Land Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Accelerated erosion and loss of surface soil (continued)
Condition (continued)
Dust storm and accelerated erosion indices [L Indicator 1.6]
Wind erosion has both a natural and human-induced component in most semi-arid and arid regions. In the past 15 000 years Australia has had a wetter climate than during the previous 5 000 years of severe aridity. Dune fields were once vegetated, but in the past 150 years they have been grazed or cleared for agriculture in some regions, and this has contributed to more dust storms than would otherwise occur with today's climates. Dust has been recorded by climatological stations across Australia for many years, but automation is now phasing out reporting of this useful indicator of wind erosion.
The 1996 State of Environment Report provided dust storm frequency data to show that the annual frequency of dust storms across Australia during the 20th century has notably decreased since the 1970s (SoE 1996, p. 6-30). This was attributed to the improved control of rabbits (through myxomatosis); the spread of 'woody weeds' and Acacia nilotica (Atlas of Australian Resources 1990); and the adoption of conservation tillage. Since that report, developments in the methodology have allowed wind erosion under natural conditions (dust storm index) to be differentiated from the component attributable to direct and indirect human activity (accelerated erosion index) (McTainsh 1998). This method of distinguishing natural from anthropogenic wind erosion was first used to identify the continental occurrence and distribution of accelerated erosion over the period 1986-1996 in a report on Australian agriculture (SCARM 1998).
Fitzroy and Murrumbidgee catchments
An example of current high impact accelerated erosion
The Fitzroy River in Queensland drains an area of 150 000 km2(15 million ha), and the flows are highly episodic. A massive 5 million tonnes of sediment, rich in nutrients and some pesticides, move through the catchment to the estuary at Rockhampton each year. Agricultural lands occupy over 95% of the catchment, of which 87% is used for pastoral grazing. This is the major contributor to runoff because, although the soil loss per hectare is small, the vast area of poorly managed grazing lands all contribute. Rain-fed cropping (0.8 million ha) is also a significant contributor to accelerated erosion, as the land is often left fallow for long periods. Small intensive industries such as irrigated agriculture and mining are of concern mainly in relation to the release of pesticides, nutrients and acid waste drainage. Integrated government agency initiatives aimed at halting these impacts are focusing on improving management practices through the extension of best practices in grazing and cropping systems. (See detail on nutrient loads in the section Nutrient and carbon cycling.)
The legacy of past accelerated erosion
The Murrumbidgee catchment is a major subcatchment of the Murray-Darling Basin, covering an area of 73 400 kmkm2. Flowing from the alpine areas of the Great Dividing Range through the southern Tablelands to the Riverine Plains, it is highly variable in topography, climate and land uses. The Snowy Mountains Hydroelectric Scheme lies principally within this catchment. Its 14 major dams and eight weirs supply water via 10 000 km of distribution channels to irrigation areas. In the upper reaches of the basin, land uses are pasture, conservation reserves, and expanding residential areas. The mid and lower areas have a patchwork of irrigated agriculture, forestry, grazing, rain-fed cropping and small urban settlements. The catchment generates more than A$1 billion of agricultural produce and A$500 million in tourism annually, but water quality deterioration from nutrient and salt contamination is causing grave concern.
A small sediment-loaded stream with steep banks, typical of active accelerated gully erosion.
Source: Ann Hamblin
Algal blooms are fed by phosphates and organic colloids bound to suspended clay in the waterways. Their source is deep gullies that have resulted from accelerated erosion of the unstable, rapidly dispersed sodic subsoils in the upper parts of the catchment (Wallbrink 1998). Although these gullies are now mainly stabilised (Wasson et al. 1997), the impact of the movement of the sediments into the river beds and the highly artificial regulation of the river flows continue to exert adverse effects on many ecosystem elements.
Planned changes in land use to help control further deterioration include increased tree plantings, major changes to water use and allocation (see the Inland Waters Theme Report) and further rural land use diversification, but recent predictions on the extension of future secondary salinity and nutrient expansion are not optimistic (see Soil and land pollution).
Algal blooms annually cause death of aquatic life-forms in the Murrumbidgee system.
Source: Ann Hamblin
Wind erosion from 1986 to 1996
Since the SCARM report on sustainable agriculture (SCARM 1998), the data available from the Bureau of Meteorology has been revised and improved, and an improved climatic model of wind erosion has been developed. The general pattern for the decade 1986-1996 is similar to that in the earlier analysis, but there are a number of significant differences which are explicable in terms of differences in the meteorological data, including the addition or removal of some recording stations. A total of 109 stations were used for the present report.
Figure 16 compares the natural wind erosion with the accelerated erosion resulting from direct and indirect human activity. Measurable natural wind erosion occurs over a larger area of the continent than accelerated erosion, and accounts for the larger proportion of total dust lifted into the atmosphere. Seasonal conditions have much to do with this. Figure 17 shows how severe the extent of natural wind erosion can be in droughts (El Nio years).
Figure 16: Dust storm index (natural erosion) and accelerated erosion index (anthropogenically related) for the decade 1986-1996.
Source: McTainsh et al.(2000)
Figure 17: Dust storm index for the El Nio year 1994.
