Qanat

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I qanat (dall'Arabo: قنات) o kareez (dal Persiano كاريز) sono un sistema di trasporto idrico usato per fornire una fonte affidabile d'approvvigionamento d'acqua per insediamenti umani e per l'irrigazione in ambienti caldi e aridi. La tecnologia su cui sono basati fu sviluppata inizialmente nell'antica Persia e quindi diffusa ad altre culture, in particolare lungo la Via della seta: verso est, fino alla Cina; verso ovest, fino ad altri territori arabi come il Marocco e la penisola iberica.

Indice 1 Qanats and settlement patterns 1.1 Features common to regions which use qanat technology 1.2 Settlement patterns 2 Costruzione 2.1 Preparativi 2.2 Scavo 2.3 Manutenzione 2.4 Valore 3 Other applications for qanats 3.1 Distribution systems 3.2 Water storage 3.3 Cooling 3.4 Ice storage 4 Common terms 5 Qanats in practical application 5.1 Asia 5.1.1 China 5.1.2 Pakistan 5.1.3 Iran 5.1.4 Syria 5.2 Arabian Peninsula 5.2.1 United Arab Emirates 5.2.2 Oman 5.3 North Africa 5.3.1 Egypt 5.3.2 Tunisia 5.3.3 Algeria 5.3.4 Morocco 5.4 Europe 5.4.1 Spain 5.4.2 Italy 5.5 The Americas 6 See also 7 References and notes 8 Collegamenti esterni

[modifica] Qanats and settlement patterns

Qanats are constructed as a series of well-like vertical shafts, connected by gently sloping tunnels. This technique:

• Taps into a 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.
• Allows water to be transported long distances in hot dry climates without losing a large proportion of the source 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 underground aquifer qanats must often be of extreme length.^[1]

[modifica] Features common to regions which use qanat technology

The qanat technology was used most extensively in areas with the following characteristics:

• An absence of larger rivers with year-round flows sufficient to support irrigation.
• Proximity of potentially fertile areas to precipitation-rich mountains or mountain ranges.
• Arid climate with its high surface evaporation rates so that surface reservoirs and canals would result in high losses
• An aquifer at the potentially fertile area which is too deep for convenient use of simple wells.

The investment and organization required by the construction and the maintenance of a qanat is typically provided by local merchants or landowners in small groups. In the middle of the twentieth century, it is estimated that approximately 50,000 qanats were in use in Iran^[1], each commissioned and maintained by local users^[2]. 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.

[modifica] Settlement patterns

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.^[1]

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 Ater-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.^[1]

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; what is sometimes viewed as chaotic to the western eye is a practical response to efficient water distribution over varying terrain.^[1]

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^[1].

[modifica] Costruzione

Tradizionalmente i qanat sono costruiti da un gruppo di lavoratori esperti, chiamati muqannī, con l'appoggio di forza lavoro semplice. Questa professione è ben pagata ed è tipicamente ereditata dai figli.

[modifica] Preparativi

Il primo passo, critico, nella costruzione di un qanat è l'identificazione di una fonte acquifera adatta. La ricerca comincia nel punto dove un ventaglio alluvionale incontra le pendici di montagne o colline; l'acqua è più abbondante ad altitudini elevate per via del sollevamento orografico e le operazioni di scavo nel ventaglio sono relativamente facili. I muqannī seguono la traccia delle principali vie d'acqua provenienti dalle montagne per identificare prove di acqua sotterranea, ad esempio vegetazione con radici particolarmente profonde e filtrazioni stagionali. Un pozzo è scavato per determinare l'esatta posizione della fonte sotterranea e per verificare che il flusso d'acqua sia tale da giustificare la costruzione del qanat. Se i prerequisiti sono raggiunti, la rotta viene tracciata sul terreno.

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.^[3][1]

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.^[3]

[modifica] Scavo

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.^[3]

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.^[3][1]

Most qanats in Iran run less than 5 km. The overall length of the qanat often runs up to 16 km, while some have been measured at ~70 km in length near Kirman. The vertical shafts usually range from 20 to 200 meters in depth, although in Iran 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.^[3][1]

The qanat's water-carrying channel is 50-100 cm wide and 90-150 cm high. The 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 result in severe erosion and 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.^[3][1]

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 overgound. This is avoided when possible to limit pollution, warming and water loss due to evaporation.^[3][1]

The construction speed depends on the depth. 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. Deep, long qanats (which many are) require years and even decades to construct.^[3][1]

The excavated material is usually transported by means of leather bags from 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. From the air, these pits look like a string of bomb craters.^[3]

[modifica] Manutenzione

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.

[modifica] Valore

The value of a qanat is directly related to the quality, volume and regularity of the water flow. Much of the population of Iran 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. ^[1]

[modifica] Other applications for qanats

[modifica] Distribution systems

Qanats were frequently split into an underground distribution network of smaller canals called kariz when reaching a major city. Like Qanats, these smaller canals were below ground to avoid contamination.

[modifica] Water storage

An Ab Anbar is a traditional qanat fed reservoir for drinking water in Persian antiquity.

[modifica] Cooling

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 to catch the prevailing wind. The tower catches the wind, driving a hot, dry breeze into the house; the flow of the incoming air is then directed across the vertical shaft from the qanat. 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.^[4]

[modifica] Ice storage

In 400 BC Iran engineers had already mastered the technique of storing ice in the middle of summer in the desert. The ice was brought in during the winters from nearby mountains in bulk amounts, 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.

