Environmental Engineering Reference
In-Depth Information
cultivation to compensate for reduced yields, or
they may try to retain flocks and herds which
have expanded during the times of plenty. If the
drought is prolonged in the arable areas, the
crops die and the bare earth is exposed to the
ravages of soil erosion. The Dustbowl in the
Great Plains developed in this way. Once the
available moisture had been evaporated, and
the plants had died, the wind removed the
topsoil—the most fertile part of the soil
profile—leaving a barren landscape, which even
the most drought-resistant desert plants found
difficult to colonize (Borchert 1950). In the
absence of topsoil there was nothing to retain
the rain which did fall. It rapidly ran off the
surface, causing further erosion, or percolated
into the ground-water system where it was
beyond the reach of most plants.
Prolonged drought in pastoral areas is
equally damaging. It reduces the forage supply,
and, if no attempt is made to reduce the animal
population, the land may fall victim to
overgrazing. The retention of larger herds
during the early years of the Sahelian drought,
for example, allowed the vegetation to be
overgrazed to such an extent that even the plant
roots died. In their desperation for food, the
animals also grazed on shrubs and even trees,
and effectively removed vegetation which had
helped to protect the land. The flocks and herds
had been depleted by starvation and death by
the time this stage was reached, but the damage
had been done. The wind, its speed unhampered
by shrubs and trees, lifted the exposed, loose soil
particles and carried them away, taking with
them the ability of the land to support plant and
animal life. In combination these human and
physical activities seemed to be pushing the
boundaries of the Sahara Desert inexorably
southwards, to lay claim to territory which only
recently supported a population living as
comfortably as it could within the constraints of
the environment. Out of this grew the image of
the 'shifting sands', which came to represent
desertification in the popular imagination. As
an image, it was evocative, but the reality of
such a representation has been increasingly
questioned in the 1990s (Nelson 1990; Pearce
1992f).
Desertification caused by human activity
Climatic variability clearly made a major
contribution to desertification, in both the Sahel
and the Great Plains—perhaps even initiating
the process—and in concert with human
activities created serious environmental
problems. An alternative view sees human
activity in itself capable of initiating
desertification in the absence of increased
aridity (Verstraete 1986). For example, human
interference, in areas where the environmental
balance is a delicate one, might be sufficient to
set in motion a train of events leading eventually
to desertification. The introduction of arable
agriculture into areas more suited to grazing, or
the removal of forest cover, to open up
agricultural land or to provide fuelwood, may
disturb the ecological balance to such an extent
that the quality of the environment begins to
decline. The soil takes a physical beating during
cultivation: its crumb structure is broken down
and its individual constituents are separated
from each other. In addition, cultivation
destroys the natural humus in the soil and the
growing crops remove the nutrients, both of
which normally help to bind the soil particles
together into aggregates. If nothing is done to
replace the organic material or the nutrients, the
soil then becomes highly susceptible to erosion.
Modern agricultural techniques, which allow
the soil to lie exposed and unprotected by
vegetation, for a large part of the growing
season, also contribute to the problem. When
wind and water erode the topsoil it becomes
impossible to cultivate the land, and even
natural vegetation has difficulty re-establishing
itself in the shifting mineral soil that remains.
The removal of trees and shrubs to be used as
fuel has had similar effects in many Third World
nations, where the main source of energy is wood.
In Sudan, for example, the growing demand for
fuelwood was a major factor in the reduction of
the total wood resource by 3.6 per cent annually
