Geoscience Reference
In-Depth Information
Figure 4.2. Cross section through a
foggara
, central Sahara. (After El Hadj,
1982
.)
lakes, like Lake Chad or the Aral Sea. The Syr Darya and Amur Darya in central Asia
are good examples of such drainage systems, with prolonged water abstraction from
the Syr Darya for cotton irrigation being the main cause of the shrinking of the Aral
Sea. Areic drainage systems are usually ephemeral and lack an integrated drainage
network, although they may have possessed such a network under a previously less
arid climate. The linear salt lakes of western Australia are the remnants of a once
integrated and extensive drainage system that was active when Antarctica and Aus-
tralia formed one large and well-watered continent more than 45 million years ago,
but they now form part of an areic drainage system.
There are strong physiographic controls on the surface and shallow sub-surface
distribution of water in deserts. Surface water is generally absent from dunes, gravel-
covered plains and plateaux. A plethora of local names has been used to describe
these landforms, with the gravel plains known as
reg
or
serir
in the Sahara,
gobi
in
Mongolia and
gibber
plains in Australia. Likewise, the rocky plateaux are known
as
hamada
in the Sahara
, mesas
in the American deserts and
stony tablelands
in
Australia. Springs and ephemeral, seasonal or even perennial streams tend to occur
within mountain valleys or along mountain fronts, while small ponds may form in
the hollows between dunes after local heavy rain. In the Badain Jaran desert of Inner
Mongolia, where the dunes attain relative heights close to 500 metres (Yang,
1991
;
Yang et al.,
2011a
), making them the highest dunes in the world, the depressions
between the dunes are occupied by permanent lakes, although many of these lakes
have been shrinking during the last few thousand years, most probably as a result of
a progressively drier climate (Yang and Williams,
2003
).
Alluvial fans at the foot of mountains have long been a major source of shallow
groundwater for plants, animals and humans and some ingenious methods of water
extraction, such as the
foggara, karez
or
qanats
discussed in
section 4.5
, are still
in use today among peasant farmers in the semi-arid world (
Figure 4.2
). Shallow
sub-surface water also occurs in the sand and gravel beds of ephemeral or seasonal
stream channels and can be accessed from wells dug one or more metres below the
surface. These temporary wells remain a major source of water for nomads and small
villages throughout the semi-arid world today. Many permanent wells have been sunk
away from the river channels to tap aquifers at greater depth. When dug through clay
or sandy clay, they are usually lined with bricks or rocks. Several remarkable wells
