4.2 Soil erosion

Although soil erosion is thought to be improving, the potential for erosion remains high where there is clearing of native vegetation, poor land management and inadequate preparation for droughts. Climate change leading to increased intensity or frequency of storms, bushfires, droughts and floods could accelerate erosion.

The best available hazard modelling indicates that 43% of the State's land has the potential to be eroded by water, causing losses of soil for agriculture that far exceed rates of soil development. Wind erosion continues to be a concern in the west of the State, particularly during droughts when ground cover is removed, and during cropping under some management practices. Exacerbating the problem is the presence of large areas of sodic soils across NSW which are particularly susceptible to erosion.

While the problem of soil erosion is thought to be gradually improving, there is still need for extensive remediation of affected areas identified in the 2002 National Land and Water Resources Audit, and ongoing changes to existing agricultural and urban development practices are needed to prevent the problem from worsening.

There is a need for information on the extent of actual soil erosion across the State.

NSW indicators

Introduction

Soil erosion by water and wind is a natural process that can be greatly accelerated by poor land-management practices. Soil erosion is a critical issue because soil is effectively a non-renewable resource (see Land 4.1), and soil health underpins the functioning of natural and agricultural landscape systems.

Many of Australia's soils are particularly susceptible to erosion if their protective ground cover is removed. In NSW, past soil and water management practices during urban development, forestry, agriculture and mining, as well as other disturbances of the soil, have greatly accelerated natural rates of soil erosion by reducing vegetation cover, which has increased runoff and reduced resistance to water and wind erosion (Edwards & Zierholz 2000). The potential for soil erosion by water is highest in landscapes with high rainfall intensity, steep slopes and inherently erodible soils. Wind erosion is potentially more of a hazard in the Western Division of NSW, in some cropping zones, particularly where land is allowed to remain fallow, and also in coastal sand dune areas. The potential for soil erosion increases during high-intensity, episodic rainfall or wind events, especially when these follow events that cause ground-cover depletion, such as droughts or bushfires.

Soil erosion leads to a loss of topsoil, organic matter and nutrients. It also degrades soil structure and decreases water storage capacity, thus reducing fertility and the availability of water to plant roots. Soil erosion is therefore a major threat to biodiversity. Soil erosion can degrade floodplains, riverine and coastal water quality, and aquatic ecosystems by significantly increasing sediment and nutrient loads (see Water 5.1 and Water 5.3). The costs to the community of restoration works and the decline in agricultural productivity from soil erosion are hard to quantify.

Current status and trends

Since 2003, there has been no new or updated statewide mapping of areas affected by actual sheet and rill erosion, gully erosion or wind erosion. The 2002 National Land and Water Resources Audit reported that, for an average year and based on the prevailing land cover at that time, up to 172 million tonnes of soil in NSW may be removed as a result of sheet and rill erosion alone. Anecdotal reports have suggested an overall improvement due to decreased rabbit numbers and rates of stocking during drought periods, and large tracts of erodible farm land being treated with structural earthworks. However, soil erosion rates are expected to have been much higher than average during conditions such as those experienced between 2002 and 2005, when drought affected up to 96% of the State. Increased erosion after bushfires that remove protective vegetation is also common, particularly in the steep forested lands of the Eastern Highlands (Chapman & Daly 2001). In the 2002–03 season alone, bushfires burnt over 1.6 million hectares of NSW and the ACT, affecting erosion and water quality (CRCCH no date).

Models of potential soil erosion can be used to predict maximum erosion rates under different ground-cover scenarios where actual erosion rates are unavailable. Map 4.2 shows the predicted potential mean annual rates for sheet and rill erosion for NSW, based on modelling undertaken in 2002. The map takes into account rainfall erosivity, soil erodibility, slope and ground cover. The mapping is considered most accurate where soil landscape mapping is available. New land-use data, when available, could be combined with potential soil erosion models to provide updated estimates of erosion rates from water.

Areas of the State where soil erosion is likely to be very high include large parts of the Western Slopes, the Monaro, Hunter Valley, the Northern and Southern tablelands and north-western NSW. To maintain soil quality and productivity in the long term, there needs to be a balance between the rates of soil formation and soil loss. While natural erosion rates would have exceeded formation rates before European settlement, modelling now indicates that soil is potentially being eroded faster than it is being formed over at least 43% of NSW. Although soil formation rates are poorly defined, it is thought that they are almost certainly less than 1 tonne per hectare per year (t/ha/yr) in upland areas, and are probably less than 0.5 t/ha/yr (Edwards & Zierholz 2000).

Map 4.2: Predicted potential mean annual sheet and rill erosion rates for NSW

Source: DNR, after NLWRA 2002

Table 4.1 summarises reported rates of soil erosion from a range of studies in Australia (Edwards & Zierholz 2000). The table highlights the correlation between erosion rates and land use, and the potential for very high rates of erosion in unfavourable circumstances and events.

