A qanāt (from Arabic: قناة) (Iran, Syria and Jordan), kārīz (or kārēz from Persian: كاريز (Iran, Afghanistan, Pakistan and Central Asia, derived from Persian: كاهریز), kahan (from Persian: کهن), khettara (Morocco); galeria (Spain); falaj (United Arab Emirates and Oman); Kahn (Baloch) or foggara/fughara (North Africa)  is a water management system used to provide a reliable supply of water to human settlements and for irrigation in hot, arid and semi-arid climates. Alternative terms for Qanats in Asia and North Africa are kakuriz, chin-avulz, and mayun. Common variant spellings/transliterations of qanat in English include kanat, khanat, kunut, kona, konait, ghanat, ghundat. Closely related to such structures is the Turfan water system in China. The technology was developed in ancient Iran.  The value of a qanat is directly related to the quality, volume and regularity of the water flow. Much of the population of Iran and other arid countries in Asia and North Africa historically depended upon the water from qanats; the areas of population corresponded closely to the areas where qanats are possible. Although a qanat was expensive to construct, its long-term value to the community, and therefore to the group who invested in building and maintaining it, was substantial. 
Qanats are constructed as a series of well-like vertical shafts, connected by gently sloping tunnels. Qanats tap into subterranean water in a manner that efficiently delivers large quantities of water to the surface without need for pumping. The water drains relying on gravity, with the destination lower than the source, which is typically an upland aquifer. Qanats allow water to be transported over long distances in hot dry climates without losing a large proportion of the water to seepage and evaporation.
It is very common in the construction of a qanat for the water source to be found below ground at the foot of a range of foothills of mountains, where the water table is closest to the surface. From this point, the slope of the qanat is maintained closer to level than the surface above, until the water finally flows out of the qanat above ground. To reach an aquifer qanats must often be of extreme length.
Qanats are sometimes split into an underground distribution network of smaller canals called kariz. Like Qanats, these smaller canals were below ground to avoid contamination. In some cases water from a Qanat is stored in a reservoir, typically storing night flow for daytime use. An Ab Anbar is an example of a traditional qanat fed reservoir for drinking water in Persian antiquity.
The qanat system has the advantage of being relatively immune to natural disasters (earthquakes, floods…) and human destruction in war. Further it is relatively insensitive to the levels of precipitation; a qanat typically delivers a relatively constant flow with only gradual variations from wet to dry years.
The qanat technology is used most extensively in areas with the following characteristics:
A typical town or city in Iran and elsewhere where the qanat is used has more than one qanat. Fields and gardens are located both over the qanats a short distance before they emerge from the ground and after the surface outlet. Water from the qanats defines both the social regions in the city and the layout of the city.
The water is freshest, cleanest, and coolest in the upper reaches and more prosperous people live at the outlet or immediately upstream of the outlet. When the qanat is still below grade, the water is drawn to the surface via water wells or animal driven Persian wells. Private subterranean reservoirs could supply houses and buildings for domestic use and garden irrigation as well. Further, air flow from the qanat is used to cool an underground summer room (shabestan) found in many older houses and buildings.
Downstream of the outlet, the water runs through surface canals called jubs (jūbs) which run downhill, with lateral branches to carry water to the neighborhood, gardens and fields. The streets normally parallel the jubs and their lateral branches. As a result, the cities and towns are oriented consistent with the gradient of the land; this is a practical response to efficient water distribution over varying terrain.
The lower reaches of the canals are less desirable for both residences and agriculture. The water grows progressively more polluted as it passes downstream. In dry years the lower reaches are the most likely to see substantial reductions in flow.
Traditionally qanats are built by a group of skilled laborers, muqannīs, with hand labor. The profession historically paid well and was typically handed down from father to son.
The critical, initial step in qanat construction is identification of an appropriate water source. The search begins at the point where the alluvial fan meets the mountains or foothills; water is more abundant in the mountains because of orographic lifting and excavation in the alluvial fan is relatively easy. The muqannīs follow the track of the main water courses coming from the mountains or foothills to identify evidence of subsurface water such as deep-rooted vegetation or seasonal seeps. A trial well is then dug to determine the location of the water table and determine whether a sufficient flow is available to justify construction. If these prerequisites are met, then the route is laid out aboveground.
Equipment must be assembled. The equipment is straightforward: containers (usually leather bags), ropes, reels to raise the container to the surface at the shaft head, hatchets and shovels for excavation, lights, spirit levels or plumb bobs and string. Depending upon the soil type, qanat liners (usually fired clay hoops) may also be required.
Although the construction methods are simple, the construction of a qanat requires a detailed understanding of subterranean geology and a degree of engineering sophistication. The gradient of the qanat must be carefully controlled—too shallow a gradient yields no flow—too steep a gradient will result in excessive erosion, collapsing the qanat. And misreading the soil conditions leads to collapses which at best require extensive rework and, at worst, can be fatal for the crew.
