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66-654: Huacachina is a village built around a small oasis and surrounded by sand dunes in southwestern Peru . It is about five kilometers from the city of Ica in the Ica District of Ica Province . The oasis was introduced as a feature on the back of the 50 nuevo sol note in 1991. Huacachina has a permanent population of around 100 people, although it hosts many tens of thousands of tourists each year. The name comes from Quechua : wakachina , lit.   'guard, conceal', possibly shortened from wakachina qucha lit.   ' hidden lagoon ' . Huacachina

132-496: A nanobubble system to decontaminate lake El Cascajo, would lead a project to restore the Huacachina lagoon. As of 2020, no further news of the proposal has been forthcoming. Huacachina is a resort geared to local families from the nearby city of Ica, but is increasingly an attraction for tourists drawn by the sports of sandboarding on the sand dunes that reach heights of up to 500 metres (1,600 ft). Another popular activity

198-471: 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

264-630: A confining layer, often made up of clay. The confining layer might offer some protection from surface contamination. If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10 ), which means that

330-489: 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

396-403: A million cubic kilometers of "low salinity" water that could be economically processed into potable water . The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100 times the amount of water extracted from other aquifers since 1900. An aquitard

462-407: A mirror she saw a male hunter approaching her from behind, startled at the intrusion, she fled the area, leaving behind her mirror, which turned into a lake. Other versions hold that she fled, leaving the pool of water she had been bathing in to become the lagoon. The folds of her mantle , streaming behind her as she ran, became the surrounding sand dunes. The woman herself is rumoured to still live in

528-807: 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

594-442: A rock unit of low porosity is highly fractured, it can also make a good aquifer (via fissure flow), provided the rock has a hydraulic conductivity sufficient to facilitate movement of water. Challenges for using groundwater include: overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, groundwater becoming saline, groundwater pollution . Aquifer depletion

660-411: 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

726-440: A two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), whereas the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas—sometimes called the pressure area). Since there are less fine-grained deposits near

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792-416: A well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. Voids in karst aquifers can be large enough to cause destructive collapse or subsidence of the ground surface that can initiate a catastrophic release of contaminants. Groundwater flow rate in karst aquifers is much more rapid than in porous aquifers as shown in the accompanying image to

858-589: Is dune buggy rides on vehicles known locally as areneros. Huacachina has been a favourite family destination for decades, and in the 60s was a popular summer resort for tourists around the world. Oasis 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

924-534: 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

990-566: Is a problem in some areas, especially in northern Africa , where one example is the Great Manmade River project of Libya . However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States. The Great Artesian Basin situated in Australia

1056-550: Is a zone within the Earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge . Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity . Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain fresh water . The Earth's crust can be divided into two regions:

1122-994: Is arguably the largest groundwater aquifer in the world (over 1.7 million km or 0.66 million sq mi). It plays a large part in water supplies for Queensland, and some remote parts of South Australia. Discontinuous sand bodies at the base of the McMurray Formation in the Athabasca Oil Sands region of northeastern Alberta , Canada, are commonly referred to as the Basal Water Sand (BWS) aquifers . Saturated with water, they are confined beneath impermeable bitumen -saturated sands that are exploited to recover bitumen for synthetic crude oil production. Where they are deep-lying and recharge occurs from underlying Devonian formations they are saline, and where they are shallow and recharged by surface water they are non-saline. The BWS typically pose problems for

1188-467: Is built around a small natural desert lake, commonly referred to as the "oasis of America". According to local legends, the water and mud of the area are therapeutic. Both locals and tourists often bathe in the waters or cover themselves with the mud in an attempt to cure ailments such as arthritis, rheumatism, asthma, and bronchitis. Legend holds that the lagoon was created by a beautiful native princess, she removed her clothes to bathe, but after looking in

1254-495: Is considered to be a high rate for porous aquifers, as illustrated by the water slowly seeping from sandstone in the accompanying image to the left. Porosity is important, but, alone , it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps (a basaltic lava) in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly,

1320-421: Is held in place by surface adhesive forces and it rises above the water table (the zero- gauge-pressure isobar ) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe ) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from

1386-480: 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,

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1452-423: 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

1518-419: Is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere. Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). An aquitard is a zone within the Earth that restricts

1584-427: The saturated zone or phreatic zone (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the unsaturated zone (also called the vadose zone ), where there are still pockets of air that contain some water, but can be filled with more water. Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of

1650-792: The Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon ranges between Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra Nevada and neighboring ranges in the United States' Southwest , have shallow aquifers that are exploited for their water. Overexploitation can lead to the exceeding of the practical sustained yield; i.e., more water is taken out than can be replenished. Along

1716-610: The Guarani people , it covers 1,200,000 km (460,000 sq mi), with a volume of about 40,000 km (9,600 cu mi), a thickness of between 50 and 800 m (160 and 2,620 ft) and a maximum depth of about 1,800 m (5,900 ft). The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily fossil water from

1782-561: 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

1848-661: The depositional sedimentary environment and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers. Sandy deposits formed in shallow marine environments and in windblown sand dune environments have moderate to high permeability while sandy deposits formed in river environments have low to moderate permeability. Rainfall and snowmelt enter

