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Friday, 29 December 2006

Oases in the Sahara: Use of Groundwater for Survival - Page 3

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?

qanatThere are great rewards for building a qanat, though. The supply of water could flow for centuries- since you are tapping into large underground aquifers.

 

Where the water comes to the surface for consumption, there are often mud tracks (weirs) that are used to separate and send the water to different family farms. Young boys typically worked on the building of the qanat – a form of employment that was often well paid.

One drawback of qanats is that they must be cleaned out periodically and can suffer from erosion. One can also get unlucky in choosing an aquifer that doesn’t have a large supply or one that does not get recharged well by the rain and condensation.

qanat

One interesting piece of fluid mechanics has to do with the choice of angle for the drainage. Usually the grade chosen is between 1:1000 and 1:1500 or very, very, shallow grades. The reason is that one is trying to avoid the boundary of subcritical flow and supercritical flow of the water – a boundary that can be crossed when the angle of the tunnel and the resulting speed of the running water change.

 

Shifting between these two different types of flow (one has water velocity less than surface wave velocity and the other more) can create a buildup of wave amplitude and the subsequent turbulent breakdown of the flow will erode the tunnel walls causing seepage and blockage. See a fluid mechanics text for the rather complicated details.

Qanats can also a have a cyclical nature to the flow of water that works like this: hot moist air from the vegetation (often palm trees) will run backward up into the qanat and release its water as it cools and travels up the vertical shafts of the qanat, thereby replenishing to some extent the water drawn out of the qanat. One wonders if a greenhouse in the desert could magnify this effect by trapping hot moist air and then being able to channel it into the qanat.

Speaking of alternative uses of qanats, they were used to cool the basements of houses since if air could be drawn out of the qanat then it would tend to be cool because of the cold temperatures of the groundwater. Bernoulli’s principle of using moving air (from an air-catcher above the house) to reduce pressure is used to pull the air up and out of the qanat tunnel and into the basement of ones home located above the qanat.

 

qanat

 

 

Qanats can be found in Kharga, Egypt (one of Egypt's four main oases) and in some oases in Algeria such as Gourara and Tonat. Iran has tens of thousand of qanats, some still in use, and some qanats can also be found in Libya, China, Spain and Latin America.

On your next trip to the desert, see if you can locate a qanat near the edge of an oases and ponder the years-long human effort required to build even one of the underground miracles, let alone thousands of them.

©Dr. Ronald Francis

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Last modified on Sunday, 16 December 2012

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