Qanats of Iran
In the early part of the first millennium BC, Persians started constructing elaborate tunnel systems called qanat for extracting groundwater in the dry mountain basins of present- day Iran. Qanat tunnels were hand-dug, just large enough to fit the person doing the digging. Along the length of a qanat, which can be several kilometers, vertical shafts were sunk at intervals of 20 to 30 meters to remove excavated material and to provide ventilation and access for repairs. The main qanat tunnel sloped gently down from pre-mountainous alluvial fans to an outlet at a village. From there, canals would distribute water to fields for irrigation. These amazing structures allowed Persian farmers to succeed despite long dry periods when there was no surface water to be had. Many qanats are still in use stretching from China on the east to Morocco on the west, and even to the Americas.
There are significant advantages to a qanat 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. The third benefit warrants additional discussion.
The rate of flow of water in qanat is controlled by the level of the underground water table. Thus a qanat cannot cause significant drawdown in an aquifer because its flow varies directly with the subsurface water supply. When properly maintained, a qanat is a sustainable system that provides water indefinitely. The self-limiting feature of a qanat, however, is also its biggest drawback when compared to the range of technologies available today.
Water flows continuously in a qanat, and although some winter water is used for domestic use, much large amounts of irrigation water are needed during the daylight hours of the spring and summer growing seasons. Although this continuous flow is frequently viewed as wasteful, it can, in fact, be controlled. During periods of low water use in fall and winter, watertight gates can seal off the qanat opening damming up and conserving groundwater for periods of high demand. In spring and summer, night flow may be stored in small reservoirs at the mouth of the qanat and held there for daytime use.
Thanks to early writers, we have excellent descriptions of the techniques used by ancient qanat builders. A recently discovered book by Mohammed Karaji, a Persian scholar of the 10th Century AD, has a chapter on qanat construction. The techniques he describes are basically the same as those practiced today, eleven centuries later.
Qanats are constructed by specialists. A windlass is set up at the surface nd the excavated soil is hauled up in buckets. The spoil is dumped around the opening of the shaft to form a small mound, the latter feature keeps surface runoff from entering the shaft bringing silt and other contamination with it. A vertical shaft I meter in diameter is thus dug out. A gently sloping tunnel is then constructed which transports water from groundwater wells to the surface some distance away. If the soil is firm, no lining is required is required for the tunnel. In loose soil, reinforcing rings are installed at intervals in the tunnel to prevent cave-ins. These rings are usually made of burnt clay. Mineral , salt, and other deposits which accumulate in the channel bed necessitate periodic cleaning and maintenance work.
Present – Day Systems
An extensive system of qanats is still in use in Iran. According to Wulff (1968): "The 22,000 qanats in Iran, with their 170,000 miles of underground conduits all built by manual labor, deliver a total of 19,500 cubic feet of water per second – an amount equivalent to 75 percent of the discharge of the Euphrates River into the Mesopotamian plain. This volume of water production would be sufficient to irrigate 3,000,000 acres of arid land if it were used entirely for agriculture. It has made a garden of what would otherwise have been an uninhabitable desert."
Qanats are still found throughout the regions that came under the cultural sphere of the Persians, Romans, and Arabs. The qanat system in Turpan, China, is still very much in use. In the Sahara region a number of oasis settlements are irrigated by qanats, and some still call the underground conduits "Persian works."
Qanats are to this day the major source of irrigation water for the fields and towering hillside terraces that occupy parts of Oman and Yemen. They have for some 2000 years allowed the villages of the desert fringes of the Arabian Peninsula a grow their own wheat as well as alfalfa to feed their livestock. In these villages, there are complex ownerships of water rights and distribution canals. In Oman, their importance was underlined in the 1980s with a government- funded repair and upgrade program.
Qanats and Disease
Qanats were frequently used for domestic purposes, as well irrigation Because of this, they can transport disease vectors. A chemical analysis Because of this, conducted in 1924, from 6 qanats as they entered Tehran revealed water of potable quality in only 2 cases. In 3 other, water purity was questionable and in 1 case the water was definitely unfit for drinking. These results were especially shocking since the samples were taken from closed qanats before they were open to contamination. It has been hypothesized that qanats were a major contributor to the cholera epidemics of the 19th century.
Throughout Iran, even if the qanat water was uninfected before entering the cities, it hasd ample opportunity to become contaminated whil traversing the urban areas in open ditches. With the lack of proper sewage and waste disposal throughout Iranian municipalities, the cholera bacterium easily made is into drinking water.
Passive Cooling Systems
Qanats can be used for cooling as well as water supply. One technology operationg in conjunction with qanat is a wind tower. The arid regions of Iran have fairly fixed seasonal and daily wind patterns. The wind tower harnesses the prevailing summer winds to cool and circulate it through a building. A typical wind tower resembles a chimney, with one end in the basement of the building and the other end rising from the roof. Wind tower technologies date back over 1000 years.
The passive cooling of a wind tower can be enhanced by connecting it to an underground stream or qanat. In the system, a shaft connects the qanat to the basement of the building to be cooled. Hot dry air enters the qanat through one of its vertical shafts and is cooled as it flows along the water. Since the underground water is usually cold, the rate of cooling is quite high. The wind tower is placed so that wind flowing through the basement door of the tower could pass over the top of the qanat tunnel. When the air flows from a large passage (the tunnel) through a smaller one (the door), its pressure decreases. The pressure of the air from the tower is still diminished when it passes over the top of the tunnel, so that cold mioist air from the shaft is entrained by the flow of cooled air from th tower. The mixture of air from the qanat and air from the tower circulates through the basement. A single qanat can serve several wind- tower systems.
A qanat system has a profound influence o the lives of the water users. It allows those living in a desert environment adjacent to a mountain watershed to create a large oasis in an otherwise stark environment. The United Nations and other organizations are encouraging the revitalization of traditional water harvesting and supply technologies in arid areas because they feel it is important for sustainable water utilization.