In ecology , an oasis ( / oʊ ˈ eɪ s ɪ s / ; pl. : oases / oʊ ˈ eɪ s iː z / ) is a fertile area of a desert or semi-desert environment that sustains plant life and provides habitat for animals. Surface water may be present, or water may only be accessible from wells or underground channels created by humans. In geography, an oasis may be a current or past rest stop on a transportation route, or less-than-verdant location that nonetheless provides access to underground water through deep wells created and maintained by humans. Although they depend on a natural condition, such as the presence of water that may be stored in reservoirs and used for irrigation, most oases, as we know them, are artificial.
35-635: Bilma is an oasis town and commune in north east Niger with, as of the 2012 census, a total population of 4,016 people. It lies protected from the desert dunes under the Kaouar Cliffs and is the largest town along the Kaouar escarpment. It is known for its gardens, for salt and natron production through evaporation ponds, date cultivation, and as the destination of one of the last Saharan caravan routes (the Azalai , from Agadez ). Its population
70-475: A "central pool of open water surrounded by a ring of water-dependent shrubs and trees…which are in turn encircled by an outlying transition zone to desert plants." Rain showers provide subterranean water to sustain natural oases, such as the Tuat . Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to
105-836: A particular route. For example, the oases of Awjila , Ghadames and Kufra , situated in modern-day Libya , have at various times been vital to both north–south and east–west trade in the Sahara Desert . The location of oases also informed the Darb El Arba'īn trade route from Sudan to Egypt, as well as the caravan route from the Niger River to Tangier , Morocco. The Silk Road "traced its course from water hole to water hole, relying on oasis communities such as Turpan in China and Samarkand in Uzbekistan." According to
140-491: A result. Wadis tend to be associated with centers of human population because sub-surface water is sometimes available in them. Nomadic and pastoral desert peoples will rely on seasonal vegetation found in wadis, even in regions as dry as the Sahara , as they travel in complex transhumance routes. The centrality of wadis to water – and human life – in desert environments gave birth to the distinct sub-field of wadi hydrology in
175-411: A shovel is needed. Wells may or may not be equipped with pumps. Frequently the pumps are broken and useless, and a rope and bucket are then necessary to obtain water. Most of the wells in this region are less than 100 feet deep, but some are deeper, and 100 feet of rope is not too much to provide. As a rule the rope and bucket at a well, if they were ever provided, soon disappear, and one should never trust
210-709: A wide range of sedimentary structures, including ripples and common plane beds. Gravels commonly display imbrications , and mud drapes show desiccation cracks. Wind activity also generates sedimentary structures, including large-scale cross-stratification and wedge-shaped cross-sets. A typical wadi sequence consists of alternating units of wind and water sediments; each unit ranging from about 10–30 cm (4–12 in). Sediment laid by water shows complete fining upward sequence. Gravels show imbrication. Wind deposits are cross-stratified and covered with mud-cracked deposits. Some horizontal loess may also be present. Modern English usage differentiates wadis from canyons or washes by
245-538: Is a direct borrowing from Demotic Egyptian . The word for oasis in the latter-attested Coptic language (the descendant of Demotic Egyptian) is wahe or ouahe which means a "dwelling place". Oasis in Arabic is wāḥa ( Arabic : واحة ). Oases develop in " hydrologically favored" locations that have attributes such as a high water table , seasonal lakes, or blockaded wadis . Oases are made when sources of freshwater, such as underground rivers or aquifers , irrigate
280-453: Is also hot during the "winter" months and extremely hot during the summer months and for prolonged periods of time. Average high temperatures in "winter" months surpass 27 °C (81 °F), and exceed 40 °C (104 °F) from April to September inclusively, peaking at 44 °C (111 °F) in June. The record high temperature is 48.2 °C (118.8 °F) on June 23, 2010. Bilma had
315-480: Is integrated into its desert environment through an often close association with nomadic transhumant livestock farming (very often pastoral and sedentary populations are clearly distinguished). The fertility of the oasis soil is restored by "cyclic organic inputs of animal origin." In summary, an oasis palm grove is a highly anthropized and irrigated area that supports a traditionally intensive and polyculture-based agriculture. Responding to environmental constraints,
350-605: Is mostly Kanuri , with smaller Toubou , Tuareg , and Hausa populations, the last being a reminder of Bilma's role as a key stop in the Trans Saharan trade . Bilma is the administrative seat of the Bilma Department , covering some 670,000 km (260,000 sq mi) of north eastern Niger . While it continues to produce salt in large natron salt pans, and this salt is still sold for livestock use throughout west Africa. Tourism (based out of Agadez and
385-451: Is the Arabic term traditionally referring to a river valley . In some instances, it may refer to a wet ( ephemeral ) riverbed that contains water only when heavy rain occurs. Arroyo ( Spanish ) is used in the Americas for similar landforms. The term wādī is very widely found in Arabic toponyms . Some Spanish toponyms are derived from Andalusian Arabic where wādī
