Geoscience Reference
In-Depth Information
has built a number of dams in the headwaters of the
Euphrates, which eventually flows through Syria and into
Iraq. The South-eastern Anatolian Project could reduce
the flow of the Euphrates by as much as 60 per cent. This
could severely jeopardize Syrian and Iraqi agriculture
downstream. Some countries have reached agreement
over shared water resources. As long ago as 1889 Mexico
and the United States agreed to an equal allocation of the
annual average flow of the Rio Grande.
Resettlement
. As well as environmental consequences,
the construction of a large reservoir will also have settle-
ment implications. Few areas of the world are unpopu-
lated, so inundation may require considerable movement
of population (
Table 26.1).
The Three Gorges project on
the Yangtze may involve the displacement of up to 1ยท9 M
people. In many projects, resettlement has caused much
hardship. The movement of 50,000 Tongans from the area
of the Kariba dam caused a major culture shock as they
were moved to a very different community and environ-
ment. Experience developed through time was no longer
relevant to the new area and food supplies became a prob-
lem. Often little is heard of these difficulties, as govern-
ments are keen to publicize the positive aspects of the
project. One of the few examples of publicity was the relo-
cation of some Egyptian temples to prevent their disap-
pearance into Lake Nasser.
watershed of the Los Angeles basin so the cost increases.
A cost of $20 per acre ft in the Sacramento valley rises to
almost $300 per acre ft by the time the water has been
pumped over the watershed to Los Angeles, though this
is still much less than would be expected using desalin-
ization of sea water. Where water commands a high price,
as drinking water, aqueducts are, therefore, an efficient
way of carrying it from surplus to deficit areas.
Irrigation
The usual reason for developing the water resources of
an area is to provide water for irrigation. The need for
irrigation may be to compensate for rainfall variability
or it may be to provide a regular water supply when and
where rainfall is low. Whatever the reason for the provi-
sion of water, irrigation allows crops to be produced
whenever temperature conditions allow.
The amount of water required by an irrigation system
will depend upon many factors, such as the type and stage
of development of the crop, temperature and rainfall lev-
els and the nature of the soil. For the most efficient use of
the water these factors have to be taken into account; too
much water or too little water does not produce maxi-
mum yields.
The methods of application of the irrigation water
vary widely. In developing countries surface methods are
most common. These can range from simple traditional
'flow diversion' techniques to large and sophisticated
'centre pivot' schemes.
Unfortunately surface and sprinkler irrigation methods
in the tropics lead to large evaporation losses. To reduce
them, attempts have been made to use underground water
transport in plastic tubing, but pumping is required to
maintain flow. Similarly, trickle irrigation can be utilized
whereby water is released through small nozzles near the
plants or trees (
Plate 26.8
).
The amounts of irrigation
water needed are less than with the traditional methods,
as losses through seepage and evaporation are reduced.
Weed competition is minimized as most of the plots are
dry. Enough water can be passed through the root zone
to prevent the build-up of salt. This method does have
potential, especially for tree crops, where the water can be
directed straight to the roots.
Although irrigation holds great potential for food
production by providing the necessary water in dryland
areas, there are many problems. Unless the soil and water
quality are good, and unless the scheme is efficiently
managed, salinization can quickly develop. Care has to be
taken over disease prevention, as many harmful insects
find irrigated fields to their liking. The world is full of
examples of major irrigation schemes which failed, in both
Aqueducts
Where no rivers are suitable it is possible for water to be
diverted into artificial channels often called aqueducts.
The Romans were great builders of aqueducts to transport
water from source areas to drier locations but many parts
of the world still use similar methods. At its simplest an
aqueduct consists of a channel allowing water to flow by
gravity to where it is needed. On Madeira and parts of the
Canary Islands rock and concrete-lined channels carry
water from the wetter parts of the mountains to the
agricultural areas near the coast. More sophisticated
aqueducts carry water in pipes and pumping may be
required to enable it to cross a watershed.
Los Angeles, situated in a dry part of southern
California, depends heavily on water brought by an
aqueduct from the Owens valley in the mountains and
from the Colorado valley. The former flows by gravity for
360 km but the latter has to be pumped along parts of this
390 km long system. As a result, costs are much higher.
Los Angeles competes with the state of Arizona for the
water available from the Colorado river. Other schemes
now link the Colorado aqueduct with the Sacramento
valley in central California. As more pumping is required
to extract water from longer distances beyond the natural
