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65-575: Badwater Basin is an endorheic basin in Death Valley National Park , Death Valley , Inyo County, California , noted as the lowest point in North America and the United States , with a depth of 282 ft (86 m) below sea level . Mount Whitney , the highest point in the contiguous United States , is only 84.6 miles (136 km) to the northwest. The site itself consists of

130-630: 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

195-788: A large portion of Europe drains to the endorheic Caspian Sea, Europe's wet climate means it contains relatively few terminal lakes itself: any such basin is likely to continue to fill until it reaches an overflow level connecting it with an outlet or erodes the barrier blocking its exit. There are some seemingly endorheic lakes, but they are cryptorheic, being drained either through manmade canals , via karstic phenomena, or other subsurface seepage. A few minor true endorheic lakes exist in Spain (e.g. Laguna de Gallocanta , Estany de Banyoles ), Italy , Cyprus ( Larnaca and Akrotiri salt lakes) and Greece . Many small lakes and ponds in North Dakota and

260-403: 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

325-709: A mountain range, cutting off water egress to the ocean. The inland water flows into dry watersheds where the water evaporates, leaving a high concentration of minerals and other inflow erosion products. Over time this input of erosion products can cause the endorheic lake to become relatively saline (a " salt lake "). Since the main outflow pathways of these lakes are chiefly through evaporation and seepage, endorheic lakes are usually more sensitive to environmental pollutant inputs than water bodies that have access to oceans, as pollution can be trapped in them and accumulate over time. Endorheic regions can occur in any climate but are most commonly found in desert locations. This reflects

390-519: A redistribution of water from these hydrologically landlocked basins such that endorheic water loss has contributed to sea level rise , and it is estimated that most of the terrestrial water lost ends up in the ocean. In regions such as Central Asia, where people depend on endorheic basins and other surface water sources to satisfy their water needs, human activity greatly impacts the availability of that water. Large endorheic regions in Africa are located in

455-442: 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

520-454: A small spring -fed pool of "bad water" next to the road in a sink ; the accumulated salts of the surrounding basin make it undrinkable, thus giving it the name. The pool does have animal and plant life , including pickleweed , aquatic insects , and the Badwater snail . Badwater Crater , the lowest place on the planet Mars , is named after the basin due to their similarities. Adjacent to

585-464: A temporary lake known as Lake Manly . Newly formed lakes do not last long though, because the 1.9 in (48 mm) of average rainfall is overwhelmed by a 150 in (3,800 mm) annual evaporation rate and usually lakes are only a couple inches deep. This is the greatest evaporation potential in the United States, meaning that a 12 ft (3.8 m) lake could dry up in a single year. When

650-517: A thin white crust over mud), and so the sign marking the low point is at the pool instead. The basin was considered the lowest elevation in the Western Hemisphere until the discovery of Laguna del Carbón in Argentina at −344 ft (−105 m). At Badwater Basin, significant rainstorms flood the valley bottom periodically, covering the salt pan with a thin sheet of standing water, forming

715-440: 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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780-416: 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

845-466: Is a drainage basin that normally retains water and allows no outflow to other external bodies of water (e.g. rivers and oceans ); instead, the water drainage flows into permanent and seasonal lakes and swamps that equilibrate through evaporation . Endorheic basins are also called closed basins , terminal basins , and internal drainage systems . Endorheic regions contrast with open lakes (exorheic regions), where surface waters eventually drain into

910-510: 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

975-550: 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:

1040-994: 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

1105-495: 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,

1170-421: 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

1235-403: Is one such case, with annual precipitation of 850 mm (33 in) and characterized by waterlogged soils that require draining. Endorheic regions tend to be far inland with their boundaries defined by mountains or other geological features that block their access to oceans. Since the inflowing water can evacuate only through seepage or evaporation, dried minerals or other products collect in

1300-574: Is that the entire region between the Colorado River in the east and Baja California in the southwest (and bordered by various uplifts and mountains around the west-northwest-northern perimeters) has seen numerous cycles since at least the start of the Pleistocene (and perhaps up to 3  Ma ). In these cycles, pluvial lakes of varying size have come and gone in a complex cycle, mainly tied to changing climate patterns (particularly, glaciation during

