Geoscience Reference
In-Depth Information
5.5.1 Water resources and salinization
populations have accelerated rates of soils erosion.
Regions bordering the Mediterranean Sea have
some of the highest erosion rates in the world
(Woodward 1995), with some 1000 t km −2 yr −1 of
suspended sediment generated. Of this, 70% can
be attributed to anthropogenic agents (Dedkov
& Mozzherin 1992). Within this region some
countries have implemented the policy of reduc-
ing the number of people involved in agriculture
and replacing them with mechanization. Such
countries have been experiencing accelerated
rates of erosion (Morgan 1994). In some cases
this is related to the abandonment of agricultural
terraces (e.g. Millington 1989). Mismanagement
of the landscape can also lead to alterations in
the hillslope hydrology, which can have an impact
on runoff pathways and exacerbate soil erosion
related to surface runoff (Case Study 5.3, the
IBERLIM example).
Soil erosion by wind is another common
phenomenon. It is particularly common in areas
where water resources have been used for
irrigation, leading to a lowering of the affected
water bodies and exposure of lake sediments to
deflation. Wind erosion is further exacerbated
where secondary salinization inhibits vegetation
growth, thus limiting the erosional protection
benefits of vegetation cover. In other cases, soil
erosion is related to overgrazing of land or over-
production in years of drought, which can lead
to a loss of arable plant protection. Deflation
can become a major issue where this coincides
with fine-grained sediments such as glacial and
river silts or loess.
Salinization is the process of accumulation of
soluble salts in the upper parts of soil horizons.
Secondary salinization (where salinization is
enhanced by human intervention) of soils in arid
areas is a major problem. This is particularly
so where groundwater levels are close to the sur-
face and salts can be drawn into the upper parts
of the soil where they naturally accumulate. A
simple removal of vegetation cover may be all
that is required to modify the hydrology of the
soil (raise the water table) and place it in danger
of salinization in an arid area. For example, in
Australia native evergreen forest removed by
settlers increased the salinity of over 200,000 ha
to the point where they were no longer produc-
tive (Mackay 1990). Secondary salinization more
commonly occurs through excessive irrigation.
This is not surprising given the rapid increase in
irrigation globally since 1800. In some countries
(e.g. Egypt) nearly 100% of land in agricultural
use is irrigated (Rhoades 1990). Approximately
half of all irrigated areas suffer from problems
of secondary salinization (Rhoades 1990).
Salinization can lead to cracking and puffing
of salt-affected soils (Mabbutt 1986), affecting
the hydrological characteristics of the soil. Where
a salt crust or accumulation forms runoff can
be exacerbated. The salts will also exacerbate
weathering rates, producing abundant fine mater-
ial which may then be deflated by wind action,
leading to the formation of 'salt scalds' (Mabutt
1986). In hyper-arid areas where salts natur-
ally occur in the soils, for example in the form
of gypcretes (Table 5.5), irrigation may lead
to rapid dissolution and collapse of the soil
surface, affecting the urban area. As salt is a
dominant weathering agent in arid landscapes
(Doornkamp & Ibrahim 1990; section 5.2.1.3)
it can also lead to significant impacts on engineer-
ing structures and urban areas (section 5.6.3).
5.5.3 Mineral resource extraction
Arid environments are often locations of mineral
resources such as the diamonds of Namibia or
the salts and nitrates of the Atacama of northern
Chile. The impact of this mining on the environ-
ment is highly variable. In the hyper-arid climate
of the Atacama of Chile extensive nitrate min-
ing occurred across the Central Depression. The
mining at its peak provided almost 50% of the
income for the Chilean Government (Wisniak &
Garces 2001). Most of the mines were abandoned
in the 1930s and now litter the Central Depression
5.5.2 Agricultural resources and soil erosion
Soil erosion by water has been recognized re-
cently as a major contributor to erosion in arid
lands. Natural vegetation clearance and increased
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