Environmental Engineering Reference
In-Depth Information
economic problem, a political problem, a cultural
problem or sometimes a multi-faceted problem
involving all of these. Whatever else it may be,
however, it is always an environmental problem,
and basic to any understanding of the situation
is the relationship between society and
environment in drought-prone areas.
Over thousands of years, certain plants and
animals have adapted to life with limited
moisture. Their needs are met, therefore no
drought exists. This is the theoretical situation
in most arid areas. In reality, it is much more
complex, for although the flora and fauna may
exist in a state of equilibrium with other
elements in the environment, it is a dynamic
equilibrium, and the balance can be disturbed.
Changes in weather patterns, for example,
might further reduce the already limited amount
of precipitation available, changing the whole
relationship. If the plants and animals can no
longer cope with the reduced water supply, they
will suffer the effects of drought. Depending
upon the extent of the change, plants may die
from lack of moisture, they may be forced out of
the area as a result of competition with species
more suited to the new conditions or they may
survive, but at a reduced level of productivity.
The situation is more complex for animals, but
the response is often easier. In addition to
requiring water, they also depend upon the
plants for food, and their fate, therefore, will be
influenced by that of the plants. They have one
major advantage over plants, however. Being
capable of movement, they can respond to
changing conditions by migrating to areas
where their needs can be met. Eventually some
degree of balance will again be attained,
although certain areas—such as the world's
desert margins—can be in a continual state of
flux for long periods of time.
The human animal, like other species, is also
forced to respond to such changing
environmental conditions. In earlier times this
often involved migration, which was relatively
easy for small primitive communities, living by
hunting and gathering, in areas where the overall
population was small. As societies changed,
however, this response was often no longer
possible. In areas of permanent, or even semi-
permanent agricultural settlement, with their
associated physical and socio-economic
structures, migration was certainly not an
option—indeed, it was almost a last resort. The
establishment of political boundaries, which took
no account of environmental patterns, also
restricted migration in certain areas. As a result,
in those regions susceptible to drought, the
tendency, perhaps even the necessity, to challenge
the environment grew. If sufficient water was not
available from precipitation either it had to be
supplied in other ways—by well and aqueduct,
for example—or different farming techniques had
to be adopted to reduce the moisture need in the
first place. The success of these approaches
depended very much on such elements as the
nature, intensity and duration of the drought,
plus a variety of human factors, which included
the numbers, stage of cultural development and
technological level of the peoples involved.
Types of drought
C.W.Thornthwaite, the eminent applied
climatologist, whose pioneering water balance
studies made a major contribution to the
understanding of aridity, recognized four types
of drought, defined in terms of agricultural
requirements (Thornthwaite 1947). These were
permanent, seasonal, contingent and invisible
drought. Agriculture is not normally possible in
areas of permanent drought, since there is
insufficient moisture for anything but the
xerophytic plants which have adapted to the arid
environment. Crops can be produced in such
areas, but only at great expense or under
exceptional circumstances such as those which
apply to the Israeli activities in the Negev Desert,
for example (Berkofsky 1986). On the margins
of the world's great deserts, there are regions of
seasonal drought, where arid conditions prevail
for part of the year, but which are balanced by a
distinct wet season. Much of India, the Sahel and
the southern parts of Africa experience such
seasonal drought. Agriculture is carried out, often
