The special climate of Yazd has made it necessary to adapt a particular architectural style and urban development schemes. Furthermore, most historic areas of the city contain various traditional structures such as the badgirs (windcatchers), ab-anbar (wind storages) and Qanats (underground tunnels). In Yazd, some parts of the city are located on several qanats and their branches. The city and its qanats have long been interwoven. The traces of some of these qanats, dating back to the fifth and sixth centuries, can also be found in ancient quarters of the city and neighboring towns and villages such as Zarch. It should be noted that these urban facilities are the main water resource for irrigation of agricultural lands of the city and neighboring areas. Many residential buildings, schools, bazaars and mosques have also been connected to the network of qanats by gutters, grooves, rivulets and ponds.
The infrastructure supplied water to different sectors of the region, like vital veins in a body. The qanat network takes source at the foothills of the Shirkuh mountain range, and runs beneath the city, accessed only at specific points, connecting the surface to the underground tunnels. Though almost derelict, most of this network still exists today. Some of the qanats still carry water, such as the thousand-year-old Zarch. Others like Firouzabad still have water infiltration upstream at their mother wells, but are dry in the city.
The Qanat of Zarch, well-known as the longest Qanat in the world, starts in a village named Fahraj located in the northeast of Yazd. It runs to a depth of 30-40 meters under the city and finally reaches Zarch, where the exit point appears. This Qanat is 80 km long, its mother well is 90 meters deep, and it consists of more than a thousand shaft wells. Despite the sever decline of the aquifer and its passage underneath Yazd, it still remains functional. According to existing documents, this Qanat dates back to pre-Islamic times. There is also proof in historical references that this Qanat had been active for over seven hundred years when the local s started using its water for drinking and sanitation.
Qanat is a gently sloping underground channel with a series of well-like vertical access shafts, used to transport water from aquifer under a hill. Qanats create a reliable supply of water for human settlements and irrigation in hot, arid and semi-arid climates 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.
The qanat technology is known to have been developed by the Persian people sometime is the early 1st millennium B.C, and seen in the forth millennium B.C, and spread from there slowly westward and eastward.
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.
Construction of a qanat is usually performed by a crew of 3-4 Mugannis (skilled laborers who dig qanats). Vertical shafts are excavated along the route, separated at a distance of 20-35 meters. Most qanats in Iran run less than 5 km, while some have been measured at nearly 70 km in length. The vertical shafts usually range from 20 to 200 meters in depth.
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 water supply. A wind tower is a chimney-like structure positioned above the house; of its four openings, the one opposite the wind direction is opened to move air out of the house. Incoming air is pulled from a qanat below the house. In dry desert climates this can result in a greater than 15 °C reduction in the air temperature coming from the qanat. Wind tower and qanat cooling have been used in desert climate for over 1000 years.
By 400 B.C, Persian engineers had mastered the technique of storing ice in the middle of summer in the dessert. The ice could be brought in during the winters from nearby mountains. But in a more usual and sophisticated method they built a wall in the east-west direction near the yakhchal (ice pit0. Large underground space with thick insulated walls was connected to a qanat and a system of windcatchers or wind towers was used to draw cool subterranean air fro mthe qanat to maintain temperatures inside the space at low levels, even during hot summer days. As a result the ice melted slowly and was available year-round.
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