Qanats are tunnels, hand-dug through unconsolidated material, which
collect underground water from nearby mountain drainages and redirect them
to arable land. Entering and working on qanats is a very dangerous
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
Written records leave little doubt that ancient Iran (Persia) was the
birthplace of the qanat. As early as the 7th century BC, the
Assyrian king Sargon II reported that during a campaign in Persia he had
found an underground system for tapping water. His son, King Sennacherib,
applied the "secret" of using underground conduits in building an
irrigation system around Nineveh.
A vertical shaft 1 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 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
There are significant advantages to a qanat water delivery system
putting the majority of the channel underground reduces water loss
from seepage and evaporation;
since the system is fed entirely by gravity, the need for pumps is
it exploits groundwater as a renewable resource. The third benefit
warrants additional discussion.
The rate of flow of water in a 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 larger 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,
water-tight 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
Qanats are constructed by specialists. A windlass is set up at the surface
and 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. Mineral, salt, and other
deposits which accumulate in the channel bed necessitate periodic cleaning
and maintenance work.
In some cities, water in flows in tunnels beneath residential areas and
surfaces near the cultivated area. Staircases from the surface reach down
to these streams. The first access is usually at a public cistern where
drinking water is available to the entire community.
Sometimes these cisterns are sizable vaults as much as 10 meters across
and 15 or more meters deep with spiral stairs leading down to small
platforms at water level. In cities like Herat in Afghanistan, these
cisterns are ancient constructions encased in tile. Other more modest
urban access points are found along major streets, and even in some
alleys, a factor that probably played an important role in the social and
physical layout of the town.
Where tunnels run beneath houses, private access points provide water for
various domestic uses. In wealthy homes, special rooms are constructed
beside the underground stream with tall shafts reaching upward to
windtowers above roof level. Air caught by the windtowers, which are
oriented to prevailing summer winds, is forced down the shaft, circulates
at water level, and provides a cool refuge from the afternoon heat of
Qanats can be used for cooling as well as water supply. One technology
operates in conjunction with 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 (a) 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 passes 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 moist air from the shaft is entrained by the flow
of cooled air from the 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.
Traditional qanats require considerable labour in dangerous conditions but
the basic design is superb and sustainable and could revitalise many
Desert agricultural products can demand higher prices because they can be
grown pesticide, mould, fungus, nematode free (once clear of pests
the intervening desert acts as a sanitising barrier to the ingress
Horizontal drilling technology and modern grouts currently used in the
coal-seam gas industry could be adapted to drill and line a modern
equivalent of qanats.