Table 4.1: Soil erosion rates for a variety of land uses in Australia

Source: Adapted from Edwards & Zierholz 2000

Notes: ^(a) Unless noted otherwise

There is little statewide quantitative evidence to indicate trends in wind erosion and gully erosion. The potential for wind erosion in NSW is highest in the semi-arid and arid lands, where annual rainfall is below 600 millimetres and lighter sandy soils prevail, or in coastal sand dune areas (EPA 2003). The soils most prone to wind erosion are the lighter soils in the southern part of the Far West; the fine, sandy wind-deposited soils such as the Mallee lands of the central and south-western plains of the Murray–Darling Basin; and the coastal dune sands. As with sheet and rill erosion, the area susceptible to wind erosion is extended during times of drought as ground cover disappears. The extent of gully erosion has not been determined across NSW since 1987, but the Sydney Catchment Authority has mapped all gullies in its area of operations and is systematically remediating and treating them based on their potential to deliver sediment.

The predicted potential mean annual rates for sheet and rill erosion for NSW, combined with continued land clearing and poor land management in some areas, means that the potential for soil erosion remains high. The impacts of climate change, such as intense storms and frequent bushfires, are likely to accelerate erosion.

Response to the issue

Since the late 1980s, the approach to soil erosion on rural lands has changed progressively, from relying on engineering solutions, to prevention through improved land-management practices, although engineering works are still an integral aspect of soil retention in many agricultural situations. There is now a greater emphasis on integrated catchment responses to soil and land degradation, recognising that soil erosion is just one of a range of degradation processes that are usually linked. These processes include a decline in soil organic matter, acidification, soil structure decline, sodicity and salinisation, all of which can affect soil erosion potential and reduce soil health (see Land 4.1).

Over the last decade, awareness of the importance of soil and water management has increased significantly for regulated activities such as urban development, infrastructure construction, forestry operations and public land management generally. At the institutional level, since 2003 CMAs have had a major role in reducing soil erosion by recognising soil erosion as a priority concern in most catchment action plans. These plans aim to ensure appropriate government and private investment in both remedial and preventive action. Statewide natural resource targets for improving soil and landscape health will catalyse further attention and actions, and are expected to result in reduced erosion.

CMAs also manage incentive funding to encourage more-sustainable land-management practices in their areas of operation. Examples adopted by landholders include maintaining and enhancing vegetative cover, rotating land use between pastures and crops, and implementing other conservation farming techniques.

The planning, design and construction of earthworks for soil and water conservation is carried out through the NSW Government's Soilworks program, with around 30,000 hectares of land managed under the program each year. This fee-for-service program uses funds from landholders, Government agencies and CMAs.

By 2006, the NSW Soil Landscape Mapping program had covered 90% of NSW. This spatial framework of soil distribution is suitable for planning and decision making by landholders and CMAs.

The most critical factor in preventing and controlling further soil erosion by water and wind is the maintenance of an adequate ground cover. 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). This has been accompanied by fencing programs and other efforts to manage grazing around planted trees and shrubs, on remnant vegetation, on creek and river banks, and in saline and other degraded areas. However, native vegetation continues to be cleared in some parts of NSW (see Biodiversity 6.1).

Pasture and crop rotations have been widely adopted in the wetter areas of central and southern NSW, but are still not universally practised in some cropping areas with lower rainfall. Incorporating a pasture phase of annual legumes can replenish soil nitrogen levels and also help to rejuvenate soil structure, but may also contribute to soil acidification (see Land 4.4).

Conservation farming techniques help to reduce the threat of soil erosion by maintaining soil structure and allowing greater infiltration of rainfall. This in turn reduces runoff that erodes soils, and also increases the soil's moisture holding capacity that allows greater vegetation cover to establish and be maintained. By 2001, Government programs to promote and extend conservation farming techniques had resulted in approximately 70% of farmers using them in preparing their land for sowing (ABS 2001). Adopting this technology for cropping in seven local government areas in north-west NSW is estimated to have saved 18 million tonnes of soil per year and added around 70 kilograms of soil carbon per hectare annually (Scott & Farquharson 2004). Farming techniques using innovative technologies such as precision farming, zone management and alley farming are also showing potential both for maximising agricultural yield and improving on-farm environmental management.

The statewide natural resource management targets relevant to soil erosion are 'By 2015 there is an improvement in soil condition' and 'By 2015 there is an increase in the area of land that is managed within its capability'.

Future directions

Climate change may result in an increased frequency and intensity of drought, bushfire and storms in NSW, with ground cover becoming more difficult to maintain, leading to increased soil erosion. The Government, landholders and the community need to plan for the greater erosion risks associated with natural events and adopt management practices that are more appropriate. New approaches that include measures for environmental recovery in drought packages are a start only, and need to be strengthened.

An update of potential erosion rates may soon be possible from advances in discriminating different types of ground cover using satellite imagery and data about land-use and land-management practices (currently in preparation at national and catchment scales). This should allow CMAs to better target local and regional investment (see Land 4.1). However, knowledge is needed of the trends in actual degradation from erosion for determining whether investment is actually improving environmental outcomes and making progress towards statewide targets.

The reduction of soil erosion rates will require the wider adoption of sustainable land management, better preparation for droughts, and the remediation of past degradation. The wider adoption of more-sustainable practices will also assist in addressing a number of other land-management issues, including land clearing, riparian vegetation management, dryland salinisation, soil structure decline and soil acidification. Taken together these measures to reduce soil erosion will help deliver multiple outcomes towards soil health, biodiversity and river health.