Construction of a qanat is usually performed by a crew of 3-4 muqannīs. For a shallow qanat, one worker typically digs the horizontal shaft, one raises the excavated earth from the shaft and one distributes the excavated earth at the top.
The crew typically begins from the destination to which the water will be delivered into the soil and works toward the source (the test well). Vertical shafts are excavated along the route, separated at a distance of 20-35 m. The separation of the shafts is a balance between the amount of work required to excavate them and the amount of effort required to excavate the space between them, as well as the ultimate maintenance effort. In general, the shallower the qanat, the closer the vertical shafts. If the qanat is long, excavation may begin from both ends at once. Tributary channels are sometimes also constructed to supplement the water flow.
Most qanats in Iran run less than 5 km, while some have been measured at ~70 km in length near Kerman. The vertical shafts usually range from 20 to 200 meters in depth, although qanats in the province of Khorasan have been recorded with vertical shafts of up to 275 m. The vertical shafts support construction and maintenance of the underground channel as well as air interchange. Deep shafts require intermediate platforms to simplify the process of removing spoils.
The construction speed depends on the depth and nature of the ground. If the earth is easy/soft to work; at 20 meters depth, a crew of 4 people can excavate a horizontal length of 40 meters per day. When the vertical shaft reaches 40 meters, they can only excavate 20 meters horizontally per day and at 60 meters in depth this drops below 5 horizontal meters per day. In Algeria, a common speed is just 2m per day at 15m depth. Deep, long qanats (which many are) require years and even decades to construct.
The excavated material is usually transported by means of leather bags up the vertical shafts. It is mounded around the vertical shaft exit, providing a barrier that prevents windblown or rain driven debris from entering the shafts. These mounds may be covered to provide further protection to the qanat. From the air, these shafts look like a string of bomb craters.
The qanat's water-carrying channel must have a sufficient downward slope that water flows easily. However the downward gradient must not be so great as to create conditions under which the water transitions between supercritical and subcritical flow; if this occurs, the waves which are established can result in severe erosion that can damage or destroy the qanat. In shorter qanats the downward gradient varies between 1:1000 and 1:1500, while in longer qanats it may be almost horizontal. Such precision is routinely obtained with a spirit level and string.
In cases where the gradient is steeper, underground waterfalls may be constructed with appropriate design features (usually linings) to absorb the energy with minimal erosion. In some cases the water power has been harnessed to drive underground mills. If it is not possible to bring the outlet of the qanat out near the settlement, it is necessary to run a jub or canal overground. This is avoided when possible to limit pollution, warming and water loss due to evaporation.
The vertical shafts may be covered to minimize in-blown sand. The channels of qanats must be periodically inspected for erosion or cave-ins, cleaned of sand and mud and otherwise repaired. Air flow must be assured before entry for human safety.
Some damaged Qanats have been restored. In order to be sustainable, restoration needs to take into account many other factors than technical factors, beginning with the process to select the Qanat to be restored. In the case of Syria, three sites were chosen based on a national inventory conduced in 2001. One of them, the Drasiah qanat of Dmeir, was completed in 2002. Selection criteria included the availability of a steady groundwater flow, social cohesion and willingness to contribute of the community using the qanat, and the existence of a functioning water rights system.
The primary applications of qanats are for irrigation, providing cattle with water and drinking water supply. Other applications include cooling and ice storage.
Qanats used in conjunction with a wind tower can provide cooling as well as a water supply. A wind tower is a chimney-like structure positioned above the house; the one of its four openings opposite the wind direction is opened to move air out of the house. Incoming air is pulled from a qanat below the house. The air flow across the vertical shaft opening creates a lower pressure (see Bernoulli effect) and draws cool air up from the qanat tunnel, mixing with it. The air from the qanat was drawn into the tunnel at some distance away and is cooled both by contact with the cool tunnel walls/water and by the giving up latent heat of evaporation as water evaporates into the air stream. In dry desert climates this can result in a greater than 15°C reduction in the air temperature coming from the qanat; the mixed air still feels dry, so the basement is cool and only comfortably moist (not damp). Wind tower and qanat cooling have been used in desert climates for over 1000 years.
The ice was brought in during the winters from nearby mountains in large quantities, and stored in specially designed, naturally cooled refrigerators called yakhchal (meaning ice pits). A large underground space with thick insulated walls was connected to a qanat, and a system of windcatchers was used to draw cool subterranean air up from the qanat to maintain temperatures inside the space at low levels, even during hot summer days. As a result, the ice melted slowly and ice was available year-round.
