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| Australia Desertification characteristics. |
AUSTRALIA
Land degradation in Australia is, from the standpoint of area affected, a
problem of overgrazing of range lands. Window and water erosion and
salinization of irrigated and nonirrigated land are serious locally, primarily in
small areas across the southern part of the continent, and their economic
impact probably exceeds that due to overgrazing. Land degradation is usual
around farming communities and popular tourist areas such as Ayers Rock.
Overgrazing began 50 to 100 years ago with settlement of the interior arid
lands at a time when there was no information on the carrying capacity of
range land, and little knowledge of climatic averages and extremes. During
the first quarter of the 20th century, problems of wind and water erosion,
salinization and waterlogging of irrigated land, and saline seepage of
dryland areas arose as cultivation spread into the drier areas. Remedial
action was undertaken on a concerted basis in the 1930s and l940s when
states enacted soil conservation legislation and established units to correct
land abuses and prevent further deterioration. At present, it probably is safe
to say that overgrazing is much less of a problem than it was before 1940,
that wind and water erosion still occur but to a lesser degree than before,
that salinization of irrigated land in the Murray River catchment (watershed)
is increasing, and that saline seepage of semiarid and subhumid lands is
expanding steadily.
Desertification Characteristics
About 25 percent of Australia's arid regions are unoccupied (Condon, 1978)
due to the unpalatability of spinifex grasses or to the remoteness of the land
from population centers. Most of the remaining 75 percent is moderately
desertified (Figure 4). Severe desertification is most extensive in the
saltbush-bluebush (Atriplex-Maireana) vegetation type occurring in New
South Wales and South Australia, where overgrazing has caused
degeneration of the plant cover. Degradation of the vegetation due to
overgrazing has also been severe on the fine-textured lowland soils
(Vertisols) of eastern Australia and in the flood plains and surrounding
slopes of coastal river valleys, particularly the Gascoyne, Ord, and Victoria
catchments in Western Australia and the Northern Territory. Gully erosion
has been severe on the Ord River catchment and significant on the other
catchments, with greatly increased runoff where the trampling of livestock
has developed water-repellent surface crusts. The presence of large
numbers of feral donkeys in the Victoria River watershed compounds the
problem of grazing control there.
Areas delineated on the map as severely desertified are not uniformly
degraded. The typical spotty character of desertification shows up in river
catchments as badly overgrazed and gullied land on flood plains and on the
gentle slopes adjoining them, with moderately desertified areas in the less
accessible parts of those shrub steppe zones.
Distribution of saline seeps on drylands, and salinized and waterlogged
irrigated lands (Mabbutt, 1978), is also spotty. Seepage of salt-laden water
on slopes has plagued cultivated lands in the semiarid and subhumid
regions of Western Australia, South Australia, New South Wales, and
Victoria for decades. The resulting saline soils on lower slopes and in
waterlogged depressions ordinarily cover a few or several hectares and are
surrounded by nonsaline soils. In Western Australia where the problem is
most acute, seepage of saline groundwater into streams has also had a
significant adverse effect on water quality.
Waterlogging in the Murray River and its tributaries is characterized by
groundwater mounds affecting tens of thousands of hectares of irrigated
land (Pels, 1978). The high groundwater levels lead to secondary
salinization of soils as the salts originally present in the subsoils are carried
to the surface through evaporation from the shallow water tables. Since
waterlogging and salinization are functions of the irrigation water
management and the presence or absence of a drainage system, salty land
is interspersed with nonsalty land.
Wind and water erosion have been severe in the past but are less so now,
by and large. Wind erosion is an old problem in the grazing and cultivated
lands of New South Wales (Wagner, 1978) and the wheat lands of the
Murray Mallee (Matheson, 1978). One of the special problems in the
semiarid Australia is the appearance of "scalds," when sandy surface soils
are blown or washed away and the slowly permeable subsoil of
texture-contrast (duplex) soils is exposed. Revegetating scalds is difficult
unless steps are taken to impound water on the surface. Mining towns such
as Broken Hill have experienced accelerated wind erosion and dust storms
as the land around the towns was denuded of vegetation.
Magnitude of Desertification
A 1969 report on land degradation in Australia (Newman and Condon,
1969) included an estimate that about 14 percent of the arid rangelands
were severely degenerated in pastoral value, 23 percent were moderately
degenerated, and 63 percent showed minor, little, or no degradation.
Condon (1978) notes that much of the serious erosion and degeneration of
rangelands that was evident until the late 1940s was a consequence of
heavy use during the late 1800s. Since the 1940s there has been
considerable improvement in range conditions, partly due to abnormally
favorable rainfall over the last thirty years. The magnitude of desertification in
Australia is shown in Table 5.
About 1 percent of the cultivated land in Western Australia is affected by
saline seepage, with lesser percentages affected in the other states.
Northcote and Skene (1972) estimate that 85,000 hectares of irrigated land
in Australia are salt-affected, most of it in the Murray Basin. They also state
that at least 197,000 hectares of dryland are affected by secondary
salinization. Waterlogging and salinization go hand-in-hand in the Murray
region.
Techniques to control desertification are well-known in Australia, the
problem is recognized as a serious one, and state agencies are available to
assist in carrying out reclamation and prevention programs. Grazing
pressure probably has been reduced by half since the late 1800s and early
1900s and wind and water erosion in the more populous southern part of the
continent is much less of a problem now. Soil salinity and waterlogging are
more difficult to control because of the cost arising from the special
conditions in the Murray irrigated area. One method used to dispose of
saline drainage water without increasing the salinity of the Murray River
water downstream is to divert it into evaporation basins. The principle is
good but the cost is high and there are uncertainties about future
contamination of groundwater by effluents from the basins.
