thumb|The desert oasis city of [[Jubbah,_Saudi_Arabia|Jubbah in Saudi Arabia as photographed from space.]]
In ecology, an oasis (; : oases ) 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.
The word oasis came into English from , from , , which in turn 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 ().
Description
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 springs, underground rivers, or aquifers irrigate 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 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 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.
Geography
Oases in the Middle East and North Africa cover about , however, they support the livelihood of about 10 million inhabitants. The stark ratio of oasis to desert land in the world means that the oasis ecosystem is "relatively minute, rare and precious".
In some oases systems, there is "a geometrical system of raised channels that release controlled amounts of the water into individual plots, soaking the soil". 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 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 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. These Palm Oases can be found in California, Arizona, Baja California, and Sonora. The oasis 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". Challenges for date palm oasis polycultures include "low rainfall, high temperatures, water resources often high in salt content, and high incidence of pests".
Distressed systems
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 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."
Morocco has lost two-thirds of its oasis habitat over the last 100 years due to heat, drought, and water scarcity.
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| || Tébessa || Negrine Oasis || 355 || Wadi || <!--Unclear as to if this refers to the lake, the oasis, or both. Taken from the list that occupied this page prior to this table's creation.-->
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| || Matruh || Qara Oasis || -41 || Depression ||
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| || Matruh || Siwa Oasis || -11 || Depression ||
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| || Nalut || Derj Oasis || 421 || Wadi ||
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| || Adrar || Amder Oasis || 248 || Wadi ||
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| || Oriental || Ain Chouater Oasis || 769 || Wadi || <!--This was taken from the list that occupied this page prior to this table's creation.-->
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| || Guelmim-Oued Noun || Aday Oasis || 695 || Wadi ||
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| || Agadez || Adaouda Oasis || 906 || Wadi ||
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| || Al Buraymi || Ajran Oasis || 623 || Wadi ||
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| || Tabuk || Tayma Oasis || 812 || Depression ||
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| || Gafsa || Gafsa Oasis || 281 || Wadi || <!--This was not a part of my source for these oases - it was here prior to my creating of this table.-->
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| || Tozeur || Tozeur Oasis || 36 || Depression || <!--I took this from the list that occupied this page before I created this table. It appears to be a conglomeration of oases, rather than a singular oasis. I felt it necessary to include it on the list, but in my albeit brief research into this region, I couldn't find any specific information on the oases that occupy it. If you can find anything, please update this part of the table.-->
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| || Abu Dhabi || Al Ain Oasis || 292 || Erg ||
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| || Abu Dhabi || Liwa Oasis || 130 || Erg ||
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| || Hadramawt || Ad-Dees Ash-Sharqia Oasis || 95 || Wadi ||
- Kitowok, Sonora, Mexico
- Zzyzx in Mojave National Preserve, California
- Cuatro Ciénegas, Chihuahuan Desert, Mexico
- Oasis Spring Ecological Reserve, Salton Sea, California
Gallery of oases
Practical matters
A 1920 USGS publication about watering holes in the deserts of California and Arizona gave this advice for travelers seeking oases:
See also
- – the world's largest irrigation project; developed in Libya to connect cities with fossil water.
- Lençóis Maranhenses National Park (Brazil)
- Great Green Wall (disambiguation)
- Aflaj Irrigation Systems of Oman
- Palmeral of Elche
- Fog oasis (South America)