (BWk). To delineate "hot desert climates" from "cold desert climates", there are three widely used isotherms: either a mean annual temperature of 18 °C (which is the most accurate and most commonly used), or a mean temperature of 0 °C or −3 °C in the coldest month, so that a location with a "BW" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot arid" (BWh), and a location with the appropriate temperature below the given isotherm is classified as "cold arid".

Most desert and arid climates receive between 25 and 200 mm (1 to 8 inches) of rainfall annually. In the Köppen classification system, a climate will be classed as arid if its mean annual precipitation in millimeters is less than ten times its defined precipitation threshold, and it will be classed as a desert if its mean annual precipitation is less than five times this threshold. The precipitation threshold is twice its mean annual temperature in degrees Celsius, plus a constant to represent the distribution of its rainfall throughout the year. This constant is 280 for regions that receive 70% or more of their rainfall during the six winter months. The constant is 0 for regions that receive 70% or more of their rainfall during the six summer months. And it is 140 for any climates falling between these two extremes.

Drought

A drought or drouth is a natural disaster of below-average precipitation in a given region, resulting in prolonged shortages in the water supply, whether atmospheric, surface water or ground water. A drought can last for months or years, or may be declared after as few as 15 days. It can have a substantial impact on the ecosystem and agriculture of the affected region and harm to the local economy. Annual dry seasons in the tropics significantly increase the chances of a drought developing and subsequent bush fires. Periods of heat can significantly worsen drought conditions by hastening evaporation of water vapour.

Many plant species, such as those in the family Cactaceae (or cacti), have drought tolerance adaptations like reduced leaf area and waxy cuticles to enhance their ability to tolerate drought. Some others survive dry periods as buried seeds. Semi-permanent drought produces arid biomes such as deserts and grasslands. Prolonged droughts have caused mass migrations and humanitarian crisis. Most arid ecosystems have inherently low productivity. The most prolonged drought ever in the world in recorded history occurred in the Atacama Desert in Chile (400 Years).

Dryland farming

Dryland farming and dry farming encompass specific agricultural techniques for the non-irrigated cultivation of crops. Dryland farming is associated with drylands, areas characterized by a cool wet season that is followed by a warm dry season. They are also associated with arid conditions or areas prone to drought or having scarce water resources. Additionally, arid-zone agriculture is being developed for this purpose.

Gran Chaco

The Gran Chaco or Dry Chaco is a sparsely populated, hot and semi-arid lowland natural region of the Río de la Plata basin, divided among eastern

Bolivia, western Paraguay, northern Argentina and a portion of the Brazilian states of Mato Grosso and Mato Grosso do Sul, where it is connected with the Pantanal region. This land is sometimes called the Chaco Plain.

Humid continental climate

A humid continental climate (Köppen prefix D and a third letter of a or b) is a climatic region defined by Russo-German climatologist Wladimir Köppen in 1900, typified by large seasonal temperature differences, with warm to hot (and often humid) summers and cold (sometimes severely cold in the northern areas) winters. Precipitation is usually distributed throughout the year. The definition of this climate regarding temperature is as follows: the mean temperature of the coldest month must be below −3 °C (26.6 °F) (or 0 °C (32.0 °F)) and there must be at least four months whose mean temperatures are at or above 10 °C (50 °F). In addition, the location in question must not be semi-arid or arid. The Dfb, Dwb and Dsb subtypes are also known as hemiboreal.

Humid continental climates are generally found roughly between latitudes 40° N and 60° N, within the central and northeastern portions of North America, Europe, and Asia. They are much less commonly found in the Southern Hemisphere due to the larger ocean area at that latitude and the consequent greater maritime moderation. In the Northern Hemisphere some of the humid continental climates, typically in Scandinavia, Nova Scotia, and Newfoundland are heavily maritime-influenced, with relatively cool summers and winters being just below the freezing mark. More extreme humid continental climates found in northeast China, southern Siberia, the Canadian Prairies, and the Great Lakes region of the American Midwest and Central Canada combine hotter summer maxima and colder winters than the marine-based variety.

