SoE 2003 > Land > 4.2 Soil erosion

4.2 Soil erosion

Ongoing land-use changes, land management practices, drought and bushfires are leaving NSW soils vulnerable to erosion

The clearing of native vegetation and agricultural activities have been major contributors to accelerated rates of erosion across NSW. Droughts, bushfires and storms of high intensity have also exacerbated the extent of the State's erosion.

On average up to 172 million tonnes of soil is estimated to be lost as a result of sheet and rill erosion in NSW each year. The estimated average rate of sheet and rill erosion is five times the estimated rate of soil formation and almost three times the natural rate of erosion.

Action by both government and landholders to address soil erosion has increased significantly over recent years. However, the potential for soil erosion remains high in many areas.

NSW Indicator

Importance of the issue

Soil erosion by water and wind is a natural process. Many of Australia's soils are old, weathered and infertile, making them particularly susceptible to erosion. The potential for soil erosion increases wherever soil is disturbed or exposed by the removal of protective vegetation cover and during high intensity rainfall or winds.

Some land uses and management practices have greatly accelerated soil erosion by reducing surface cover, increasing runoff and reducing the resistance of soil to water and wind erosion (Edwards & Zierholz 2000). The clearing of native vegetation, excessive cultivation, bare soil and fallowing practices, overgrazing by introduced animals and pests, and poor matching of land-use activities with land capability have been directly associated with increased rates of soil erosion in NSW. Extreme events, such as drought, bushfires and floods, have also increased erosion. Degradation of riparian vegetation and changes in catchment hydrology have accelerated the erosion of creek and river banks. In urban areas, land clearing, soil disturbance and earthworks associated with development and redevelopment continue to expose soil to potential erosion.

The full extent of soil erosion in NSW has not been comprehensively documented. However, it is clear that in many areas soil loss has caused substantial land degradation. Soil erosion can lead to a loss of topsoil, organic matter and soil nutrients from the surface; degrade soil structure; and decrease water storage capacity, thus reducing availability of water to plants. Soil erosion can also degrade water quality and aquatic ecosystems by increasing sediment and nutrient loads to rivers (see Water 5.1 and Water 5.3). The costs to the community of restoration works and the decline in agricultural productivity are hard to quantify but are undoubtedly very high.

The indicator 'potential for soil erosion' has previously been quantified using inherent soil and site characteristics, such as soil type, slope and rainfall. Recent work by the National Land and Water Resources Audit (NLWRA 2001a) has improved reporting for this indicator by incorporating data on vegetation cover. While it is acknowledged that other forms of erosion, such as gully and streambank, affect NSW, this report focuses only on sheet and rill erosion and wind erosion.

Sheet and rill erosion

The National Land and Water Resources Audit has conducted a national assessment of water-borne erosion. Map 4.2 shows estimated mean annual sheet and rill erosion rates for NSW, based on erosion potential. For an average year, it is estimated that up to 172 million tonnes of soil in NSW may be moved as a result of sheet and rill erosion. This figure is expected to be much higher during drought conditions when ground cover is low. Map 4.2 highlights significant areas of the State where soil erosion is likely to be very high, including large parts of the Western Slopes, the Monaro, Hunter Valley, Northern and Southern Tablelands and north-western NSW.

Map 4.2: Estimated mean annual sheet and rill erosion rates

Source: Derived from Lu et al. 2001a and Lu et al. 2001b

Increased erosion after bushfires have removed protective vegetation is also a concern, particularly in the steep forested lands of the Eastern Highlands. For example, in the Blue Mountains, there was substantial sheet erosion and the formation of new gullies when heavy thunderstorms followed fires in 1997 (Chapman et al. 1999). Erosion is also higher when heavy rain falls after drought has depleted the ground cover. With 96% of NSW declared in drought at December 2002, much land is at risk of extreme erosion if heavy rains occur before vegetative cover is re-established.

To maintain soil quality and productivity in the long term, there has to be a balance between the rates of soil formation and soil loss. Although formation rates are poorly defined, it is thought that they are almost certainly less than 1 tonne per hectare per year (t/ha/yr) and more likely to be less than 0.5 t/ha/yr (Edwards & Zierholz 2000). While there has been progress in obtaining soil loss data, actual rates of erosion across NSW are largely undocumented (Edwards & Zierholz 2000; Olive & Walker 1982). No benchmarks for acceptable soil erosion rates have been adopted.

Table 4.4 summarises reported rates of soil erosion from a range of studies in Australia (Edwards & Zierholz 2000). A number of general conclusions have been made in relation to this data, particularly the correlation between high erosion rates and the intensity of land uses, and the potential for very high rates of erosion in unfavourable circumstances.

