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Friday, 29 December 2006

Oases in the Sahara: Use of Groundwater for Survival

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?

Oases.

An oasis comes about because of the availability of fresh water coming from underground reservoirs that are able to supply the surface because oases are at lower elevations than surrounding areas. Some oases use surface water as well from lakes and rivers.

Oases are often below sea level (although what is really important is how the oases surface level compares to the local groundwater table). The availability of groundwater makes the oases fertile so date palms, citrus, and other vegetation can thrive–also providing drinking water for humans and animals. It follows that oases in the Sahara Desert are essential stopping points for caravans; a caravan going North to South through the Sahara must go from oasis to oasis.

oasis

 

Some interesting things about the groundwater: it can be high enough to make pools of water, sometimes it is close enough that deep plant roots can get to it, and occasionally artesian wells can be used to extract the water. If shallow wells do not produce water, however, then a qanat will have to be built (see below). Modern day oases are moving toward the use electric pumps to get water up from deep wells dug using modern mechanical equipment.

To understand how oases use water, consider an artesian well. An artesian well works since water pressure builds with depth; you can get water to flow out of a well if the output of the well is located at a point that is lower than the highest points of the surrounding groundwater (called the water table).

This seems simple enough, but the problem is that the largest supplies of underground water are typically located in the mountains surrounding the oases. This is because the air flowing upward from the alluvial fan will cool and is unable to hold water, therefore there tends to be more clouds and precipitation above the mountains (as mountain travelers are well aware).

Thus the water aquifers are in the wrong spot! But they are also higher because they are in mountains, though underground. In order to get enough water for your oasis, which may be rather large in area, you will need a qanat.


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.


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

Last modified on Sunday, 16 December 2012

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