Geoscience Reference
In-Depth Information
and prevented rain infiltration. All of these practices
have enhanced the negative feedback mechanisms,
favoring continued drought and desertification.
Similar physiographic conditions exist at present in
Australia, but without the social outcomes. Here,
humans have interfered more broadly with the eco-
system. The reduction of the dingo population allows
the kangaroo population to increase, competing with
domestic stock for food and water during drier
conditions. Construction of large numbers of dams and
ponds for stock watering allows native animals to
survive into a drought; and introduction of feral
animals - such as rabbits, pigs, goats, cats, cattle, water
buffalo and donkeys - means passive native species
have been replaced with more aggressive species.
Many of these introduced species are ungulates
(hoofed animals) and cause soil compaction. Wholesale
ringbarking of trees has occurred, to such an extent
that in many areas there are not enough mature trees
left to naturally re-seed a region. Dieback is
endemic
.
Without deep-rooted trees and evapotranspiration,
watertables have risen to the surface, flushing salt from
the ground to be precipitated in insoluble form at
the surface. This salt scalds vegetation, leaving bare
ground susceptible to wind erosion. Soil structure is
altered and surface crusting takes place with the result
that wetting and water penetration of the ground
during rain becomes more difficult. In Victoria, salinity
affects about 5200 km
2
of land and is increasing
by 2-5 per cent per year, while in Western Australia,
3000 km
2
of arable land has been abandoned. Large
sections of the Murray River drainage basin are
threatened by this increasing problem, which is
exacerbated by irrigation. As a result, arable land in
some agricultural river basins within Australia is being
ruined at a rate ten times greater than Babylonian
civilization's degradation of the Euphrates-Tigris
system, and five times faster than it took to change the
Indus River floodplain into the Rajputana Desert.
DROUGHT CONDITIONS
EXACERBATED BY MODERN
SOCIETIES
(Bryson & Murray, 1976; Glantz, 1977; Lockwood, 1986)
Despite these technological solutions, human activity
can greatly exacerbate droughts through over-cropping
of marginalized land, massive vegetation clearing, and
poor soil management. These activities have affected
all semi-arid regions of the world. In the Rajputana
Desert of India, which was a cradle of civilization,
successive cultures have flourished, but each time
collapsed. The region is humid and affected seasonally
by the monsoon. However, the atmosphere is
extremely stable because overgrazing leads to dust
being continually added to it. Dust in the upper
troposphere absorbs heat during the day, lessening the
temperature gradient of the atmosphere and pro-
moting atmospheric stability. At night, dust absorbs
long wave radiation, keeping surface temperatures
elevated above the dew point. An artificial desert has
been produced that is self-maintaining as long as dust
gets into the atmosphere.
Even without massive dust production, clearing of
vegetation can itself establish a negative, bio-geophysical
feedback mechanism, locking a region into aridity.
In the Sahel of Africa, decreasing precipitation since
1960 has reduced plant growth, leading to reduced
evapotranspiration
, decreased moisture content in the
atmosphere, and a further decrease in rainfall. Over
time, soil moisture slowly diminishes, adding to the
reduction in evaporation and cloud cover. As the soil
surface dries out and vegetation dies off, the surface
albedo
- the degree to which short wave solar radiation is
reflected from the surface of plants - is reduced, leading
to an increase in ground heating and a rise in near-
ground air temperatures. This process also reduces pre-
cipitation. The destruction of vegetation exposes the
ground to wind, permitting more dust in the atmosphere.
Drought in the Sahel occurred concomitantly with
an increase in population and deleterious economic
conditions. In the 1970s, world fuel prices soared and
people switched from kerosene to wood for cooking
and heating. Such a change led to rapid harvesting of
shrubs and trees. With diminishing crop yields, land
was taken out of fallow, further reducing soil moisture.
Western techniques of plowing - introduced to increase
farming efficiency - destroyed soil structure, leading to
the formation of surface crusts that increased runoff
MODERN RESPONSE TO
DROUGHTS
Societies that expect drought: the
Unit
ed States
(Rosenberg & Wilhite, 1983; Warrick, 1983)
The above examples of national response to drought
would imply that underdeveloped countries have great
