Oasis

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. == Agroforestry ==

People who live in an oasis must manage land and water use carefully. The most important plant in an oasis is the date palm (Phoenix dactylifera L.), which forms the upper layer. These palm trees provide shade for smaller understory trees like apricots, 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 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. The Ferkla Oases in Morocco once drew on water from the Ferkla, Sat and Tangarfa Rivers but they are now dry but for a few days a year. ==List of places called oasis==

New World dryland systems with oasis-like attributes • Huacachina, Peru • Kitowok, Sonora, Mexico • Zzyzx in Mojave National Preserve, California • Cuatro Ciénegas, Chihuahuan Desert, Mexico • Oasis Spring Ecological Reserve, Salton Sea, California == Gallery of oases ==

A 1920 USGS publication about watering holes in the deserts of California and Arizona gave this advice for travelers seeking oases: == See also ==