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

eluctance to cut back on stock numbers in the first dry year, and a tendency to hang on until
drought is seen to be established. Unfortunately by this time dryland pastures are probably
being overgrazed toward a state that threatens eventual regeneration. By this time, too, prices
for surplus stock will probably have shrunk because of a glutted market, and destocking through
sale of surplus numbers will be opposed by economic forces. For the same reasons,
destocking may be prevented during the periods critical to the regeneration of pastures, that is,
those periods following the rains that end drought.
Extending Farming in Marginal Areas
Dryland farmers, too, have a tendency after a period of good years to extend their cropping onto
ever more marginal lands. Moving into areas of higher climatic risks, they push back the
pastoralists. This is especially prevalent when pressure on the land is increased by population
growth, restrictive systems of land tenure or shortsighted introduction of mechanization. The
expectable but unpredictable onset of drought will find the marginal land prepared for planting,
stripped of its protective natural vegetation and vulnerable to erosion. Such land enters a run of
dry years without defenses and may emerge in too degraded a condition to support even
livestock.
Thus, the delayed response of the land user through cycles of good and insufficient rainfall may
convert periodic drought into an engine of long-term desertification. But this need not be so.
Land-use practices should combine with efficient marketing systems to make possible an
appropriate response to drought, which is a natural and inevitable factor of dryland agriculture
and a recurring event that must be taken into account. Land users should also take advantage
equally of the rainier years, employing them to replenish the ultimate agricultural resources, the
fertility of the soil and the production of vegetation.
Climatic Preconditions for Desertification
What is certain is that the direct physical consequences of changes on the local, effective
climate, such as the adverse effects of surface denudation on the soil-water balance, are many
times more important than any indirect large-scale climatic effects.
However, the climatic effects can not be ignored since the climatic boundaries in the drylands
are subject to short-term shifts corresponding to sequences of lean years and fat years. In
general, the drier the climate, the greater the rainfall variability and the higher the drought risk.
Such fluctuations are expressed geographically in expansions and contractions of the dryland
belts, such that a semiarid region may experience arid conditions at one time, and subhumid
conditions at another.
These fluctuations, although not so regular as to be predictable, can be divided into shortperiod,
two- to four-year changes that introduce periodic stress into livelihood systems, and
those of greater amplitude and duration that can lead to significant changes in the patterns and
structure of land use, such as the extension of cultivation in good years or large build-ups in
stock numbers in runs of good years. It may not be possible to adjust these expansions
promptly when drought inevitably follows. When drought strikes land-use systems that are
stretched beyond their usual limits, the consequences can be disastrous and of their maximum,
and long-lasting degradation can occur. Recovery from such degradation will be slow at best,
and if land-use pressures continue unabated, recovery may be partial only, to a lower plane of
productivity than formerly. Desertification will then have occurred.