Geoscience Reference
In-Depth Information
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 pasture around the borehole.
Water quality
. Unfortunately not all these supplies of
water are of pure 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 consump-
tion needs water quality within the range of 500-1,000
parts per million by volume (ppm). If 1 m of water of, say
500 ppm, were added to a field of 1 ha in area as irrigation
water, 5,000 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.
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 26.9
).
Figure 26.10
illustrates the factors affecting the types of soil erosion by
water. The volumes of soil lost are difficult to estimate,
especially for wind-borne material, but studies suggest
values of up to 300 tonnes per hectare in the Ethiopian
Highlands, where rainfall erosivity is high, compared with
less than five tonnes on grazing land and less than one
tonne in forested areas. Although these figures may sound
severe, they need to be balanced against the rates of soil
formation, as it is the net loss which is of greatest signifi-
cance. Even then interpretation is not straightforward, as
the erosion may take place in narrow channels which can
rapidly expand, whereas soil formation will take place
slowly over the whole catchment.
Estimates of the extent of soil degradation in suscep-
tible dry lands are shown in
Table 26.2
.
We can see that
Africa and Asia have the largest areas affected by moderate
or severe degradation. Interestingly, the relatively small
Desalinization
Desalinization sounds an ideal way to produce fresh water
from the infinite resources of the sea. Unfortunately the
energy required to remove the minerals in the water is
considerable and so only feasible where energy costs are
low or water is unavailable from other sources, such as
the Middle East. Estimated costs of desalinized water are
variable but normally well over US $1,000 per acre-foot,
which means it is likely to be used for drinking water only.
See additional case study, 'Water supply problems in Saudi
Arabia' on the support website at
www.routledge.com/
Soil erosion
The problem
Soil erosion is not unique to dryland areas but its effects
may be more apparent there than elsewhere. Erosion