The Qanats, called Kariz (rhymes with "breeze")  in Pashto and Dari, have also been in use for hundreds of years. Kariz structures are especially found in the Southern Afghanistan provinces of Kandahar, Uruzgan, Nimroz and Hilmand. The incessant war for the last 30 years has destroyed a number of these ancient structures. In the troubled times maintenance was not always possible. To add to the troubles, at present (2008) the cost of labour has become very high and maintaining the Kariz structures is no longer possible. Lack of skilled artisans who have the traditional knowledge also poses difficulties. A number of the large farmers are abandoning their Kariz which has been in their families sometimes for centuries, and moving to tube and dug wells backed by diesel pumps.
However the government of Afghanistan is aware of the importance of these structures and all efforts are being made to repair, reconstruct and maintain (through the community) the kariz. The Ministry of Rural Rehabilitation and Development along with National and International NGOs is making the effort.
Still, there are functional qanat systems in 2009. American forces are reported to have unintentionally destroyed some of the channels during expansion of a military base, creating tensions between them and the local community. Some of these tunnels have been used to store supplies, and to move men and equipment underground.
An oasis at Turpan in the deserts of northwestern China uses water provided by qanat (locally karez). Turfan has long been the center of a fertile oasis and an important trade center along the Silk Road's northern route, at which time it was adjacent to the kingdoms of Korla and Karashahr to the southwest. The historical record of the karez system extends back to the Han Dynasty. The Turfan Water Museum (see photos on this page) is a Protected Area of the People's Republic of China because of the importance of the local karez system to the history of the area. The number of karez systems in the area is slightly below 1,000 and the total length of the canals is about 5,000 kilometers in length.
In Karnataka, India, a Qanat-type structure called Suranga is used to tap underground water. But these are rarely in use these days.
In the middle of the twentieth century, it is estimated that approximately 50,000 qanats were in use in Iran, each commissioned and maintained by local users. Of these only 25,000 remain in use as of 1980.
The oldest and largest known qanat is in the Iranian city of Gonabad which after 2700 years still provides drinking and agricultural water to nearly 40,000 people. Its main well depth is more than 360 meters and its length is 45 kilometers. Yazd, Khorasan and Kerman are the known zones for their dependence with an extensive system of qanats.
In traditional Persian architecture, a Kariz (کاریز) is a small Qanat, usually within a network inside an urban setting. Kariz is what distributes the Qanat into its final destinations.
A survey of Qanat systems in the Kurdistan region of Iraq conducted by the Department of Geography at Oklahoma State University (USA) on behalf of UNESCO in 2009 found that out of 683 karez systems, some 380 were still active in 2004, but only 116 in 2009. Reasons for the decline of qanats include "abandonment and neglect" previous to 2004, "excessive pumping from wells" and, since 2005, drought. Water shortages are said to have forced over 100,000 people who depended on karez systems for their livelihoods to leave their homes since 2005. The study says that a single karez has the potential to provide enough household water for nearly 9,000 individuals and irrigate over 200 hectares of farmland. UNESCO and the government of Iraq plan to rehabilitate Karez through a Karez Initiative for Community Revitalization to be launched in 2010. Most of the karez are in Sulaymaniyah Governorate (84%). A large number are also found in Erbil Governorate (13%), especially on the broad plain around and in Erbil city.
Among the qanats built in the Roman Empire was possibly the longest continuous qanat ever built, the 94km long Gadara Aqueduct in northern Jordan. Partly following the course of an older Hellenistic aqueduct, excavation work arguably started after a visit of emperor Hadrian in 129-130 AD. The Gadara Aqueduct was never quite finished, and was put in service only in sections.
The Chagai district is in the north west corner of Balochistan, Pakistan, bordering with Afghanistan and Iran. Karez's are found more broadly in this region. They are spread from Chaghai district all the way up to Zhob district. A number of them are present in Qilla Abdullah and Pishin districts. Karez's are also extensively found in the neighbouring areas of Afghanistan like Kandahar. The remains of qanats (called karezes) found in different parts of the district are attributed to the Arabs.
Qanats were found over much of Syria. The widespread installation of groundwater pumps has lowered the water table and qanat system. Qanats have gone dry and been abandoned across the country.
According to UNESCO some 3,000 qanat systems, called Aflaj (plural) of Falaj (singular), are still in use in Oman today. Nizwa, the former capital city of Oman, was built around a falaj which is in use to this day. In July 2006, five representative examples of this irrigation system were inscribed as a World Heritage Site.
There are 4 main oases in the Egyptian desert. The Kharga Oasis is one of them which has been extensively studied. As early as the second half of the 5th century BC there is evidence that water was being used via qanats. The qanat is excavated through water-bearing sandstone rock which seeps into the channel to collect in a basin behind a small dam at the end. The width is approximately 60 cm, but the height ranges from 5 to 9 meters; it is likely that the qanat was deepened to enhance seepage when the water table dropped (as is also seen in Iran). From there the water was used to irrigate fields. 
