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| Desertification characteristics. |
AFRICA
The severe Sahelian drought that extended from through l969 focused the
world's attention on the human aspects of land degradation and led to the
convening of the 1977 United Nations Conference on Desertification. Drought,
however, is not the cause of desertification; man is. The drought served only to
place additional stress on the biological resources of the Sahel. If resource
management has been good, little, if any, permanent damage is done by
droughts. However, if resource management has been unwise, a drought
accentuates the adverse impact of that management and accelerates land
degradation (Weaver and Albertson, 1940). The latter is what has occurred
widely in the Sahel and elsewhere.
All of the usual forms of desertification are present in the arid regions of the
African continent and are manifested as serious local or regional problems
(Figure 2). Overgrazing has reduced range productivity virtually everywhere
outside the tsetse fly regions, in north, west, east, and south Africa. Wind and
water erosion have devastated landscapes in the cultivated regions and in
much of the rangelands (Rapp, 1974). Shortened fallow periods in the shifting
cultivation system south of the Sahara have led to severe depletion of plant
nutrients. Salinization and waterlogging of irrigated land is worst in the Nile
Valley and North Africa but also occurs elsewhere. Mining has left surface
scars wherever it is practiced. The environmental degradation continues and
shows no sign of slowing down.
Desertification Characteristics
A number of factors have increased land degradation and the vulnerability of
the African arid regions to desertification. Most of them have had similar
effects in Asia and Latin America. They can be grouped in three categories:
(1) increased human and animal population, (2) improved health services, and
(3) injudicious use of technology.
Due to the increased sedentary population, pressures on cultivated land led to
a shortening of the fallow period in the shifting cultivation cycle and the
extension of cropping into the more precarious drier regions. Crop harvests
became less reliable and more variable as the desert edge was approached.
Concurrently, nomadic pastoralists were deprived of some of their best
grazing lands as the cultivators moved in (Delwaulle, 1977). At the same time
the rangeland area was contracting, populations of pastoralists and their
livestock were increasing and the provision of improved veterinary services
and the lack of a viable marketing system helped assure that animal numbers
would grow rapidly (Widstrand, 1975). The result was inevitable: overgrazing
and accelerated desertification.
Overgrazing inadvertently was made worse, particularly in the Sahel, by the
drilling of additional wells that provided drinking water for livestock throughout
the year. Without the rest period that intermittent water supplies previously
assured, forage conditions deteriorated around the wells where water was no
longer a limiting factor in livestock survival. Local authorities did not or could
not impose a control system that would allow forage plants to recover from
heavy grazing.
A crude estimate of the amount of crop yield reduction that could be attributed
to salinization and waterlogging in the Nile Valley indicates it to be at least 17
percent on 80 to 90 percent of the old irrigated land. In the newly irrigated
lands east and west of the delta, the figure is likely to be a minimum of 25
percent yield reduction. Both problems are serious ones that can become
even more serious in the future if remedial measures are not taken. The
situation in Algeria and Tunisia is similar to that in Egypt with respect to yield
reductions.
Solutions to desertification problems in Africa are known-- and in general--can
be implemented readily if resources are available to do so. An exploding
population in the developing countries, however, means that land pressures
will continue to build. Reducing livestock numbers in the grazing lands until
forage productivity can be improved is very difficult, at best, and introducing
range use controls on communal land is not easy. There is little likelihood that
marginal dryfarm lands will be returned to pastoral use unless production on
the better lands is increased enough to offset the loss of cultivated land. While
the latter can be done, progress in that direction is very slow.
Destruction of woody vegetation has been hastened by the ever-increasing
need for firewood to meet the demands of the larger population. The
destruction is especially noticeable around the rapidly growing urban centers,
where the circle of deforested lands gets larger every year (Delwaulle, 1973).
While desertification was a long-standing problem even in the absence of
droughts, the gradually increasing vulnerability of the land made the impact of
the inevitable droughts worse than ever (Dahl and Hjort, 1979). The factors
responsible for that vulnerability are still operating, desertification continues,
and future droughts will have ever-greater damaging effects.
Salt-affected soils are common in Tunisia and Algeria where both surface and
well irrigation waters frequently are saline. Salinization is important in other
irrigated areas but on a small scale.
Magnitude of Desertification
About 18 percent of the arid region of Africa is severely desertified, and most
of that represented by grazing lands and rain-fed cropping lands on the south
side of the Sahara (Table 3). The other large area that is severely affected is
the mountain slopes and the plains of North Africa. Moderate to high salinity
affects about 30 percent of the irrigated land in Egypt (Aboukhaled et al.,
1975).
Wind erosion is dominant in the drier regions and water erosion on the wetter
sloping lands. Ethiopia, Kenya, and the Maghreb countries of Algeria,
Morocco, and Tunisia have been subjected to especially serious water
erosion, whereas wind erosion has been most damaging in sub-Saharan
West Africa. While good data on the effect of land degradation on crop and
livestock yields are not available, it seems likely that soil fertility losses, alone,
have reduced dryland crop yields by 25 to 50 percent in the severely
desertified areas. Animal productivity may well have declined by at least 50
percent nearly everywhere that domestic livestock are raised. In many areas
south of the Sahara, rangeland forage production probably is less than 25
percent of the potential.
