This irrigation system is distributed in Turpan area in Xinjiang, northwest China. It was built by the people according to the local climate astd hydrological conditions in basin areas. This irrigation system has a history of more than 2,000 years.
The Turpan area has a rainfall of only 16 mm a year but its evaporation is 3,000 mm. It is impossible to use the water in the surface to irrigate farmland. Fortunately, Turpan is located in a basin and the mountains around are covered by snow year around so the area is rich in water resources underground. Therefore the local people brought into full play the tilted terrain of the basin and dug many karez and connected them together into a subterranean irrigation system.
Such irrigation system has a simple structure but the whole project is hard to build. First really vertical wells were dug at an interval of 10 meters or several dozen meters. These wells were connected through a canal underground. The sands and stones from the shafts or canal were piled around the mouth of the well, like a small crate in distance. The vertical wells have different depths according to the slope of the canal. The water will automatically flow to the outlets on the far away oases. Then the water is led to the ditches on the ground and finally to the farmland. In general the karez is three kilometers long and the longest 10 kilometers. In Turpan, you can see horses are drinking the water from such karez on the Gobi deserts. Historical records show this longest irrigation system once totaled more than 5,000 kilometers. Such irrigation system is a symbol of oasis civilization of Xinjiang. Its ecological and cultural values are irreplaceable. Karez irri- gation system is praised as the longest sub tetranean canal in the world.