1365-419: 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

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1430-427: 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

1495-737: 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

1560-610: The Bonneville flood . The Malheur / Harney lake system in Oregon is normally cut off from drainage to the ocean, but has an outflow channel to the Malheur River . This is presently dry, but may have flowed as recently as 1,000 years ago. Examples of relatively humid regions in endorheic basins often exist at high elevation. These regions tend to be marshy and are subject to substantial flooding in wet years. The area containing Mexico City

1625-610: 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

1690-481: The Quaternary Period in the area, with an intertwined history of various larger bodies of water subsuming smaller ones during water table maxima and the subsequent splitting and disappearance thereof during the evaporative part of the cycles. Although these local cycles are now somewhat modified by human presence, their legacy persists; despite appearances much to the contrary, Death Valley actually sits atop one of

1755-906: The Sahara Desert , the Sahel , the Kalahari Desert , and the East African Rift : Endorheic lakes exist in Antarctica's McMurdo Dry Valleys , Victoria Land , the largest ice-free area. Much of Western and Central Asia is a giant endorheic region made up of a number of contiguous closed basins. The region contains several basins and terminal lakes, including: Other endorheic lakes and basins in Asia include: Australia , being very dry and having exceedingly low runoff ratios due to its ancient soils, has many endorheic drainages. The most important are: Though

1820-661: 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

1885-658: 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 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

1950-449: The French word endoréisme , which combines endo- ( Ancient Greek : ἔνδον éndon 'within') and ῥεῖν rheîn 'flow'. Endorheic lakes (terminal lakes) are bodies of water that do not flow into an ocean or a sea. Most of the water that falls to Earth percolates into the oceans and the seas by way of a network of rivers, lakes, and wetlands . Analogous to endorheic lakes is

2015-521: The Northern Great Plains are endorheic, and some have salt encrustations along their shores. Some of Earth's ancient endorheic systems and lakes include: Aquifer Groundwater from aquifers can be sustainably harvested by humans through 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

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2080-662: 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,

2145-421: 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

2210-493: The balance between tectonic subsidence and rates of evaporation and sedimentation. Where the basin floor is dropping more rapidly than water and sediments can accumulate, any lake in the basin will remain below the sill level (the level at which water can find a path out of the basin). Low rainfall or rapid evaporation in the watershed favor this case. In areas where rainfall is higher, riparian erosion will generally carve drainage channels (particularly in times of flood), or cause

2275-463: The basin is flooded, some of the salt is dissolved; it is redeposited as clean crystals when the water evaporates. A popular site for tourists is the sign marking "sea level" on the cliff above the Badwater Basin. Similar to Owens Lake , it is characterized by a deep bed of unconsolidated valley fill from which the salt crust emerges. The current best understanding of the area's geological history

2340-408: The basin, eventually making the water saline and also making the basin vulnerable to pollution. Continents vary in their concentration of endorheic regions due to conditions of geography and climate. Australia has the highest percentage of endorheic regions at 21 per cent while North America has the least at five per cent. Approximately 18 per cent of the Earth's land drains to endorheic lakes or seas,

2405-537: The class of bodies of water located in closed watersheds (endorheic watersheds) where the local topography prevents the drainage of water into the oceans and the seas. These endorheic watersheds (containing water in rivers or lakes that form a balance of surface inflows, evaporation and seepage) are often called sinks. Endorheic lakes are typically located in the interior of a landmass, far from an ocean, and in areas of relatively low rainfall. Their watersheds are often confined by natural geologic land formations such as

2470-409: 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

2535-487: 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

2600-526: 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

2665-564: The degree that a lake no longer forms. Even most permanent endorheic lakes change size and shape dramatically over time, often becoming much smaller or breaking into several smaller parts during the dry season. As humans have expanded into previously uninhabitable desert areas, the river systems that feed many endorheic lakes have been altered by the construction of dams and aqueducts. As a result, many endorheic lakes in developed or developing countries have contracted dramatically, resulting in increased salinity, higher concentrations of pollutants, and