[modifica] Common terms

Qanat is from the Persian word qanāt, pronounced as ‘kanat’ in Arabic and ‘karez’ in Pashto. A qanat is referred to by different names in different regions: qanat (Iran); karez (Afghanistan and Pakistan); kanerjing (China); qanat romani (Jordan and Syria); khettara (Morocco); galeria (Spain); falaj (United Arab Emirates); Kahn (Baloch). foggara/fughara is the French translation of the Arabic qanat, used in North Africa.^[5] 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 karez.

[modifica] Qanats in practical application

[modifica] Asia

[modifica] China

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 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.^[6]

[modifica] Pakistan

The Chagai district is in the north west corner of Balochistan, Pakistan, bordering with Afghanistan and Iran. The remains of qanats (called karezes) found in different parts of the district are attributed to the Arabs.

[modifica] Iran

About four-fifths of the water used in the plateau regions of Iran is brought to use in this way. However, because agriculture is less and less practiced in Iran, the qanats that are being made now are not as effective as those made in the past because knowledge of how to make them is being lost. Also, the construction and maintenance of a qanat is unpleasant and dangerous, and modern technology allows water to be pumped from a drilled well. Hence although qanats still exist, they are falling out of use.

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 and Kerman are the also 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. (see also Traditional water sources of Persian antiquity and Ab Anbar)

[modifica] Syria

Qanats were found over much of Syria. The widespread installation of groundwater pumps has lowered the water table and antiquated the old qanat system. Qanats have gone dry and been abandoned across the country.^[7]

[modifica] Arabian Peninsula

[modifica] United Arab Emirates

The oasis of Al Ain in the United Arab Emirates continues traditional falaj (qanat) irrigations for the palm-groves and gardens. Evidence suggests the technology has been in use for 3000 years here.^[8]

[modifica] Oman

A ribbon of oases, watered by wells and underground channels (falaj), extends the length of the Oman plain, extending about ten kilometers inland. Nizwa was the capital city of Oman proper was built around a falaj (qanat) which is in use to this day.

In July 2006, the five representative examples of this irrigation system were inscribed as a World Heritage Site.

[modifica] North Africa

[modifica] Egypt

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. ^[9][3]

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 rosk—are evident.^[9]

[modifica] Tunisia

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.^[10]

[modifica] Algeria

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 imposed the Islamic religion upon the residents.

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. ^[11]

[modifica] Morocco

In southern Morocco the qanat (locally khettara) is also used. On the margins of the Sahara Desert, lies the isolated oasis of Tafilaft has relied on qanat water for irrigation since the late-14th century (locally khettara). These qanats have been abandoned since the early 1970s as they've dried up; their loss is attributed to the Hassan Adahkil Dam's impacts on local water tables.^[7]

The haritin are the hereditary class of qanat diggers in Morocco who build and repair these systems. Their work is hazardous.^[5]

[modifica] Europe

[modifica] Spain

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.^[12]

[modifica] Italy

[modifica] The Americas

Qanats in the Americas can be found in the Atacama regions of Peru, and Chile at Nazca and Pica.^[7] The Spanish introduced qanats into Mexico in 1520 AD.^[13]

[modifica] See also

• Yakhchal, ancient natural refrigerators in Persia.
• Iranian architecture
• Kariz
• Traditional water sources of Persian antiquity
• Menara gardens

[modifica] References and notes

1. ↑ ^{1,00 1,01 1,02 1,03 1,04 1,05 1,06 1,07 1,08 1,09 1,10 1,11 1,12} Kheirabadi, Masoud. Iranian Cities: Formation and Development. University of Texas Press, 1991. ISBN 0-292-78517-8.
2. ↑ Of these only 25,000 remain in use as of 1980.
3. ↑ ^{3,0 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8 3,9}
4. ↑ Bahadori, M. N. titled "Passive Cooling Systems in Iranian Architecture" Scientific American, February 1978, pages 144-154.
5. ↑ ^5,0 5,1 Article titled Etymological Conduit to the Land of Qanat by Dr. V. Sankaran Nair, 2004
6. ↑ Oasis at Turpan in northwestern China uses water provided by karez.
7. ↑ ^{7,0 7,1 7,2} http://www.waterhistory.org/histories/qanats/ History from Waterhistory.org]
8. ↑ Arabian wildlife article
9. ↑ ^9,0 9,1 Michel Wuttmann, "The Qanats of 'Ayn-Manâwîr, Kharga Oasis, Egypt", in Jasr 2001, p. 1 (pdf).
10. ↑ "Water: symbolism and culture"
11. ↑ An excellent UNESCO article with numerous clear photographs showing the Foggara in Algeria
12. ↑ http://www.ucm.es/BUCM/revistas/ghi/02143038/articulos/ELEM8484120249A.PDF Water supplies in Granada Template:Sp icon]
13. ↑ Libyan web site on qanats

[modifica] Collegamenti esterni

• The Aqueducts of Iran (CAIS)
• WaterHistory.org Article on Karez in Turpan, Xinjiang, China
• World Wildlife Fund Editorial on Karez in Afghanistan
• The Internet Archive
• Useful information on Qanat provided by Farzad Kohandel, in arabic) and in english
• Qanat
• Information on Qanats (includes photo of access shafts from above)
• Site includes discussion of use of qanats in Libya Template:Fr icon
• International Center on Qanats and Historic Hydraulic Structures
• - The origin and spread of qanats in the Old World - by PW English, in Proceedings of the American Philosophical Society Volume 112, Number 3 June 21, 1968.
• The art and science of water, in Saudi Aramco May/June 2006