Qanat
Qanat is an underground gallery that conveys water from an aquifer or a water source to less elevated fields. In practice, a Qanat consists of a series of vertical shafts in sloping ground, interconnected at the bottom by a tunnel with a gradient more gentle than that of the ground. The first shaft (mother well) is sunk, usually into an alluvial fan, to a level below the groundwater table. Shafts are sunk at intervals of 20 to 200 meters in a line between the groundwater recharge zone and the irrigated land. From the air, a Qanat system looks like a line of anthills leading from the foothills across the desert to the greenery of an irrigated settlement.
Structure
Some parts of the Qanat are described as follows:
Gallery: the Qanat gallery or tunnel which is an almost horizontal tunnel dug to get access to groundwater reserves, and to transfer this water to the earth’s surface. The dimensions of the tunnel are such that the workers can easily go through and work in it: between 90 and 150 centimeters high and its width is less than half the height.
Exit point of the Qanat: Where the tunnel and the ground surface eventually intersect is the exit point of the Qanat which is called the "Mazhar" meaning where water appears.
Shaft wells: there are some vertical shafts sunk along the tunnel to connect the surface to the horizontal gallery. The main application of these wells is to haul the debris and excavated materials from the tunnel on to the surface. They also provide access and help ventilate the tunnel and provide more oxygen for the workers. These wells play an important role in repairing the Qanat, by making it possible to send down the needed facilities and tools and remove the debris. The shaft wells cut short the time needed for Qanat construction or repair, and reduce the relevant expenses. A shaft well is between 80 and 100 centimeters in diameter, and the distance between the wells vary from 20 to 200 meters. In fact, the deeper the shaft wells, the further they are from each other. The distance between the shallow wells (up to 40 meters) is two times as their depth, but in terms of deeper wells the distance is equal to their depth.
Mother well: the furthest shaft well from the exit point, sunk upstream is called the "mother well". The mother well is usually the deepest well, in which a large inflow of water shows the Qanat is in a satisfactory state. If the water table goes down so much that it is located below the bottom of mother well, no water can seep into the gallery, and if this situation persists, the Qanat will inevitably dry up. If a Qanat is extended so far that another well is needed, the new well would now be the mother well and the former one would be a normal shaft well. In a nutshell, the last well is always called the mother well. The depth of the mother well varies from Qanat to Qanat, and the deepest one in Iran has been recorded in the Qanat of Gonabad at 300 meters.
Farm: the farm is a cultivated area which is less elevated than the exit point of the Qanat, irrigated by the water coming out of the Qanat. The extent of the cultivated area depends on several factors such as the Qanat discharge, soil quality, soil permeability, local climatic conditions, etc. If the water flowing from the Qanat is insufficient, the water is stored in a pool to increase the volume and head of water so that it can be delivered to the land at a higher flow rate and thus irrigate the farms. The irrigation cycle differs from area to area but is usually between 12 and 15 days. It should be noted that an irrigation cycle is a water management order according to which the shareholders take turns irrigating their farms. For example, if the irrigation cycle is 12 days, every farmer has the right to take his share just once every 12 days.
Qanat diagram
A Qanat Profile
A Qanat Plan
Terms
There are more than 27 terms for Qanat, being used in these countries:
“ Qanat” and “Kariz” in Iran, “ Falaj” pl “Aflaj” in Oman, “Kariz” or “Karez” in Afghanistan, Pakistan, Azerbaijan and Turkmenistan, “ Ain” in Saudi Arabia, “Kahriz” in Iraq, “ Kanerjing” in China, “ Foggara” in Algeria, “Khattara” or “Khettara” and “ Rhettara” in Morocco, “Galleria” in the Spain, “ Qanat Romoni” in Syria and Jordan, “Foggara” and “Khettara” and “Iffeli” in North Africa, “Galerias” in the Canary Islands, “ Mambo” in Japan, “Inguttati” in Sicily. Some other terms used for qanats are: Ghundat, Kona, Kunut, Kanat, Khad, Koniat, Khriga, Fokkara, etc.
Wonders of Qanats in Iran
Inside qanats there are wonders unbelievable to modern world. According to the Iranian Ministry of Energy, the number of qanats in Iran is about 36,300. The average length of these qanats is about 6 kilometers and the average depth of all shaft wells of a Qanat together is some 4 kilometers. So if we take it as a sample of all qanats, we have about 376,068 kilometers gallery and shafts in Iran and if we consider the mouth of each Qanat is about 1 meter in diameter so if we collect all the soil extracted from all these qanats we will have a pile 160 km long, 185 meter wide and 20 m high. The total length of the qanats equals to 9.4 times as long as the Equator, and about 97.9 percent of the distance between the moon and the earth. If we consider the wages for those who have created such hydraulic systems it exceeds billions of dollars.
In Ardestan, Iran, there is a Qanat with two levels of water lying over each other, called the Moon Qanat. The 1st level is 30 meters deep, and the 2nd level is 27 meters deep, so the high difference is 3 meters. The soil formation of this Qanat is such that the water from 2nd level does not penetrate the lower level.
Vazvan Qanat in Meimeh city, Iran; this Qanat has an underground dam. They dug the whole of the gallery toward the dam through stone, and so this gallery is constructed in stone and they built a dam between the wet zone and the dry zone of this Qanat. The dam stores the Qanat water in winter they get access to the dam through a gallery down the front and from the other gallery they go behind this dam. This dam dates back to Sassanid era about 1600 years ago.
The dams store the Qanat water in winter, when there is no need of that water for agriculture. In spring the outlet of the dam is opened so that stored water can be used for agricultural purposes.
