Karez an under ground city
A karez consists of one or more water-catchment tunnels 1.0–1.4 m high and 0.5–0.6 m wide, with reinforced walls laid through a water-bearing horizon, as well as vertical ventilation wells and a water tunnel connected to a discharge channel.
Karez tunnels can be as much as several kilometers long. The average yield of a water-catchment tunnel is 0.3–0.6 liters per sec per meter of the tunnel.
Karez irrigation systems are the crystallization of ancient people's diligence and intelligence. Without the Karez, there would be no Turpan culture.
Constructing the kareez as an underground water channel rather than an above ground canal, helps to prevent evaporation. When the kareez enter an agricultural area, they often become above ground channels or streams.
The underground sections are evidenced by excavation craters at the top of shafts that go down to the tunnel. It is not unusual to find several underground kareez running parallel to one another, or to find a freshly constructed kareez constructed close to an older disused one.
Establishing the size and gradient of the kareez requires experience and knowledge of surveying, geology and hydrology. The kareez cannot be too steep because the erosion by swift water can cause irreparable damage. Kareez tunnels can have a 0.5 to 0.1% slope gradient. A tunnel needs to be of a size sufficient enough to accommodate the water flow as well as a person working and maintaining the tunnel.
The karez system consists of four parts: a hole as deep as 50 to 60 meters, an underground canal, an aboveground canal and a small reservoir. It has many advantages, such as little evaporation from season to season and little percolation and can provide a stable water supply that does not consume energy or cause pollution.
Wandering underground over 5,000 kilometers, the system has also been called "the underground Great Wall."
Karez systems also functions as the natural air conditioner for the locals because they can sit in one while chatting or doing some chores. When exhausted during the scorching weather, sitting in a Karez, picking some grapes as they hang over the vertical wells and savoring their taste, you will be thoroughly refreshed
.To explain how the karez or qanat technology works: It is a system that taps into the ground water sources (or natural springs) and carries it through an underground tunnel to a settlement, and accumulates in the surface canals and pools in the village to be used further for various purposes.
Karez are constructed as a series of well-like vertical shafts, connected by gently sloping tunnels. 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.