ASIA
Desertification in the arid regions of Asia is characterized by overgrazing of
the rangelands of the Middle East and Central Asia, water erosion of
cultivated lands from eastern China to the Mediterranean Sea, and
salinization and waterlogging on a large scale in Iraq, Pakistan, China, and
the Soviet Union. Mining (including oil and gas production) has caused severe
land damage wherever it is carried on.
Overgrazing, soil erosion, and salt damage to irrigated land are long-standing
problems in the Middle East and Central Asia, as is water erosion on the
rain-fed cultivated lands of India, Pakistan, and the loessial plateau of China.
Waterlogging and salinization are problems centuries old in the lower plain of
the Yellow River in China, but are of relatively recent origin in the Indus Basin
of Pakistan and India. Ameliorative measures to reduce land degradation
have been undertaken in every country affected, with varying degrees of
success. The problem is an immense one in terms of the area involved and
the amount of degradation that has occurred.
Desertification Characteristics
Man first placed his imprint on the landscapes of Asia thousands of years
ago. Historical records make it clear that the destructive impact of man was
already apparent two or three thousand years ago in China and the Middle
East. It was then that the first period of accelerated, man-made erosion began
(Dregne, in press). Woodcutting and cultivation of sloping lands were
responsible for most of the land degradation that occurred in the uplands of
China and around the Fertile Crescent. Sedimentation of canals,
waterlogging, and salinization combined to raise havoc in the Tigris and
Euphrates valleys of Mesopotamia (Jacobsen and Adams, 1958).
Grazing pressure seems to have become a much greater problem in the past
two or three decades than it was before. There has been an explosion of
human and livestock population recently from India (Office of Environmental
Planning and Coordination, 1977) to the Middle East (Pearse, 1971).
Cultivation has been extended into pastoral lands, wells have been
constructed to make pastures usable throughout the year, and improved
transportation facilities have made formerly remote areas accessible to
livestock. As a consequence, cropping has become a high-risk land use in the
climatically marginal areas that formerly were the best grazing lands. At the
same time, overgrazing of the remaining range lands is accelerated by the
increased concentration of livestock on drier lands. Destruction of vegetative
cover on sandy lands has led to widespread wind erosion, depletion of soil
fertility, and the undertaking of expensive erosion control projects in China,
Iran, Saudi Arabia, and elsewhere.
Magnitude of Desertification
Asia has large areas where desertification is so severe that it borders on
being classed as very severe desertification, but about half of the land is only
slightly desertified (Table 4). Water erosion is spectacular in the loessial
plateau in the middle reaches of the Yellow River. The eroded silt is
sometimes so thick in the river that at Shanhsien, east of Xian, the sediment
load has been measured at 46 percent. The river annually carries 1.3 billion
tons of sediment, enough to cover 1,800 square kilometers of land 30
centimeters deep (de Crespigny, 1971). Virtually all of that represents loess
eroded from 300,000 square kilometers of highlands. Efforts are being made
to reduce erosion but the problem is so huge that progress is slow. Only the
great thickness of the aeolian deposits has allowed cultivation to be continued
on the eroding slopes.
Water erosion also approaches the spectacular on the slopes of the dryland
farming areas of western India and northern Pakistan. Fortunately, the soils
there are more resistant to displacement than in loessial material.
Nevertheless, the damage--in the form of reduced yields--is serious nearly
everywhere, and it is getting worse.
Salinization and waterlogging are as bad--and as difficult to control--in the
irrigated region of southern Iraq as they are anywhere in the world. Deep
fine-textured soils and almost flat topography make it hard to provide drainage
that is adequate to maintain a deep water table and prevent salt accumulation.
Techniques of soil and water management that are suitable for conditions in
Iraq have been developed and tested successfully, but the technical problems
are less difficult to resolve than the social ones (Dougrameji and Clor, 1977).
Land reclamation is much easier on the more permeable soils of the Indus
Valley and Central Asia.
Range productivity over most of the Middle East and western Pakistan is
among the lowest in the arid world. A small beginning has been made in
bringing the pastoral lands up to their potential productivity. Much remains to
be done, however, because range management is only a young science in
Asia. Recent emphasis on sand dune stabilization has led to a number of
successful efforts to find effective methods and to identify adapted plants.
Accomplishments have been outstanding, under very difficult circumstances.
A rapidly growing population and limited land resources mean that combating
desertification will be difficult for the developing nations of Asia and very
difficult for the poor countries. Land pressures must somehow be relieved,
and that can only be accomplished by raising productivity and yields per unit
area. A start can be made with presently available knowledge, but research
and training will be needed to broaden the knowledge base and to provide
guidance to the pastoralists and cultivators.