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

The Disaster and its Impact on the Natural Environment
Impact of Land Use on Dryland Equilibrium
Under natural conditions and through appropriate conservation strategies, the dryland
ecosystems maintain a balanced exchange between water and energy. Unfortunately this
favorable equilibrium is readily disturbed when people unwisely use the land. For example,
where meager vegetation is reduced to expose the ground surface, the organic portion of the
soil will be mineralized by exposure to the sun, and the soil structure will be broken down by rain
falling directly on the soil. In addition, the sun will bake the top layer of soil into a thin crust that
prevents infiltration of additional water. As the equilibrium of subsurface water deteriorates, the
level of groundwater in nearby wells may fall. The water lost to the soil store now contributes to
over-rapid runoff. Where the surface has been loosened or disturbed as by the trampling of
animals, the topmost soil layer, that with the best structure and containing the bulk of plant food,
may be washed away, or blown away in dust storms. The denuded soil is essentially infertile,
with poor structure and water relations. All these changes constitute a shift towards a more
hostile environment for plants, with the result that the vegetation responds less well to rain and
produces less biomass. Many plants tend to die off at an increasingly early stage of drought.
Such changes are typical of desertification.
Process and Stages of Desertification
The main process and stages of desertification can be summarized as follows. In pastoral
rangelands, there is an initial deterioration in the composition of pastures subject to excessive
grazing in dry periods, particularly a reduction in the proportion of edible perennial plants and an
increase in the proportion of annual and inedible species. The thinning and death of vegetation
in dry seasons increases the extent of bare ground. This is followed in turn by a deterioration of
the surface conditions that are vital to plant growth. Impoverishment of plant-water relations is
especially pronounced, and ephemerals now respond poorly to rain. With consequent increase
in runoff, sheet and gully erosion set in on sloping ground, and the topsoil and its store are lost.
These changes result in an environment inhospitable to plant growth and less suitable as
pasture. With continuing erosion, formerly productive lands may be lost through soil stripping
and gully extension. These changes are even more drastic where devegetation occurs in
strategic areas, as on watershed uplands, and the processes are advanced where soils are
exposed and disturbed in dryland cultivation.
Water and Wind Erosion
In areas of rainfed farming, desertification often originates on fallow land or on land cleared for
cultivation. Removal of the original ground cover exposes the soil to accelerated wind and
water erosion. The beating action of rain on naked soil puddles the surface, which crusts when
the sun comes out, reducing infiltration of water into the soil and further increasing runoff. This
in turn leads to increased soil erosion, which, unless halted by protective measures, ultimately
strips away the fertile surface soil and exposes infertile subsoils. Gullies may form on the lower
parts of slopes, impeding or preventing farming operations. Sediment deposited at the foot of
slopes covers plants, fills waterways and aggravates flooding in low-lying areas. The flooding
follows increased runoff from the slopes above.
Water and wind erosion work together, as redeposited silts from surfaces stripped by water
erosion are particularly vulnerable to wind transport. Wind erosion starts with the movement of
coarse soil particles in one part of a field, then progresses downwind with increasing severity as
bouncing soil particles knock other particles into the air in a kind of progressive, increasing