Qanat (karez or foggara) irrigation systems are ancient (circa 800 BC) and consist of underground tunnels constructed into a cliff, scarp or base of a mountainous area, following an aquifer, or from rivers, to bring water out to the surface. The tunnels are straight and horizontal with a slope to allow the water to drain out into an oasis or irrigation system.

The volume of water produced by qanats depends on the type and extent of the aquifer, and its recharge rate. When tunnelling horizontally, air shafts 15-55 meters deep are constructed every 15-47 meters to remove the mined soil, clean the tunnels of silt, and aerate the tunnels.
There are significant advantages to a qanats water delivery system including: (1) putting the majority of the channel underground reduces water loss from seepage and evaporation; (2) since the system is fed entirely by gravity, the need for pumps is eliminated; and (3) it exploits groundwater as a renewable resource.

During the period 550-331 BC, when Persian rule extended from the Indus to the Nile, qanats technology spread throughout the empire. To the west, qanats were constructed from Mesopotamia to the shores of the Mediterranean, as well as southward into parts of Egypt. To the east of Persia, qanats were constructed in Afghanistan, the Silk Route oases settlements of central Asia, and Chinese Turkistan.

During Roman-Byzantine era (64 BC to 660 AD), many qanats were constructed in Syria and Jordan. From here, the technology appears to have diffused north and west into Europe. There is evidence of Roman qanats as far away as the Luxembourg area.

The expansion of Islam initiated another major diffusion of qanat technology. The early Arab invasions spread qanats westward across North Africa and into Cyprus, Sicily, Spain, and the Canary Islands. Evidence of New World qanats can be found in western Mexico, in Peru and Chile.

Qanats irrigated agriculture is threatened by silt sedimentation in canals, moving sand dunes, urban migration of youth, and decline of experts for managing such systems.

In Iran alone there are an estimated 50,000 qanats. Assembled end to end, they would reach two-thirds of the way to the moon.

Information from:
the International Center on Qanats and Historic Hydraulic Structures website
the Globally Important Agricultural Heritage Systems website section ‘Qanat Irrigation Systems and Homegardens (Iran)’
the qanats section of the WaterHistory.org website

Source: UNESCO Water Portal, May 2007