Köppen climate classification

The Köppen climate classification is one of the most widely used climate classification systems. It was first published by the Russian climatologist Wladimir Köppen (1846–1940) in 1884, with several later modifications by Köppen, notably in 1918 and 1936. Later, the climatologist Rudolf Geiger (1954, 1961) introduced some changes to the classification system, which is thus sometimes called the Köppen–Geiger climate classification system.The Köppen climate classification divides climates into five main climate groups, with each group being divided based on seasonal precipitation and temperature patterns. The five main groups are A (tropical), B (dry), C (temperate), D (continental), and E (polar). Each group and subgroup is represented by a letter. All climates are assigned a main group (the first letter). All climates except for those in the E group are assigned a seasonal precipitation subgroup (the second letter). For example, Af indicates a tropical rainforest climate. The system assigns a temperature subgroup for all groups other than those in the A group, indicated by the third letter for climates in B, C, and D, and the second letter for climates in E. For example, Cfb indicates an oceanic climate with warm summers as indicated by the ending b. Climates are classified based on specific criteria unique to each climate type.Köppen designed the system based on his experience as a botanist, so the main climate groups are based on the different variety of vegetation that grows in climates belonging to each group. In addition to identifying climates, the system can be used to analyze ecosystem conditions and identify the main types of vegetation within climates. Due to its link with the plant life of a region, the system is useful in predicting future changes in plant life within a region.The Köppen climate classification system has been further modified, within the Trewartha climate classification system in the middle 1960s (revised in 1980). The Trewartha system sought to create a more refined middle latitude climate zone, which was one of the criticisms of the Köppen system (the C climate group was too broad).

Mediterranean climate

A Mediterranean climate or dry summer climate is characterized by rainy winters and dry summers, with less than 40 mm of precipitation for at least three summer months. While the climate receives its name from the Mediterranean Basin, these are generally located on the western coasts of continents, between roughly 30 and 45 degrees north and south of the equator, typically between oceanic climates towards the poles (where they tend to be wetter and cooler), and semi-arid and arid climates towards the equator (where they tend to be drier and hotter).

In essence, and due to the seasonal shift of the subtropical high-pressure belts with the apparent movement of the Sun, a Mediterranean climate is an intermediate type between these other climates, with winters warmer and drier (and sunnier) than oceanic climates and summers imitating sunny weather in semi-arid and arid climates.

The resulting vegetation of Mediterranean climates are the garrigue or maquis in the Mediterranean Basin, the chaparral in California, the fynbos in South Africa, the mallee in Australia, and the matorral in Chile. Areas with this climate are where the so-called "Mediterranean trinity" of agricultural products have traditionally developed: wheat, vine and olive.

Most large, historic cities of the Mediterranean basin also lie within Mediterranean climatic zones, including Algiers, Athens, Beirut, İzmir, Jerusalem, Marseille, Naples, Rome, Tunis, and Valencia. Examples of major cities with Mediterranean climates that lie outside of the historic Mediterranean basin include major examples as Adelaide, Cape Town, Casablanca, Dushanbe, Los Angeles, Lisbon, Perth, San Francisco, Santiago and Victoria.

Namib

The Namib is a coastal desert in southern Africa. The name Namib is of Khoekhoegowab origin and means "vast place". According to the broadest definition, the Namib stretches for more than 2,000 kilometres (1,200 mi) along the Atlantic coasts of Angola, Namibia, and South Africa, extending southward from the Carunjamba River in Angola, through Namibia and to the Olifants River in Western Cape, South Africa. The Namib's northernmost portion, which extends 450 kilometres (280 mi) from the Angola-Namibia border, is known as Moçâmedes Desert, while its southern portion approaches the neighboring Kalahari Desert. From the Atlantic coast eastward, the Namib gradually ascends in elevation, reaching up to 200 kilometres (120 mi) inland to the foot of the Great Escarpment. Annual precipitation ranges from 2 millimetres (0.079 in) in the most arid regions to 200 millimetres (7.9 in) at the escarpment, making the Namib the only true desert in southern Africa. Having endured arid or semi-arid conditions for roughly 55–80 million years, the Namib may be the oldest desert in the world and contains some of the world's driest regions, with only western South America's Atacama Desert to challenge it for age and aridity benchmarks.

The desert geology consists of sand seas near the coast, while gravel plains and scattered mountain outcrops occur further inland. The sand dunes, some of which are 300 metres (980 ft) high and span 32 kilometres (20 mi) long, are the second largest in the world after the Badain Jaran Desert dunes in China. Temperatures along the coast are stable and generally range between 9–20 °C (48–68 °F) annually, while temperatures further inland are variable—summer daytime temperatures can exceed 45 °C (113 °F) while nights can be freezing. Fogs that originate offshore from the collision of the cold Benguela Current and warm air from the Hadley Cell create a fog belt that frequently envelops parts of the desert. Coastal regions can experience more than 180 days of thick fog a year. While this has proved a major hazard to ships—more than a thousand wrecks litter the Skeleton Coast—it is a vital source of moisture for desert life.