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#1732765321858420-423: Is the date palm ( Phoenix dactylifera L. ), which forms the upper layer . These palm trees provide shade for smaller understory trees like apricots , dates , figs , olives , and peach trees, which form the middle layer. Market-garden vegetables, some cereals (such as sorghum , barley , millet , and wheat ), and/or mixed animal fodder , are grown in the bottom layer where there is more moisture. The oasis
455-629: The Aïr Mountains some 560 km (350 mi) to the west) is of growing importance. Bilma features a hot desert climate ( Köppen climate classification BWh ). The town lies deep in the heart of the Sahara Desert , more precisely in the Ténéré , a hyper-arid desert region lying over northeastern Niger and western Chad , and as such is extremely dry, averaging only 12.7 mm (0.50 in) of measurable precipitation annually. Bilma
490-573: The Western Desert of Egypt ( Kharga , Dakhla , Farafra , Baharyia , and Siwa ) once had "flowing spring and wells" but due to the decline of groundwater heads because of overuse for land reclamation projects those water sources are no more and the oases suffer as a result. Morocco has lost two-thirds of its oasis habitat over the last 100 years due to heat, drought, and water scarcity . The Ferkla Oases in Morocco once drew on water from
525-484: The Ferkla, Sat and Tangarfa Rivers but they are now dry but for a few days a year. A 1920 USGS publication about watering holes in the deserts of California and Arizona gave this advice for travelers seeking oases: The usual watering places are springs or wells. Springs are frequently clogged with gravel or rubbish or sometimes even with the bodies of dead animals, and it may be necessary to clean them out. For this work
560-837: The United Nations, "Oases are at the very heart of the overall development of peri-Saharan countries due to their geographical location and the fact they are preferred migration routes in times of famine or insecurity in the region." Oases in Oman , on the Arabian Peninsula near the Persian Gulf , vary somewhat from the Saharan form. While still located in an arid or semi-arid zone with a date palm overstory, these oases are usually located below plateaus and "watered either by springs or by aflaj , tunnel systems dug into
595-425: The action and prevalence of water. Wadis, as drainage courses, are formed by water, but are distinguished from river valleys or gullies in that surface water is intermittent or ephemeral. Wadis are generally dry year round, except after a rain. The desert environment is characterized by sudden but infrequent heavy rainfall, often resulting in flash floods . Crossing wadis at certain times of the year can be dangerous as
630-441: The chance of finding them there. Open wells are sometimes contaminated in the same way as springs and need to be cleaned out, particularly in little-frequented places where they are unused for months at a time. Wadi Wadi ( Arabic : وَادِي , romanized : wādī , alternatively wād ; Arabic : وَاد , Maghrebi Arabic oued , Hebrew : וָאדִי , romanized : vadi , lit. 'wadi')
665-435: The city [ medina ] or village [ ksar ] with its surrounding feeding source, the palm grove, within a relational and circulatory nomadic system." The location of oases has been of critical importance for trade and transportation routes in desert areas; caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on
700-504: The country's lowest ever recorded temperature, −2.4 °C (27.7 °F), on 13 January 1995. The sunshine duration is extremely high year-round with some 4,000 hours of bright sunshine annually. The region excels at hot, sunny and dry weather. The poet Abu Ishaq Ibrahim al-Kanemi was born in Bilma in the 12th century. During French Colonial rule, Bilma was the site of a major—if isolated—military post at Fort Dromard . After independence,
735-504: The distal portions of alluvial fans and extend to inland sabkhas or dry lakes . In basin and range topography , wadis trend along basin axes at the terminus of fans. Permanent channels do not exist, due to lack of continual water flow. They have braided stream patterns because of the deficiency of water and the abundance of sediments . Water percolates down into the stream bed, causing an abrupt loss of energy and resulting in vast deposition. Wadis may develop dams of sediment that change
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#1732765321858770-513: The dull bluish or gray green of the feathery crowns of foliage. It is small wonder that a whole folklore of poetic legends and proverbs has grown up around the date palm in the regions where it flourishes. Many historic oases have struggled with drought and inadequate maintenance. According to a United Nations report on the future of oases in the Sahara and Sahel , "Increasingly ... oases are subject to various pressures, heavily influenced by
805-510: The effects of climate change , decreasing groundwater levels and a gradual loss of cultural heritage due to a fading historical memory concerning traditional water management techniques. These natural pressures are compounded by demographic pressures and the introduction of modern water pumping techniques that can disrupt traditional resource management schemes, particularly in the North Saharan oases." For example, five historic oases in
840-649: The garden resembles a tropical jungle. Very beautiful are these gardens in the spring, when the apricot and peach trees are in blossom here and there among the palms and the figs and vines are putting forth their leaves. In autumn, when the dates are ripening, the color effects, especially when the tops of the palms are lit up by the last rays of the setting sun, are something that once seen can never be forgotten. The great clusters of fruit, displaying every tint from bright yellow, through orange, vermilion, and maroon, to plum purple and chestnut brown, with their brilliant yellow or rich orange ivory-like stalks, contrast superbly with