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2730-440: The disruption of ecosystems. Even within exorheic basins, there can be "non-contributing", low-lying areas that trap runoff and prevent it from contributing to flows downstream during years of average or below-average runoff. In flat river basins, non-contributing areas can be a large fraction of the river basin, e.g. Lake Winnipeg 's basin. A lake may be endorheic during dry years and can overflow its basin during wet years, e.g.,

2795-439: 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

2860-557: 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

2925-478: 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

2990-774: The former Tulare Lake . Because the Earth's climate has recently been through a warming and drying phase with the end of the Ice Ages, many endorheic areas such as Death Valley that are now dry deserts were large lakes relatively recently. During the last ice age, the Sahara may have contained lakes larger than any now existing. Climate change coupled with the mismanagement of water in these endorheic regions has led to devastating losses in ecosystem services and toxic surges of pollutants. The desiccation of saline lakes produces fine dust particles that impair agriculture productivity and harm human health. Anthropogenic activity has also caused

3055-526: 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

3120-610: 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)

3185-621: The largest aquifers in the world. Throughout the Quaternary's wetter spans, streams running from nearby mountains filled Death Valley, creating Lake Manly , which during its greatest extents was approximately 80 mi (130 km) long and up to 600 ft (180 m) deep. Numerous evaporation cycles and a lack of outflow caused an increasing hypersalinity , typical for endorheic bodies of water. Over time, this hypersalinization, combined with sporadic rainfall and occasional aquifer intrusion, has resulted in periods of "briny soup", or salty pools, on

3250-422: The largest of these land areas being the interior of Asia. In deserts, water inflow is low and loss to solar evaporation high, drastically reducing the formation of complete drainage systems . In the extreme case, where there is no discernible drainage system, the basin is described as arheic . Closed water flow areas often lead to the concentration of salts and other minerals in the basin. Minerals leached from

3315-481: 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

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3380-457: The lowest parts of Death Valley's floor. Salts (95% table salt – NaCl ) began to crystallize, coating the surface with the thick crust, ranging from 3 to 60 in (8 to 152 cm), now observable at the basin floor. Notes Further reading Endorheic basin An endorheic basin ( / ˌ ɛ n d oʊ ˈ r iː . ɪ k / EN -doh- REE -ik ; also endoreic basin and endorreic basin )

3445-462: 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

3510-604: The numerous recent Ice Age cycles ), but also influenced by the progressive depositing of alluvial plains and deltas by the Colorado River, as can also be seen in the case of the Salton Sea . These alternate with periodic water body breakthroughs and rearrangements due to erosion and the proximity of the San Andreas Fault . This process has resulted in a high number of evaporating and reforming endorheic lakes throughout

3575-456: The ocean. In general, water basins with subsurface outflows that lead to the ocean are not considered endorheic; but cryptorheic . Endorheic basins constitute local base levels , defining a limit of the erosion and deposition processes of nearby areas. Endorheic water bodies include the Caspian Sea , which is the world's largest inland body of water. The term endorheic derives from

3640-416: 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

3705-428: The pool, where water is not always present at the surface, repeated freeze–thaw and evaporation cycles gradually push the thin salt crust into hexagonal shapes. The pool is not the lowest point of the basin: the lowest point (which is only slightly lower) is several miles to the west and varies in position, depending on rainfall and evaporation patterns. The salt flats are hazardous to traverse (in many cases being only

3770-402: 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

3835-429: 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

3900-425: 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

3965-420: 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

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4030-599: The surrounding rocks are deposited in the basin, and left behind when the water evaporates. Thus endorheic basins often contain extensive salt pans (also called salt flats, salt lakes, alkali flats , dry lake beds, or playas). These areas tend to be large, flat hardened surfaces and are sometimes used for aviation runways , or land speed record attempts, because of their extensive areas of perfectly level terrain. Both permanent and seasonal endorheic lakes can form in endorheic basins. Some endorheic basins are essentially stable because climate change has reduced precipitation to

4095-605: 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

4160-487: The water level in the terminal lake to rise until it finds an outlet, breaking the enclosed endorheic hydrological system's geographical barrier and opening it to the surrounding terrain. The Black Sea was likely such a lake, having once been an independent hydrological system before the Mediterranean Sea broke through the terrain separating the two. Lake Bonneville was another such lake, overflowing its basin in

4225-410: 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

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