In ecology, an oasis (; pl.: 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 Latin: oasis, from Ancient Greek: ὄασις, óasis, 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 (Arabic: واحة).
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 West Asia and North Africa cover about 1,000,000 hectares (2,500,000 acres; 10,000 km2), 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". 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 the water into individual plots, soaking the soil".
History Oases often have human histories that are measured in millennia. Archaeological 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 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. 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 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 the ground or carved into the rock to tap underground aquifers". This rainwater harvesting system "never developed a serious salinity problem".
Palm Oasis 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.
Agroforestry
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