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Friday, 29 December 2006

Oases in the Sahara: Use of Groundwater for Survival - Page 2

Written by Dr. Ronald Francis
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Since so much of human existence depends on water, how have people survived crossing deserts where there is precious little precipitation?

Qanats

qanat

A qanat (also known as a foggara in North Africa and karez in Afghanistan) is an underground, barely tilted drainage tunnel, that uses gravity to bring water from high underground water aquifer into a lower area. The tunnel is located up to 100 meters (typically 50 meters) below ground and can be as short as 10 meters in length but more typically a mile or two and occassionally dozens of miles. See the diagram for a rough sketch of a typical qanat .

The qanats have equally spaced vertical shafts that provided an air supply for workers, and also a way to remove the dirt that must be moved to make the tunnel. The vertical shafts are spaced typically from 30 to 50 meters apart and each vertical shaft as a ring of dirt around it near the opening to the land surface above. At the surface, a qanat looks like a line of life-sized lifesaver candies.

 

qanat

 

That the qanats are underground means that there are few losses due to evaporation and virtually no contamination from the surface.

Though difficult to build, there were 21,000 operating qanats in Iran in the early 70s and another 16,000 that were in need of repair. Qanats began in Persia and their use spread in all directions- reaching North Africa and the Sahara dessert on one side and China on the other side. Some are also found in Latin America.

Qanats are often necessary because of the limited ability of shallow wells in the oases to bring up water The qanat takes advantage of stored water under mountains at the edge of the alluvial fan where air masses have shed water due to cooling upon rising.

The building of a qanat is an engineering feat that lasts several years and comes with great risk to workers. It starts by carefully selecting the site; one looks for vegetation with deep roots or the presence of a wadi (dried river bed) traceable to underground sources. The “mother well” is dug vertically to locate the aquifer - typically near the edge of the alluvial fan.

Next, and often several miles away, they begin digging a nearly horizontal tunnel into the hills on the side of the alluvial fan in which the oasis is located. Then, every 15 to 50 meters they make a vertical shaft that goes down and intersects the tunnel and use a windlass to lift the dirt out.

For workers, the building of a qanat is perilous and uncomfortable. They must always check for oxygen, the possibility of a collapse is ever present, and when the mother well is finally pierced there is the risk of drowning or suffocation.

(Page 2 of 3)
Last modified on Sunday, 16 December 2012

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