Table 4.4: Soil erosion rates for a variety of land uses in Australia

Source: Edwards & Zierholz 2000

Map 4.2 shows a breakdown of NSW in erosion rate classes. From these estimates, approximately 9% of NSW experiences high to very high sheet and rill erosion, with about 17% of the area eroding at a greater rate than the State average of 2.5 t/ha/yr. If an acceptable soil erosion rate is considered to be equivalent to the soil formation rate of 0.5 t/ha/yr, around 43% of the State's land is being eroded at a rate which is unsustainable. It is clear that soil loss rates from all but the most conservative land uses are likely to exceed those rates of formation and consequently lead to further degradation of the State's soils.

Comparing the current estimates of soil erosion with those of pre-European settlement conditions provides an understanding of the sheet erosion that can be attributed to the changes in land use (NLWRA 2001a). Data derived from the National Land and Water Resources Audit shows that, on average, the current rate of sheet erosion in NSW exceeds the natural rate by 2.7 times. Areas which exhibit the greatest change in erosion rates are in coastal catchments, congregated around the estuaries and extending further inland to the eastern margins of the Great Dividing Range. High to very high erosion, ranging from 25 to greater than 50 times that observed under natural conditions, occurs in significant areas of the Tweed, Richmond and Hunter catchments as well as the Snowy catchment in the south-eastern part of the State.

The greatest change in erosion rates is associated with agriculture, particularly the grazing of livestock on native and improved pastures and permanent cropping. Grazing is the land use that contributes most to total erosion across assessed river basins because of the vast areas involved (NLWRA 2001a).

In the Murray–Darling Basin, many of the areas which were cleared, overstocked or plagued by rabbits in the 19th century are showing signs of stabilisation as vegetation cover is re-established and the rates of erosion are slowly declining from their peaks. However, the current levels of erosion are still many times higher than those of pre-European times, and the legacy of the past 180 years continues to have a significant impact on both the environment and agricultural productivity (Scott 2001).

Wind erosion

Most wind erosion in NSW occurs in the semi-arid and arid lands, where annual rainfall is below 600 millimetres and lighter sandy soils prevail. The area susceptible to wind erosion is extended during times of drought. In October 2002, with drought conditions affecting over 90% of the State, dust storms swept over south-eastern Australia. It was estimated that the storms contained up to 20 million tonnes of soil (DLWC 2002a).

Map 4.3 shows groupings of wind erodibility for soil across the State. These groupings assess the susceptibility of land to wind erosion if the soil is dry and exposed. The soils that are most prone to wind erosion are the lighter soils in the southern part of the Far West, the fine sandy soils such as the mallee lands of the central and south-western plains of the Murray–Darling Basin, and the coastal dune sands. Isolated occurrences of soils prone to wind erosion are found in other areas. These areas need careful management to keep susceptible soils in place, especially when affected by drought.

Map 4.3: Wind erodibility of soils

Source: DLWC data, as at 2002

Notes: Wind erodibility, as calculated here, is independent of site conditions such as climate, vegetative cover or surface roughness. There is no data for approximately 14% of NSW and much of this area is in western parts of the State. Additionally, many catchments, especially those in western NSW, had inadequate data on land characterisation.

Response to the issue

Institutional responses to the problem of soil erosion have changed progressively over many years, with a shift recently from remediation to prevention through better land management practices. There has been a greater emphasis on total catchment responses to soil and land degradation rather than engineering solutions, although the latter are still an integral aspect of soil retention in many agricultural situations. This change in emphasis reflects a growing recognition that soil erosion is just one of a range of degradation processes that are usually linked. These include soil organic matter decline, acidification, structure decline, sodicity and salinisation, all of which can affect soil erosion potential.

Another recent change of emphasis has been a greater involvement by the community in identifying and managing environmental degradation, including soil erosion. Following the success of the Landcare program and using the concepts of Total Catchment Management, the NSW Government recently established catchment management boards (CMBs), regional vegetation committees and water management committees to oversee and guide decision-making and resource allocation. The primary change is that CMBs have been entrusted with the responsibility to plan and implement natural resource management activities using a series of explicit guidelines. The resulting Catchment Blueprints have been prepared in consultation with agencies and the local community, which continues to have a key role in identifying and prioritising the most significant environmental issues within each catchment.