875-613: The ground or carved into the rock to tap underground aquifers." This rainwater harvesting system "never developed a serious salinity problem." In the drylands of southwestern North America, there is a habitat form called Palm Oasis (alternately Palm Series or Oasis Scrub Woodland) that has the native California fan palm as the overstory species. These Palm Oases can be found in California , Arizona , Baja California , and Sonora . People who live in an oasis must manage land and water use carefully. The most important plant in an oasis
910-732: The isolation of Bilma made it the destination for disgraced officials under the authoritarian regime of Seyni Kountché , and a prison was built there by the government. Political leaders were held there in the 1980s, such as Sanoussi Tambari Djakou , today president of the PNA-AL , a Nigerien political party. In 1989, UTA Flight 772 crashed into the desert near the town after a bomb exploded on board, killing all 170 people aboard. 18°41′N 12°55′E / 18.683°N 12.917°E / 18.683; 12.917 Oasis The word oasis came into English from Latin : oasis , from Ancient Greek : ὄασις , óasis , which in turn
945-688: The potential evapotranspiration of the areas was reduced by 30 to 50 percent within the oasis." The keystone date palm trees are "a main income source and staple food for local populations in many countries in which they are cultivated, and have played significant roles in the economy, society, and environment of those countries." Challenges for date palm oasis polycultures include "low rainfall, high temperatures, water resources often high in salt content, and high incidence of pests." The oases consist of almost unbroken forests of date palms, divided up into many gardens that are separated by mud walls and intersected by innumerable irrigation and drainage ditches… In
980-399: The shade of the palms are grown many other kinds of fruit trees—oranges, olives, figs, apricots, peaches, pomegranates, and jujubes—interlaced with large grape vines that often hang in festoons from the palm trunks. Beneath the trees are small plots of garden vegetables, barley, and alfalfa. Neither date palms nor other trees are planted with any regularity, and the growth is often so dense that
1015-485: The stream patterns in the next flash flood . Wind also causes sediment deposition. When wadi sediments are underwater or moist, wind sediments are deposited over them. Thus, wadi sediments contain both wind and water sediments. Wadi sediments may contain a range of material, from gravel to mud, and the sedimentary structures vary widely. Thus, wadi sediments are the most diverse of all desert environments. Flash floods result from severe energy conditions and can result in
1050-664: The surface naturally or via man-made wells. The presence of water on the surface or underground is necessary and the local or regional management of this essential resource is strategic, but not sufficient to create such areas: continuous human work and know-how (a technical and social culture) are essential to maintain such ecosystems. Some of the possible human contributions to maintaining an oasis include digging and maintaining wells, digging and maintaining canals, and continuously removing opportunistic plants that threaten to gorge themselves on water and fertility needed to maintain human and animal food supplies. Stereotypically, an oasis has
1085-474: The surface. Any incidence of water is then used by migrating birds , which also pass seeds with their droppings which will grow at the water's edge forming an oasis. It can also be used to plant crops. Oases in the Middle East and North Africa cover about 1,000,000 hectares (10,000 km ), however, they support the livelihood of about 10 million inhabitants. The stark ratio of oasis to desert land in
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1120-426: The three strata create what is called the " oasis effect ". The three layers and all their interaction points create a variety of combinations of "horizontal wind speed, relative air temperature and relative air humidity." The plantings—through a virtuous cycle of wind reduction, increased shade and evapotranspiration —create a microclimate favorable to crops; "measurements taken in different oases have showed that
1155-648: The water into individual plots, soaking the soil." Oases often have human histories that are measured in millennia. Archeological digs at Ein Gedi in the Dead Sea Valley have found evidence of settlement dating to 6,000 BC. Al-Ahsa on the Arabian Peninsula shows evidence of human residence dating to the Neolithic . Anthropologically , the oasis is "an area of sedentary life, which associates
1190-400: The world means that the oasis ecosystem is "relatively minute, rare and precious." There are 90 “major oases” within the Sahara Desert. Some of their fertility may derive from irrigation systems called foggaras , khettaras , lkhttarts , or a variety of other regional names . In some oases systems, there is "a geometrical system of raised channels that release controlled amounts of
1225-428: Was used to mean a permanent river, for example: Guadalcanal from wādī al-qanāl ( Arabic : وَادِي الْقَنَال , "river of refreshment stalls"), Guadalajara from wādī al-ḥijārah ( Arabic : وَادِي الْحِجَارَة , "river of stones"), or Guadalquivir , from al-wādī al-kabīr ( Arabic : اَلْوَادِي الْكَبِير , "the great river"). Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on
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