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The technology of kārēz exploits a difference in grade between a tunnel and the groundwater table. The grade of the tunnel is less steep than the grade of the water table, so that the tunnel ends at an elevation distinctly higher than that of the water table.
In a few exceptional cases, the channels are quite long. The known maximum lengths are some 50 km in the region of Yazdand about 35 kilometers at Gonābād in Khorasan
The channel is the essential functional element of the kārēz, but vertical shafts, which connect the tunnel to the surface, are needed for digging the tunnels and play no role in the actual operation of the kārēz. The vertical shafts are laid out at more or less regular intervals, which are relatively close in order to facilitate the removal of debris.
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
Although the Arabic name ‘Qanat Firaun’ means ‘Canal of the Pharaohs’, the massive canal was not Egyptian but Imperial Roman, and stands as a testament to their incredible engineering abilities. The 170-kilometre pipeline is not only the world’s longest underground aqueduct of the antiquity, it is also the most complex, and represents a colossal work of hydroengineering.
they don't seem the traditional linear arrangement or particularly rational
Sometimes there seems the creation of pattern in laying out the course of the Qanat and the above ground marks as in the recent example from Kazakhstan.