Source: McTainsh et al. (2000)
In 1994 there was very active wind erosion in the Lake Eyre Basin region, the Mallee region, Eyre Peninsula and south-western Western Australia (Figure 17). In response to the drought conditions in eastern and southern Australia, wind erosion spread east into central Queensland and New South Wales, and south into Victoria. The 1994 season was classified as exceptional (occurring only 5 years in 100) in much of eastern Australia (Figure 17). The areas where dust storms were recorded expanded to cover nearly the whole continent other than the areas receiving over 500 mm average annual rainfall. The extension of visible dust storms expanded well into south-western Western Australia, an area that has had relatively less wind erosion over the past 20 years because of better than average seasons.
Wind erosion from 1996 to 1999
The main region of wind erosion during 1996-1999 is bounded by the 25-year average 500 mm rainfall isohyet (Figure 18), which includes most of the Lake Eyre Basin and the lower Murray-Darling Basin. Western Australia has had less active wind erosion.
Figure 18: Dust storm index with 300mm and 500mm median rainfall lines, and accelerated erosion index for 1996-1999.
Source: McTainsh et al. (2000)
The areas with higher probable accelerated erosion index (AEI) have tended to be within the semi-arid zone (300-500 mm rainfall) rather than in the arid heart (<300 mm rainfall) of the Lake Eyre Basin. 'Hotspots' are around:
- Thargomindah (south-western Queensland),
- Tibooburra and White Cliffs (north-western New South Wales),
- the Hay-Balranald-Mildura region in south-western New South Wales,
- Eyre Peninsula in South Australia,
- Southern Cross in southern Western Australia, and
- Marble Bar in north-western Western Australia.
These areas have the vulnerable combination of erodible soils and land uses that accelerate wind erosion. Cattle grazing is the main land use in south-western Queensland, north-western New South Wales and north-western Western Australia. The high AEI values in south-western Queensland and north-western New South Wales may also, to a certain extent, reflect the active wind erosion occurring in the Strzelecki and Simpson Deserts' dunefields to the west, because most dust storms in that region originate from the western sector.
From 1994 to 1999, overall climatic conditions ameliorated, with persistent drought only in southern Victoria (Figure 18 shows the continued drought conditions there). Wind erosion activity decreased during this period and contracted to the south, largely in response to changes in rainfall, leaving the arid and semi-arid western region of the continent more stable. In contrast, the semi-arid regions of South Australia, east of Port Pirie and Marree, had a slight increase in AEI. Persistent drought in Victoria, however appears to have maintained erosion activity in the Mallee area and southern New South Wales.
By 1999 the northern loci of accelerated wind erosion shifted east and south to White Cliffs. The hot spot in the eastern wheatbelt of Western Australia dramatically reduced, but the rate of erosion increased in the Mallee of Victoria, with a new locus appearing around Ouyen in 1999.
Possible land management effects
Rabbits
One of the major management-induced changes in semi-arid regions in the late 1990s resulted from the release of the rabbit calicivirus. This effect was most pronounced in the Strzelecki and Simpson Deserts regions, where there was widespread vegetation recovery, aided by increased rainfall (Neave, 1999).
The general southward shift in the epicentre of wind erosion during the late 1990s is very likely to be in response to rainfall-induced improvements in vegetation cover. However, the accelerated erosion index also moved south, indicating that wind erosion rates were reduced by more than can be explained by climatic conditions. This evidence lends support to view that rabbit depopulation may have resulted in measurable reductions in accelerated wind erosion in the semi-arid regions of south-western Queensland, northern South Australia and far western New South Wales.
Conservation-oriented agricultural methods
During the late 1990s expansion of conservation-oriented agricultural methods continued in cereal cropping regions. The reduction in wind erosion in south-western Western Australia since 1994 was accompanied by increases in rainfall, but as the accelerated erosion index in this region also decreased, erosion rates have evidently been reduced by more than would be expected from climatic conditions alone. It is reasonable to infer that conservation-oriented agricultural methods are reducing wind erosion.
Both total wind erosion and the accelerated component decreased in the Eyre Peninsula of South Australia, suggesting a lesser effect from conservation cropping. In the Mallee area and further east in central New South Wales, however, both total wind erosion rates and the accelerated component have remained reasonably static, except in parts of western Victoria. In these areas fallowing is still practised much more widely than in either Western Australia or South Australia (Latta 1997).
A long-term trend analysis of the AEI over the period 1965-1996 (McTainsh et al. 2000) showed that there is a very close relationship between widespread droughts and increased wind erosion caused by human-related activities. This highlights the need to maintain surface cover and reduce the effects of stock grazing during these periods.
Implications
Climate trends have been responsible for some of the changes in the natural erosion rate, but improvements in data richness and computation have resulted in a revision of earlier estimates. Changes to management practices and vegetation cover (resulting from both human activities and climatic variation) have had a measurable effect in certain areas. It can now be claimed with some confidence that a distinction can be made between areas in the southern grain belt where conservation cropping is practised and areas where it is not. Consequently, the patchy adoption of conservation practices in the eastern Mallee regions is a cause for concern.
It is very encouraging that there has been a reduction in accelerated wind erosion in the central arid areas where rabbit populations have been most severely reduced. The challenge will be to maintain these advances.