1914-480: 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

1980-807: 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

2046-662: The United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs." An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer in central Texas . This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today,

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2112-421: The aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically called specific yield ) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to

2178-490: 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. Aquifer An aquifer is an underground layer of water -bearing material, consisting of permeable or fractured rock, or of unconsolidated materials ( gravel , sand , or silt ). Aquifers vary greatly in their characteristics. The study of water flow in aquifers and

2244-612: The characterization of aquifers is called hydrogeology . Related terms include aquitard , which is a bed of low permeability along an aquifer, and aquiclude (or aquifuge ), which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. The classification of aquifers is as follows: Saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; transboundary aquifer. Groundwater from aquifers can be sustainably harvested by humans through

2310-435: 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

2376-409: The coastlines of certain countries, such as Libya and Israel, increased water usage associated with population growth has caused a lowering of the water table and the subsequent contamination of the groundwater with saltwater from the sea. In 2013 large freshwater aquifers were discovered under continental shelves off Australia, China, North America and South Africa. They contain an estimated half

2442-487: The complexity of karst aquifers. These conventional investigation methods need to be supplemented with dye traces , measurement of spring discharges, and analysis of water chemistry. U.S. Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Linear alignment of surface features such as straight stream segments and sinkholes develop along fracture traces . Locating

2508-526: The compound Kh and Kv values are different (see hydraulic transmissivity and hydraulic resistance ). When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty. To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and contamination . Considerations include where and how much water enters

2574-439: The drainable porosity of the aquifer material, or the minimum volumetric water content ). In isotropic aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense. Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because

2640-509: 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

2706-503: 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

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2772-557: The fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings. Abundant small openings store a large quantity of water. The larger openings form a conduit system that drains the aquifer to springs. Characterization of karst aquifers requires field exploration to locate sinkholes, swallets , sinking streams , and springs in addition to studying geologic maps . Conventional hydrogeologic methods such as aquifer tests and potentiometric mapping are insufficient to characterize

2838-478: The flow of groundwater from one aquifer to another. A completely impermeable aquitard is called an aquiclude or aquifuge . Aquitards contain layers of either clay or non-porous rock with low hydraulic conductivity . In mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium , composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at

2904-649: 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

2970-607: 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

3036-526: The groundwater from rainfall and snowmelt, how fast and in what direction the groundwater travels, and how much water leaves the ground as springs. Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the hydrology has been characterized . Porous aquifers typically occur in sand and sandstone . Porous aquifer properties depend on

3102-610: The groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water. Analyzing this type of information over an area gives an indication how much water can be pumped without overdrafting and how contamination will travel. In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day (0.3 m/d)

3168-481: The left. For example, in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.5 to 7 miles per day (0.8 to 11.3 km/d). The rapid groundwater flow rates make karst aquifers much more sensitive to groundwater contamination than porous aquifers. In the extreme case, groundwater may exist in underground rivers (e.g., caves underlying karst topography . If

3234-462: The micro-porous (Upper Cretaceous ) Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring. Karst aquifers typically develop in limestone . Surface water containing natural carbonic acid moves down into small fissures in limestone. This carbonic acid gradually dissolves limestone thereby enlarging

3300-464: The oasis as a mermaid . The lake was created by natural seepage from underground aquifers, but the increase in well-drilling started to threaten that supply in the early 2000s. To compensate for the loss, and preserve the oasis as a destination for tourists, local business groups began pumping water into the lake in 2015. It was announced in 2016 that the Peruvian scientist Marino Morikawa , who created

3366-416: The phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than 1.8 m (6 ft) but can range between 0.3 and 10 m (1 and 33 ft). The capillary rise of water in a small- diameter tube involves the same physical process. The water table

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3432-688: 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

3498-402: The recovery of bitumen, whether by open-pit mining or by in situ methods such as steam-assisted gravity drainage (SAGD), and in some areas they are targets for waste-water injection. The Guarani Aquifer , located beneath the surface of Argentina , Brazil , Paraguay , and Uruguay , is one of the world's largest aquifer systems and is an important source of fresh water . Named after

3564-429: The same geologic unit may be confined in one area and unconfined in another. Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface (see Biscayne Aquifer ). Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and

3630-399: 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

3696-425: The source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface. An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in

3762-419: The surface are not only more likely to be used for water supply and irrigation, but are also more likely to be replenished by local rainfall. Although aquifers are sometimes characterized as "underground rivers or lakes," they are actually porous rock saturated with water. Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources. Part of

3828-664: 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

3894-468: 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

3960-420: The surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller). Confined aquifers are aquifers that are overlain by

4026-426: 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

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4092-605: The time of the last glaciation . Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals. According to a 2013 report by the United States Geological Survey (USGS), the depletion between 2001 and 2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century. In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction. "Cumulative total groundwater depletion in

4158-443: The use of qanats leading to a well. This groundwater is a major source of fresh water for many regions, however can present a number of challenges such as overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, and the salinization or pollution of the groundwater. Aquifers occur from near-surface to deeper than 9,000 metres (30,000 ft). Those closer to

4224-624: 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

4290-410: The water table is the surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0). Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under suction . The water content in the unsaturated zone

4356-400: 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

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