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

deserts of the Sahara and Arabia, on the one hand, and the mid-latitude deserts of central
Australia or the Gobi of central Asia, on the other.
The dearth of rainfall in the arid zone may be caused by one or more factors. First, aridity may
result from location within a continental interior far removed from the moisture-laden oceanic
winds, as in the case of central Asia. Second, the region may lie in the rain shadow of a major
mountain range, so that much of the precipitation from the moist airstream is removed in
passage across the mountain zone. The Kalahari and parts of Patagonia fall into this category.
Third, coastal deserts such as the Atacama of Chile exist because winds, having blown across a
cool ocean current onto a heated land, do not condense their moisture into anything more than
mist. Last, but most important, are the major tropical deserts that coincide with large,
permanent high-pressure systems in which air is slowly subsiding before blowing outwards at
the surface. In this way rain-bearing cyclones are virtually excluded and precipitation totals are
practically nil in wide areas of the central Sahara and Arabia. The driest place in the world,
however, occurs near Calama, in Chile’s Atacama Desert, where no rain has fallen for over 400
years. 6
Natural and Human Preconditions for Disaster Occurrence
Factors Leading to Desertification
Vulnerability to desertification and the severity of its impact are partly governed by climate, in
that the lower and more uncertain the rainfall, the greater the potential for desertification. Other
natural factors include the seasonal occurrence of rainfall, as between hot season, when it is
quickly evaporated, and cool season. Also important are nonclimatic factors such as the
structure and texture of the soil, topography, and types of vegetation. Above all, susceptibility to
desertification is a function of pressure of land use, as reflected in density of population or
livestock or in the extent of agricultural mechanization.
Background: The Soil-Water Energy Balance
To see precisely what happens when desertification occurs, attention should be focused on that
shallow meeting place, between soil and atmosphere, where plants thrive and where a balance
is maintained between incoming and outgoing energy and between water received and lost.
When rain falls, some of the water is directly taken up by plants. Some filters into the soil,
where it may remain in storage. The rest evaporates or runs off. Some soil moisture, being
intercepted by plants, is put back into the atmosphere by the transpiration of plants. Some of
the moisture may seep into deeper layers, collecting in underground reservoirs or aquifers,
where it may remain for thousands of years. Or it may migrate slowly from plateau to
depression or back to the ocean itself.
The soil-air meeting place participates in an energy balance activated by the rays of the sun or
by atmospheric heating. The surface layer reflects some of this energy back into the
atmosphere and into space. Some is held in storage by the soil, thereby warming the earth.
This energy as well as that directly from the sun is used by plants to carry out the processes of
photosynthesis and growth. Some of the plants are eaten by grazers or browsers, and these
animals in turn may be eaten by carnivores. Through respiration all animals return energy and
moisture to the atmosphere. They also return humus to the soil. The excreta of animals, their
decomposing carcasses, and the decomposition of plants supply the soil with organic nutrients
that are most dense in the topmost layers and thinner below. This balance is lost when land is
denuded by desertification.