The Persian Qanat
A qanāt  is a gently sloping underground channel with a series of vertical access shafts, used to transport water from an aquifer under a hill. Qanāts create a reliable supply of water for human settlements and irrigation in hot, arid, and semi-arid climates.

The qanat technology is known to have been developed in Iran by the Persian people sometime in the early 1st millennium BC, and spread from there slowly westward and eastward.

The value of the qanat is directly related to the quality, volume, and regularity of the water flow. Much of the population of Iran and other arid countries in Asia and North Africa historically depended upon the water from qanats; the areas of population corresponded closely to the areas where qanats are possible. Although a qanat was expensive to construct, its long-term value to the community, and thereby to the group that invested in building and maintaining it, was substantial.
 

Technical features
Qanats are constructed as a series of well-like vertical shafts, connected by gently sloping tunnels. Qanats tap into subterranean water in a manner that efficiently delivers large quantities of water to the surface without need for pumping. The water drains by gravity, with the destination lower than the source, which is typically an upland aquifer. Qanats allow water to be transported over long distances in hot dry climates without loss of much of the water to evaporation.

The qanat should not be confused with the spring-flow tunnel, typical to the mountainous area around Jerusalem. Although there are similarities in the construction techniques (both are excavated tunnels designed to extract water by gravity flow), there are crucial differences between the two. Firstly, the origin of the qanat was a well that was turned into an artificial spring. In contrast, the origin of the spring-flow tunnel was the development of a natural spring to renew or increase flow following a recession of the water table. Secondly shafts, which are essential to qanats, are not essential to spring-flow tunnels.

It is very common in the construction of a qanat for the water source to be found below ground at the foot of a range of foothills of mountains, where the water table is closest to the surface. From this point, the slope of the qanat is maintained closer to level than the surface above, until the water finally flows out of the qanat above ground,i.e., the qanat is an underground tunnel, beginning from an underground water source, with a gentle slope made in such a way that water is pulled by gravity to the surface (which is at a lower level than that of the underground water source). To reach an aquifer, qanats must often extend for long distances.

Qanats are sometimes split into an underground distribution network of smaller canals called kariz. Like qanats, these smaller canals are below ground to avoid contamination. In some cases water from a qanat is stored in a reservoir, typically with night flow stored for daytime use. An ab anbar is an example of a traditional qanat-fed reservoir for drinking water in Persian antiquity.

The qanat 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. From a sustainability perspective, Qanats use the force of gravity to surface up groundwater with no energy requirement and, thus, have low life cycle operation & maintenance costs once built. Qanats transfer freshwater from the mountain plateau to the lower lying plains that have a saltier soil. This helps to control the salinity of soil and prevent desertification.

 

Excavation
Construction of a qanat is usually performed by a crew of 3-4 muqannīs. For a shallow qanat, one worker typically digs the horizontal shaft, one raises the excavated earth from the shaft and one distributes the excavated earth at the top.

The crew typically begins from the destination to which the water will be delivered into the soil and works toward the source (the test well). Vertical shafts are excavated along the route, separated at a distance of 20–35 m. The separation of the shafts is a balance between the amount of work required to excavate them and the amount of effort required to excavate the space between them, as well as the ultimate maintenance effort. In general, the shallower the qanat, the closer the vertical shafts. If the qanat is long, excavation may begin from both ends at once. Tributary channels are sometimes also constructed to supplement the water flow.

Most qanats in Iran run less than 5 km, while some have been measured at ~70 km in length near Kerman. The vertical shafts usually range from 20 to 200 meters in depth, although qanats in the province of Khorasan have been recorded with vertical shafts of up to 275 m. The vertical shafts support construction and maintenance of the underground channel as well as air interchange. Deep shafts require intermediate platforms to simplify the process of removing soil.

The construction speed depends on the depth and nature of the ground. If the earth is soft and easy to work, at 20 meters depth a crew of four workers can excavate a horizontal length of 40 meters per day. When the vertical shaft reaches 40 meters, they can excavate only 20 meters horizontally per day and at 60 meters in depth this drops below 5 horizontal meters per day. In Algeria, a common speed is just 2 m per day at 15 m depth. Deep, long qanats (which many are) require years and even decades to construct.

The excavated material is usually transported by means of leather bags up the vertical shafts. It is mounded around the vertical shaft exit, providing a barrier that prevents windblown or rain driven debris from entering the shafts. These mounds may be covered to provide further protection to the qanat. From the air, these shafts look like a string of bomb craters.

The qanat's water-carrying channel must have a sufficient downward slope that water flows easily. However the downward gradient must not be so great as to create conditions under which the water transitions between supercritical and subcritical flow; if this occurs, the waves that result can result in severe erosion that can damage or destroy the qanat. In shorter qanats the downward gradient varies between 1:1000 and 1:1500, while in longer qanats it may be almost horizontal. Such precision is routinely obtained with a spirit level and string.

In cases where the gradient is steeper, underground waterfalls may be constructed with appropriate design features (usually linings) to absorb the energy with minimal erosion. In some cases the water power has been harnessed to drive underground mills. If it is not possible to bring the outlet of the qanat out near the settlement, it is necessary to run a jub or canal overground. This is avoided when possible to limit pollution, warming and water loss due to evaporation.

 

 
Maintenance
The vertical shafts may be covered to minimize blown-in sand. The channels of qanats must be periodically inspected for erosion or cave-ins, cleaned of sand and mud and otherwise repaired. For safety, air flow must be assured before entry.

Cooling
Qanats used in conjunction with a wind tower can provide cooling as well as a water supply. A wind tower is a chimney-like structure positioned above the house; of its four openings, the one opposite the wind direction is opened to move air out of the house. Incoming air is pulled from a qanat below the house. The air flow across the vertical shaft opening creates a lower pressure (see Bernoulli effect) and draws cool air up from the qanat tunnel, mixing with it. The air from the qanat is drawn into the tunnel at some distance away and is cooled both by contact with the cool tunnel walls/water and by the transfer of latent heat of evaporation as water evaporates into the air stream. In dry desert climates this can result in a greater than 15 °C reduction in the air temperature coming from the qanat; the mixed air still feels dry, so the basement is cool and only comfortably moist (not damp). Wind tower and qanat cooling have been used in desert climates for over 1000 years.