Natural Hazards: Causes and Effects - Disaster Management Center ...

Adaptation of the Hydrological Cycle in Arid Climates
In arid situations the cycling of water and energy takes on special characteristics because of
infrequent rainfall and abundant solar energy from cloudless skies. Vegetation is generally
sparser than in humid areas, providing less cover to the ground surface and returning less
organic matter to the topsoil. During occasional intense rainfalls flash floods may occur.
However, this surface water is rapidly lost through evaporation, and in the long intervening dry
spells the soil is parched and heated by the sun.
Vegetation in Arid Environments
However scanty it may be, the dryland vegetation constitutes a fundamental resource that
transforms solar energy into food and protects and stabilizes the surface of the ground.
Part of the plant population consists of short-lived ephemerals, plants that germinate and
complete their life cycles rapidly after rain, remaining as seed through intervening dry periods.
Such plants are commonly fleshy and palatable; they are preferred by grazing animals. Other
plants, such as perennial grasses, wither during dry spells and die back to the root stock or to
bulbs; with fresh rains they shoot forth anew. These plants form more durable pastures. When
green, they are attractive and palatable to stock. They may provide valuable hay, although
when thoroughly dried they have little pastoral value. Nevertheless, their extensive, fine root
systems remain to bind the topsoil and to contribute to its organic content. Last are the longerlived
perennial plants that resist water loss by such adaptations as woody stems and leathery
leaves. These include the larger plants, such as shrubs and trees, which remain nutritious
during the dry periods when they can provide an important food source to grazing animals.
However, their adaptations may reduce for some stock their palatability and attractiveness.
These plants have the additional role of protecting the ground surface, providing shade and
preserving an environment that favors the response of important shorter-lived plants.
Dryland Mismanagement
Misuse and eventual desertification arise through any use of drylands that does not take into
account their limitations and the patchwork contrasts in productivity and vulnerability that
accompany them. The problem is compounded by fluctuating climate and land conditions, and
a tendency toward over-optimistic assessments of the potential of drylands for sustained
production. Such assessments are often made on the basis of remembered best years.
Optimism is often a response to external pressures to which the users of drylands are
increasingly subjected—from commercial markets, from their own rising expectations, and from
population growth. Optimism is often associated with the introduction of inappropriate
technology in the hope of short-term gains, for instance deep ploughing of croplands or
excessive pulverization through mechanical tillage of topsoils.
Positive and Negative Land Use
Traditional systems of land use have met these environmental challenges in various ways in
order to maintain flexibility and spread the risks. For instance, pastoralists may herd several
kinds of animals, each capable of profiting from different parts of the ecosystem, or they may
range widely as nomads to spread and lighten the grazing load. But with increasing
technological inputs arising from the push towards higher productivity, there is a trend towards
diminished flexibility. For instance, as dryland farmers extend towards the climatic limits of
cropping, they must depend increasingly on the hardiest cereals, wheat and barley, and
fallowing to conserve soil moisture. Lack of flexibility is particularly evident in large-scale
commercial ranching and farming operations, which tend to be highly specialized; nevertheless,