QANATS OF IRAN

with commentary at http://video.google.com/videoplay?doc...
A qanat (from Arabic: قنات‎) or kareez (from Persian: كاريز‎) is a water management system used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates. The technology is known to have developed in ancient Persia, and then spread to other cultures, especially along the Silk Road as far east as China as well as by Arabic cultures as far west as Morocco and the Iberian Peninsula. 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. 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. http://en.wikipedia.org/wiki/Qanat«

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