A qanat or kariz, is a system for transporting water from an aquifer or water well to the surface, through an underground aqueduct. Constructed in Iran, Iraq and numerous other societies, this is an ancient system of water supply which allows water to be transported over long distances in hot dry climates without loss of much of the water to evaporation. The system has the advantage of being resistant to natural disasters such as earthquakes and floods, and to deliberate destruction in war. Furthermore, it is almost insensitive to the levels of precipitation, delivering a flow with only gradual variations from wet to dry years.
Karez are constructed as a series of well-like vertical shafts, connected by a gently sloping tunnel. This taps into underground water and delivers it to the surface by gravity, without need for pumping. The vertical shafts along the underground channel are purely for maintenance purposes, and water is typically used only once it emerges from the daylight point.
The qanats still create a reliable supply of water for human settlements and irrigation in hot, arid and semi-arid climates, but the value of this system is directly related to the quality, volume and regularity of the groundwater. Much of the population of Iran and other arid countries in Asia and North Africa historically depended upon the water from qanats, with many populated areas corresponding closely to the areas where qanats are possible.