The Namib is almost completely uninhabited by humans except for several small settlements and indigenous pastoral groups, including the Ovahimba and Obatjimba Herero in the north, and the Topnaar Nama in the central region. Owing to its antiquity, the Namib may be home to more endemic species than any other desert in the world. Most of the desert wildlife is arthropods and other small animals that live on little water, although larger animals inhabit the northern regions. Near the coast, the cold ocean water is rich in fishery resources and supports populations of brown fur seals and shorebirds, which serve as prey for the Skeleton Coast's lions. Further inland, the Namib-Naukluft National Park, the largest game park in Africa, supports populations of African Bush Elephants, Mountain Zebras, and other large mammals. Although the outer Namib is largely barren of vegetation, lichens and succulents are found in coastal areas, while grasses, shrubs, and ephemeral plants thrive near the escarpment. Several types of trees are also able to survive the extremely arid climate.

Outback

The Outback is the vast, remote interior of Australia. "The Outback" is more remote than those areas named "the bush" which is any location outside the main urban areas.

While often envisaged as being arid, the Outback regions extend from the northern to southern Australian coastlines, and encompass a number of climatic zones; including tropical and monsoonal climates in northern areas, arid areas in the "red centre" and semi-arid and temperate climates in southerly regions.Geographically, the Outback is unified by a combination of factors, most notably a low human population density, a largely intact natural environment and, in many places, low-intensity land uses such as pastoralism (livestock grazing) in which production is reliant on the natural environment.Culturally, the Outback is deeply ingrained in Australian heritage, history and folklore. In 2009 as part of the Q150 celebrations, the Queensland Outback was announced as one of the Q150 Icons of Queensland for its role as a "natural attraction".

Psammobates

Psammobates is a genus of tortoise. This genus contains three member species, all of which are indigenous to Southern Africa.The genus name means "Sand-loving", and these tortoises typically inhabit the arid and semi-arid areas of southern Africa. Their specific diets and adaptations for this environment mean that these species do not generally survive outside their habitats and soon die when kept in captivity.

All three species suffer from illegal collecting and habitat destruction, but the Geometric tortoise has historically been the worst affected and is now endangered.

Semi-arid climate

A semi-arid climate or steppe climate is the climate of a region that receives precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-arid climates, depending on variables such as temperature, and they give rise to different biomes.

Steppe

In physical geography, a steppe (Russian: степь, IPA: [stʲepʲ]) is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes. In South Africa, they are referred to as veld. The prairie of North America (especially the shortgrass and mixed prairie) is an example of a steppe, though it is not usually called such. A steppe may be semi-arid or covered with grass or shrubs or both, depending on the season and latitude. The term is also used to denote the climate encountered in regions too dry to support a forest but not dry enough to be a desert. The soil is typically of chernozem type.

Steppes are usually characterized by a semi-arid or continental climate. Extremes can be recorded in the summer of up to 45 °C (113 °F) and in winter, −55 °C (−67 °F). Besides this huge difference between summer and winter, the differences between day and night are also very great. In both the highlands of Mongolia and northern Nevada, 30 °C (86 °F) can be reached during the day with sub-zero °C (sub 32 °F) readings at night.

The mid-latitude steppes can be summarized by hot summers and cold winters, averaging 250–510 mm (10–20 in) of precipitation per year. Precipitation level alone is not what defines a steppe climate; potential evapotranspiration must also be taken into account.

Stone-curlew

The stone-curlews, also known as dikkops or thick-knees, consist of nine species within the family Burhinidae, and are found throughout the tropical and temperate parts of the world, with two species found in Australia. Despite the group being classified as waders, most species have a preference for arid or semi-arid habitats.

Western broad-nosed bat

The inland broad-nosed bat (Scotorepens balstoni), also known as the western broad-nosed bat, is a species of vesper bat. It is endemic to Australia and is widespread throughout the inland, especially in arid and semi-arid regions. This insectivorous microbat, measuring 12 cm in length, roosts in tree hollows during the day and forages over woodland and water at night.

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