Qanat
A qanāt or kārīz is a water supply system that was developed in ancient Iran for the purpose of transporting usable water to the surface from an aquifer or a well through an underground aqueduct. Originating approximately 3,000 years ago, its function is essentially the same across the Middle East and North Africa, but it is known by a variety of regional names beyond today's Iran, including: kārēz in Afghanistan and Pakistan; foggāra in Algeria; ' in the Atlas Mountains; the daoudi-type falaj in Oman and the United Arab Emirates; and ʿuyūn' in Saudi Arabia. In addition to those in Iran, the largest extant and functional qanats are located in Afghanistan, Xinjiang in China, Oman, and Pakistan.
Proving crucial to water supply in areas with hot and dry climates, a qanat enables water to be transported over long distances by largely eliminating the risk of much of it evaporating on the journey. The system also has the advantage of being fairly resistant to natural disasters, such as floods and earthquakes, as well as to man-made disasters, such as wartime destruction and water supply terrorism. Furthermore, it is almost insensitive to varying levels of precipitation, delivering a flow with only gradual variations from wet to dry years.
The typical design of a qanat is a gently sloping tunnel accessed by a series of well-like vertical shafts visible at ground level. This taps into groundwater and delivers it to the surface at a lower level some distance away, via gravity, therefore eliminating the need for pumping. The vertical shafts along the underground channel are for maintenance purposes, and water is typically used only once it emerges from the daylight point.
To date, the qanat system still ensures a reliable supply of water for consumption and irrigation across human settlements in hot, arid, and semi-arid climates, but its value to a population is directly related to the quality, volume, and regularity of the groundwater in the inhabited region. Since their adoption outside of the Iranian mainland in antiquity, qanats have come to be heavily relied upon by much of the Middle Eastern and North African populations for sustenance. Likewise, many of the continuously inhabited settlements in these regions are established in areas where conditions have historically been favourable for creating and sustaining a qanat system.
Image:Qanat Kashan.jpg|thumb|Qanat in Iran's Kashan surfacing within the Fin Garden in 2005; it is thought to have served Tepe Sialk for thousands of years.
Image:Qanat Niavaran.jpg|thumb|Kariz surfacing in Niavaran, a district in the Iranian capital city of Tehran, currently used for watering the grounds of the National Library and Archives of Iran, 2005.
Names
Common variants of qanat in English include kanat, khanat, kunut, kona, konait, ghanat, ghundat and quanat.is an Arabic word that means "channel". In Persian, two words are used: ' or ', which is derived from the earlier word ; and . Other names for qanat include ; ; ; ; galerías, or ; daoudi-type ; . Alternative terms for qanats in Asia and North Africa are kakuriz, chin-avulz, and mayun.
Origins
According to most sources, qanat technology was developed by the ancient Iranians sometime in the early 1st millennium BCE and slowly spread westward and eastward from there. Other sources suggest a Southeast Arabian origin. Analogous systems appear to have been developed independently in China and in South America.A cotton species, Gossypium arboreum, is indigenous to South Asia and has been cultivated on the Indian subcontinent for a long time. Cotton appears in the Inquiry into Plants by Theophrastus and is mentioned in the Laws of Manu. As transregional trade networks expanded and intensified, cotton spread from its homeland to the Middle East. One theory is that the qanat was developed to irrigate cotton fields, first in what is now Iran, where it doubled the amount of available water for irrigation and urban use. Because of this, Persia enjoyed larger surpluses of agricultural products, thus increasing urbanization and social stratification. The qanat technology subsequently spread from Persia westward and eastward.
In the arid coastal desert of Peru, a technology of water supply similar to that of the qanats, called puquios, was developed. Most archaeologists believe that the puquios are indigenous and date to about 500 CE, but a few believe they are of Spanish origin, brought to the Americas in the 16th century. Puquios were still in use in the Nazca region in the 21st century.
Features
Qanats are constructed as a series of well-like vertical shafts, connected by a gently sloping tunnel which carries a water canal. Qanats efficiently deliver large amounts of subterranean water to the surface without need for pumping. The water drains by gravity, typically from an upland aquifer, with the destination lower than the source. Qanats allow water to be transported over long distances in hot dry climates without much water loss to evaporation.It is very common for a qanat to start below the foothills of mountains, where the water table is closest to the surface. From this source, the qanat tunnel slopes gently downward, slowly converging with the steeper slope of the land surface above, and the water finally flows out above ground where the two levels meet. To connect a populated or agricultural area with an aquifer, qanats must often extend for long distances.
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 and evaporation. 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 Persian qanat-fed reservoir for drinking water.
The qanat system has the advantage of being resistant to natural disasters such as 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 are powered only by gravity and thus have low operation and maintenance costs. Qanats transfer fresh water from the mountain plateau to the lower-lying plains with saltier soil. This helps to control soil salinity and prevent desertification.
The qanat should not be confused with the spring-flow tunnel typical to the mountainous area around Jerusalem. Although both are excavated tunnels designed to extract water by gravity flow, there are crucial differences. Firstly, the origin of the qanat was a well that was turned into an artificial spring. In contrast, the origin of the spring-flow tunnel was the development of a natural spring to renew or increase flow following a recession of the water table. Secondly, the shafts essential for the construction of qanats are not essential to spring-flow tunnels.
Impact on 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 below the surface outlet. Water from the qanats define 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 ground, the water is drawn to the surface via wells or animal driven Persian wells. Private subterranean reservoirs could supply houses and buildings for domestic use and garden irrigation as well. Air flow from the qanat is used to cool an underground summer room found in many older houses and buildings.
Downstream of the outlet, the water runs through surface canals called jubs 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.
Construction
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.Preparations
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 depth of the water table and determine whether a sufficient flow is available to justify construction. If these prerequisites are met, the route is laid out aboveground.Equipment must be assembled. The equipment is straightforward: containers, ropes, reels to raise the container to the surface at the shaft head, hatchets and shovels for excavation, lights, and spirit levels or plumb bobs and string. Depending upon the soil type, qanat liners 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 and 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 are fatal for the crew.