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11-01-2015, 01:02 PM
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A qanāt (Arabic: قناة‎, Persian: قنات‎‎/کاریز) is a gently sloping underground channel with a series of vertical access shafts, used to transport water from an aquifer under a hill. Qanāts create a reliable supply of water for human settlements and irrigation in hot, arid, and semi-arid climates.

The qanat technology is known to have been developed by the Persian people sometime in the early 1st millennium BC, and spread from there slowly westward and eastward.[1][2][3][4][5][6]

The value of the 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 thereby to the group that invested in building and maintaining it, was substantial.

Etymology

Qanat is the Arabic word for "channel". Qanats are also called kārīz (or kārēz from Persian: كاريز‎‎) (Iran, Afghanistan, Pakistan and Central Asia, derived from Persian: كاهریز‎‎), kahan (from Persian: کهن‎‎), kahriz/kəhriz (Azerbaijan); khettara (Morocco); galería (Spain); falaj (from Arabic: فلج‎) (United Arab Emirates and Oman); Kahn (Baloch) or foggara/fughara (North Africa).[8] Alternative terms for qanats in Asia and North Africa are kakuriz, chin-avulz, and mayun. Common variants of qanat in English include kanat, khanat, kunut, kona, konait, ghanat, ghundat.

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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 by 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 loss of much of the water to 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 extend for long distances.[7]

Qanats are sometimes split into an underground distribution network of smaller canals called kariz. Like qanats, these smaller canals are below ground to avoid contamination. In some cases water from a qanat is stored in a reservoir, typically with night flow stored 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 resistant to natural disasters such as earthquakes and floods, and to deliberate destruction in war. Furthermore, it is almost insensitive to the levels of precipitation, delivering a flow with only gradual variations from wet to dry years. From a sustainability perspective, Qanats use the force of gravity to surface up groundwater with no energy requirementand, thus, have low life cycle operation & maintenance costs once built. Qanats transfer freshwater from the mountain plateau to the lower lying plains that have a saltier soil. This helps to control the salinity of soil and prevent desertification.[9]

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 below the surface outlet. Water from the qanats defines both the social regions in the city and the layout of the city.[7]

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.[7]

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.[7]

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.[7]

Construction[edit]
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.[7]

Preparations