Specific government initiatives that are contributing to the management of soil erosion include:

• financial assistance to landholders in the Warragamba, Shoalhaven, Upper Nepean and Blue Mountains water supply catchments to reduce soil loss from properties into rivers and streams through the Catchment Protection Scheme
• planning, design and construction of soil and water conservation earthworks through the Soilworks program with around 30,000 hectares of land managed under the program each year
• the National Land and Water Resources Audit, which has provided data for modelling at the national scale and mapping of a range of soil and land properties, including the potential for soil erosion
• mapping the soils of eastern and central NSW through the NSW Soil Landscape Mapping program with a further 15% of the mapping target area completed to at least reconnaissance level since 2000 and the program due for completion in 2010.

The initial response to soil erosion in NSW was the widespread use of engineering solutions, such as earthworks, contour banks and waterways, to trap, divert and slow down the surface flow of water. These measures provide important insurance against the risk of erosion when soil is bare following cultivation. Over time, however, it has become more widely recognised that there is an urgent need to adopt land management practices that will minimise the rate of soil erosion in conjunction with these engineering solutions. Maintenance and enhancement of vegetative cover, pasture-crop rotations and adoption of conservation farming systems have been important aspects of this approach (Pratley & Robertson 1998).

The most critical factor in preventing and controlling further soil erosion by water and wind is the maintenance of an adequate vegetation cover. To protect against water erosion it is recommended that landholders maintain 70% plant cover in higher rainfall areas on the slopes and the coast, 30–40% in the far west and 40–50% on the eastern edge of the rangelands (Lang 1990; Eldridge 2000). There has been a significant increase over recent years in the number of tree and shrub seedlings being planted by farmers on agricultural lands specifically for nature conservation and to protect land and water resources (see Biodiversity 6.2). Efforts to protect planted trees and shrubs, remnant vegetation, creeks and rivers, saline and other degraded areas on farm properties from grazing activities have also increased, primarily through the construction of fencing.

A pasture phase based on annual legumes replenishes soil nitrogen levels and also rejuvenates soil structure. However this practice may cause acidification of soil (see Land 4.4). Pasture and crop rotations have been widely adopted in the wetter areas of central and southern NSW but are still not widely practised in some cropping areas with lower rainfall, such as north-western NSW, as few pasture species are suited to these areas (Pratley & Robertson 1998).

Conservation farming techniques reduce the threat of soil erosion by maintaining soil structure and allowing infiltration of rainfall rather than runoff and erosion. The technique is a viable tillage system that creates a suitable soil environment for growing a crop, but which also conserves soil and other resources. Conservation farming promotion and extension programs by government have largely been taken up by farmer groups. Adoption of conservation farming techniques is illustrated by the finding of the 2001 ABS Agricultural Census that approximately 70% of farmers were using them in the preparation of their land for sowing.

The gradual adoption of new farming techniques using innovative technology, such as precision farming, zone management and alley farming, are also showing potential both for maximising agricultural yield and improving on-farm environmental management.

Effectiveness of responses

There has been a significant increase over recent years in activities by both government and landholders to address soil erosion. However the potential for erosion remains high in many areas as a result of continued clearing of native vegetation, inappropriate agricultural land-use activities, droughts, bushfires and storms.

Recent NSW Government initiatives such as setting up catchment management boards, regional vegetation committees and water management committees are in their early stages. These rely on strong community involvement and education to prevent further soil erosion. Based on previous programs involving the farming community, such as Landcare, these are likely to generate positive results.

Future directions

Soil erosion is linked very closely to a number of other land management issues, including land clearing, riparian vegetation management, dryland salinisation, soil structure decline and soil acidification.

Landholders can adopt land management practices that will minimise erosion and other forms of soil degradation. Examples include maintenance and enhancement of vegetative cover, pasture-crop rotations and conservation farming techniques. In some instances, financial incentives are available. More information on sustainable land management practices and programs to address soil erosion is available from the Department of Infrastructure, Planning and Natural Resources and NSW Agriculture. Landholders need to ensure that they are prepared and able to manage the erosion risks associated with natural events, such as droughts, bushfires and storms.

Government agencies can continue to address soil erosion by supporting the activities of catchment management boards, regional vegetation committees and water management committees and by providing resources and assistance to implement Catchment Blueprints.

The Government can also provide support by increasing knowledge and awareness of soil erosion and ways to address it. Soil erosion issues could be fully integrated within other land management extension services. There is also a need for further work to provide appropriate soils data at the local catchment level.

Linked issues

4.1 Land-use changes

5.1 Freshwater riverine ecosystem health

5.3 Surface water quality

5.6 Marine and estuarine water quality

6.2 Native vegetation clearing

6.5 Fire

6.6 Aquatic ecosystems

6.7 Aquatic species diversity

6.9 Aquatic harvesting