Kariz (also known as qanat) is an ancient water supply system that can be described as one of the biggest ontributions made by Iranians to hydraulics. This system must have been started at least 5,000 years ago in Iran.
The system is usually found in central Iran toward the east and southeast of Iran. Some of the existing examples in Iran include the Old Zavareh Kariz dating back to 5,000 years and the 350-meter deep well of Gonabad Qanat dating back to 2,500 years.
Since antiquity, the supply of water for drinking, irrigating and other household uses has been essential for survival.
To meet these needs, kariz, water reservoirs, icehouses, water mills, water dams, bridges and diversion dams have been built.
Kariz is a mining installation using galleries or canals to extract water from the depths of the earth to the ground. In fact, water is brought by gravity flow from the upper end, where it seeps into the gallery to a ground surface outlet and irrigation canal at its lower end. This is done by means of a gently sloping tunnel.
Kariz Mining Technique
The first step in making a kariz is to sink a Gamaneh (trial shaft) to estimate the presence of water and determine its depth.
When the trial shaft is sunk and water is reached, it must be determined whether the well has struck a constant flow of water in an impermeable stratum. If so, the alignment and slope of the kariz from the shaft have to be established. This shaft becomes the mother well.
The gradient of the gallery must not be too steep, because the water will then flow too fast and erode the walls and the tunnel will fall in. Work on kariz usually begins at the lower end where its water is to come to the surface.
Using spike and shovel, workers dig the tunnel toward the mother well. At times, it begins simultaneously at both ends.
Vertical shafts are sunk from the surface to the tunnel approximately every 20 meters, or are sunk first and then connected by a tunnel. Mud or stone linings at the upper parts strengthen these shafts.
The soil excavated is moved to the surface in a bucket by a windlass. If the shaft is too deep, a second windlass may be set halfway down in a niche.
Usually, there accumulates a ring of soil around the shaft on the surface. So, looking from the air, sets of wells look like a line of small craters.
The gradient of a kariz is established by the use of a spirit level suspended between two pieces of cord, each about nine meters long. In a short kariz, the gradient varies from 1:1000 to 1: 1500, but in a long one, it is nearly horizontal.
In some cases, when the kariz slope should be steeper, they usually break the route line at one point and allow water to flow lower than the original level.
Therefore, an underground waterfall was created. Iranians also built water mills to make use of water energy for other purposes like grinding cereals.
Discharge of water from kariz varies according to ground water characteristics, type of soil and season.
Those that tap a permanent aquifer usually have a constant flow throughout the year. If a kariz does not tap a stable groundwater source or is in porous soil, its flow may be reduced to a trickle in summer, or in a dry year.