Environmental Engineering Reference
In-Depth Information
sources are limited. Some deep aquifers are regarded as 'fossil', since they are no longer
being significantly recharged. Much of the ground water under the Sahara and the Middle
East is fossil. The rate of exploitation of this non-renewable resource means that water
table levels will decline rapidly. At the rates of exploitation by Saudi Arabia in the 1990s,
and assuming that 80 per cent of the ground water can be extracted, the supply will be
exhausted in about fifty years. Much of this water is pumped to the surface, using
abundant and cheap local fuel supplies, and used for irrigating crops of wheat. By heavily
subsidizing land, equipment and the irrigation water, and by buying the wheat at several
times the world price, the Saudi government has encouraged large-scale wheat farming in
the desert based on fossil ground water. Ironically the wheat could have been purchased
on the world market at a quarter of the price.
On a much smaller scale many dryland countries have drilled small boreholes to the
groundwater level to supply drinking water or water for animals. In theory this provides a
more stable supply of water, but in practice it generates overgrazing of pastures around
the borehole as farmers congregate to water their animals.
Water quality . Unfortunately not all these supplies of water are of perfect quality.
Impurities in water can take the form of solids or salts. The World Health Organization
sets an upper limit of 500 mg litre −1 for the solid content of drinking water, though often
levels much higher than this will be consumed by humans and livestock. Even more
severe is the problem of salt content. Domestic consumption needs water quality within
the range of 500-1000 parts per million by volume (ppm). If 1 metre of water of, say 500
ppm, were added to a field of 1 ha in area as irrigation water, 5000 kg of salt would be
deposited. To remove the salt, larger quantities of irrigation water would be required to
flush the salt into drainage water. As the salt concentration of the water increases, so the
frequency with which soil leaching is required also increases and the greater is the
proportion of drainage water.
In some cases, salty irrigation water can react with the soil. On an experimental farm
in Queensland water from the Great Artesian Basin was used for crop irrigation. After
two years the experiment was abandoned, as the highly alkaline water had reacted
chemically with the clay soils to produce a crust so hard it had to be broken up by
dynamite to allow seeds to be planted! The water was used only for livestock
subsequently. Similar, though less extreme, reactions have occurred in some of the
calcareous soils of the Middle East. It has been estimated that within fourteen years of
irrigation the salts deposited in the soil will have reached levels that are toxic to many
plants.
SOIL EROSION
THE PROBLEM
Soil erosion is not unique to dryland areas but its effects may be more apparent there than
elsewhere. Erosion can take place as a general deflation of surface material together with
nutrients or it can occur as gullying and sheet erosion, where large amounts of material
may be removed following heavy rain (Plate 27.9). Figure 27.10
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