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The word oasis came into English from Latin: oasis, from Ancient Greek: , óasis, which in turn is a direct borrowing from Demotic Egyptian. The word for oasis in the later attested Coptic language (the descendant of Demotic Egyptian) is wahe or ouahe which means a "dwelling place".
Oases are made fertile when sources of freshwater, such as 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.
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.
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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 Silk Road across Central Asia also incorporated several oases.
In North American history, oases have been less prominent since the desert regions are smaller, however several areas in the deep southwestern United States have oases regions that served as important links through the hot deserts and vast rural areas. While present day desert cities like Las Vegas, Phoenix, Palm Springs, and Tucson are large modern cities, many of these locations were once small, isolated farming areas that travelers through the western desert stopped for food and supplies. Even today, there are several roads that go through western deserts like U.S. Route 50 through southern Nevada, and the Mojave Desert that feature small green fields, citrus groves and small isolated supply towns.
People who live in an oasis must manage land and water use carefully; fields must be irrigated to grow plants like apricots, dates, figs, and olives. The most important plant in an oasis is the date palm, which forms the upper layer. These palm trees provide shade for smaller trees like peach trees, which form the middle layer. By growing plants in different layers, the farmers make best use of the soil and water. Many vegetables are also grown and some cereals, such as barley, millet, and wheat, are grown where there is more moisture. In summary, an oasis palm grove is a highly anthropized and irrigated area that supports a traditionally intensive and polyculture-based agriculture. 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). However, the oasis is emancipated from the desert by a very particular social and ecosystem structure. Responding to environmental constraints, it is an integrated agriculture that is conducted with the superposition (in its typical form) of two or three strata creating what is called the "oasis effect":
Rubaksa in a dry limestone environment in north Ethiopia is an oasis thanks to the existence of karstic springs
Twentynine Palms sign