There is another instructive structure located at the Kharga Oasis. A well which apparently dried up was improved by driving a side shaft through the easily penetrated sandstone (presumably in the direction of greatest water seepage) into the hill of Ayn-Manâwîr to allow collection of additional water. After this side shaft had been extended, another vertical shaft was driven to intersect the side shaft. Side chambers were built and holes bored into the rock—presumably at points where water seeped from the rocks—are evident.
David Mattingley reports foggara extending for hundreds of miles in the Garamantes area near Jarma in Libya: "The channels were generally very narrow - less than 2 feet wide and 5 high - but some were several miles long, and in total some 600 foggara extended for hundreds of miles underground. The channels were dug out and maintained using a series of regularly-spaced vertical shafts, one every 30 feet or so, 100,000 in total, averaging 30 feet in depth, but sometimes reaching 130." ("The 153 Club Newsletter", July 2007 No. 112, pp. 14-19; reprinted from Current world Archaeology.
The foggara water management system in Tunisia, used to create oases, is similar to that of the Iranian qanat. The foggara is dug into the foothills of a fairly steep mountain range such as the eastern ranges of the Atlas mountains. Rainfall in the mountains enters the aquifer and moves toward the Saharan region to the south. The foggara, 1 to 3 km in length, penetrates the aquifer and collects water. Families maintain the foggara and own the land it irrigates over a ten meter wide, with width only by the size of plot that the available water will irrigate.
Qanats (designated foggaras in Algeria) are the source of water for irrigation at large oases like that at Gourara. The foggaras are also found at Touat (an area of Adrar 200 km from Gourara). The length of the foggaras in this region is estimated to be thousands of kilometers.
Although sources suggest that the foggaras may have been in use as early as 200 AD, they were clearly in use by the 11th century after the Arabs took possession of the oases in the 10th century and the residents embraced Islam.
The water is metered to the various users through the use of distribution weirs which meter flow to the various canals, each for a separate user.
The humidity of the oases is also used to supplement the water supply to the foggara. The temperature gradient in the vertical shafts causes air to rise by natural convection, causing a draft to enter the foggara. The moist air of the agricultural area is drawn into the foggara in the opposite direction to the water run-off. In the foggara it condenses on the tunnel walls and the air passed out of the vertical shafts. This condensed moisture is available for reuse. 
In southern Morocco the qanat (locally khettara) is also used. On the margins of the Sahara Desert, the isolated oases of the Draa River valley and Tafilalt have relied on qanat water for irrigation since the late-14th century. In Marrakech and the Haouz plain the qanats have been abandoned since the early 1970s as they've dried; in the Tafilaft area half of the 400 khettaras are still in use. The Hassan Adahkil Dam's impact on local water tables is said to be one of the many reasons given for the loss of half of the khettara.
Qanats have preserved in Armenia in the community of Shvanidzor, in the southern province of Syunik, bordering with Iran. Qanats are named kahrezes in Armenian. There are 5 kahrezes in Shvanidzor. Four of them were constructed in XII-XIVc, even before the village was founded. The fifth kahrez was constructed in 2005. Potable water runs through I, II and V kahrezs. Kahrez III and IV are in quite poor condition. In summer, especially in July and August, the amount of water reaches its minimum, creating critical situation in the water supply system. Still, kahrezes are the main source of potable and irrigation water for the community.
The 5653 m long Claudius Tunnel, meant for draining the largest Italian inland water, Fucine Lake, was constructed in the qanat technique. It featured shafts up to 122 m deep. The entire ancient town of Palermo in Sicily has been built over a huge qanat system built during the Arab period (827–1072). [This seems unlikely, given that the city was founded by the Phoenicians over 2700 years ago.] Many of the qanat are now mapped and some can be visited. An interesting building is the famous Scirocco room, which has an air refreshing system using the flux of waters of a qanat and a "wind tower", a structure able to catch the wind and direct it into the room.
The Raschpëtzer near Helmsange in southern Luxembourg is a particularly well preserved example of a Roman qanat. It is probably the most extensive system of its kind north of the Alps. To date some 330 m of the total tunnel length of 600 m have been explored. Thirteen of the 20 to 25 shafts have been investigated. The qanat appears to have provided water for a large Roman villa on the slopes of the Alzette valley. It was built during the Gallo-Roman period, probably around the year 150 and functioned for about 120 years thereafter.
There are still many examples of galeria or qanat systems in Spain, most likely brought to the area by the Moors during their occupation of the Iberian peninsula. Turrillas in Andalusia on the north facing slopes of the Sierra de Alhamilla has evidence of a qanat system. Granada is another site with an extensive qanat system.