Land: Water erosion
Author
Bruce Carey, Department of Natural Resources and Water
Reviewer
David Freebairn, Department of Natural Resources
and Water
Key findings
- Most land in Queensland, depending on how it is used and managed, is susceptible to erosion by water. Activities such as grazing, cropping, forestry, mining, urban development and the construction of infrastructure can all make the land vulnerable to erosion.
- The drought conditions experienced by much of the state in the past four years have reduced the incidence of water erosion. However, the significant rainfalls that have occurred in parts of north Queensland would have resulted in water erosion in vulnerable areas. Bare ground is a common feature in drought-affected areas, and any rainfall that occurs during the drought or when the drought is broken can cause erosion. Lack of erosion-producing rainfall leads to complacency among land managers, agencies and industries responsible for soil conservation; this may lead to disastrous outcomes related to a lack of skills and resources when erosion becomes an issue again.
- As grazing is carried out on around 85% of the state it has the potential to have the most significant impact on soil erosion. Erosion control in grazing lands relies primarily on the management of stocking rates to maintain adequate levels of cover on the soil surface. The extended drought has led to high grazing pressures over large areas, and native and pest animals have compounded the problem.
- Significant progress continues to be made in the control of erosion in cropping lands, especially in relation to the adoption of improved methods of maintaining the soil surface cover through various forms of conservation tillage. The drought conditions, however, have reduced cropping frequencies in some areas, leading to lower than normal cover levels. The lack of runoff in recent years has contributed to a lower level of interest in the implementation and maintenance of control measures such as contour banks and strip cropping, although the dry seasons have emphasised the value of conservation tillage in maintaining profitability.
- Queensland's increasing population and continued economic development are responsible for a high level of projects associated with urban development and the provision of infrastructure. Such projects are at risk of erosion during the construction process. Awareness of the need to implement erosion and sediment control practices during these activities is increasing.
Indicators and summary of status
Indicator |
Status of indicator |
Rate of soil loss from |
The recording of soil loss for all erosion events in a state as large as Queensland would be an impossible undertaking, so no data to quantify trends in the status of this indicator are available. The drought conditions experienced over much of the state in the past four years are likely to have reduced the overall incidence of water erosion. However, drought leaves the land vulnerable to erosion should the drought be broken by heavy rainfall. |
Area under bare-surface production systems |
Inadequate data to quantify trends in the status of this indicator are available. |
Grazing pressures |
Inadequate data to quantify trends in the status of this indicator are available. However, the widespread drought conditions would have led to high grazing pressures over large areas. |
Actual stocking rates compared to carrying capacity |
Inadequate data to quantify trends in the status of this indicator are available. However, the widespread drought conditions would have led to high grazing pressures over large areas. |
Percentage of cropped land with stubble retention (reduced or no tillage) |
Reduced or no tillage practices are now commonly adopted by farmers. For example, in sugarcane areas, 75% of farmers have adopted green cane trash blanketing. In the cropping land in the main grain growing areas of southern and central Queensland, the overall area under no tillage is about 50% of the cropped area. |
Importance
Soil erosion affects the productivity of land by removing fertile topsoil, leaving less fertile subsoil for growth of crops and pastures. It has off-site effects including sediment deposition and diminished water quality in streams and oceans. It results in additional costs to the community associated with increased costs for food production, sediment removal and additional water treatment.
Most land in Queensland, depending on how it is used
and managed, is susceptible to erosion by water. Erosion takes many forms, including sheet, rill, tunnel and gully. The amount of erosion likely to occur depends on the amount of rainfall and its intensity, the soil type, the length and steepness of slope, and the amount of cover on the soil surface.
The results of erosion can be very dramatic: for example, a gully that is several metres deep; or the process of scalding, which is the removal of the topsoil, exposing subsoil that is impermeable and naturally high in salts. The results of some erosion events are less visible, however, and the evidence may soon be masked by the growth of crops, pastures or weeds. In urban areas, a well-landscaped development may replace an erosion-prone construction site.
Water erosion poses a threat in any areas disturbed for development. Construction activities lead to the removal of native vegetation followed by major soil disturbance and may involve the altering of natural drainage lines. Contaminated stormwater runoff from construction and building sites can have significant adverse impacts on downstream environmental values through the discharge of sediment and other associated pollutants, including cement residue, nutrients, metals and litter. The increased rates of runoff arising from developed sites may lead to erosion of stream banks and scouring of stream beds.
Pressure and condition
It is not possible to provide a direct assessment of the condition of the land in relation to an issue such as water erosion for a state as large as Queensland. The accurate monitoring of a site to assess erosion rates involves the installation of measuring and sampling equipment and needs to be continued for an extended period because of the episodic nature of erosion events. Such monitoring has been carried out on selected sites in the past, but current work in this area is very limited.
The episodic nature of erosion events adds to the difficulties associated with the monitoring of soil erosion. For example, one detailed erosion study found that, over a 14-year period, 81 rainfall events produced runoff. The total soil movement for the period was 556 t/ha, and more than 70% of this erosion resulted from only six of the rainfall events (Wockner and Freebairn 1991).
An alternative approach to the direct measurement of soil loss is to monitor management practices that have impacts on soil erosion. Such practices include those aimed at the maintenance of a suitable level of cover on the surface of the soil as well as practices that manage runoff. Another alternative is to monitor the amount of sediment in downstream drainage lines, creeks and rivers. However, such measurement provides only a crude estimate of the amount of erosion occurring in the catchments that contribute to the streams. Much of the sediment lost from a paddock may travel a very short distance before deposition occurs and it may never reach the stream. A high proportion of the sediment in streams may come from a limited area of active gullies immediately adjacent to the streams or from stream bank erosion, rather than from areas of pasture or crop within the catchment. For information on sediment levels in streams, see 'Condition of rivers' (page 155) in Chapter 5, Inland waters.
Grazing
The challenge in grazing lands is to use strategic stocking strategies that match stock numbers to available levels of pasture. During the extended drought, large areas of grazing lands have come under high levels of grazing pressure, resulting in low levels of surface cover. These lands are therefore very susceptible to erosion by water, the extent depending on the amount and intensity of the rainfall that will finally break the drought. Indicators relating to pasture production and condition are discussed later in this chapter (page 127).
Both sheet and gully erosion are prevalent in grazing lands. A serious form of sheet erosion is scalding, which results in the loss of the topsoil layer. Scalding occurs on heavily grazed, fragile soils in the state's arid and semi-arid regions (rainfall less than 500 mm a year). A 1982 study reported that 590 000 ha of land in Queensland is affected to some degree by erosion scalding (DNR 1997). No assessments of the extent of scalding have been carried out since then. This process, however, along with gully erosion, is being monitored regularly by traverses along roads in extensive areas of inland Queensland by the Department of Natural Resources and Water. Figure 4.2 shows where gully erosion was observed along roads in part of the Burdekin catchment in May 2006 (R. Hassett, pers. comm.).
Figure 4.2Observations of incidences of gully erosion along roads in the Charters Towers district
Source: DNRW
Cultivation
Soil erosion poses a serious threat to the approximately 2% of land in Queensland that is used for growing crops. Significant gains have been made in the control of erosion in cropping areas since control programs began in the 1950s. Many steep paddocks have been retired from cultivation, stubble retention on the soil surface is becoming standard practice, and large areas have been treated with contour banks and strip cropping.
In sugarcane areas, 75% of farmers have adopted green cane trash blanketing (Wrigley 2005). In the main grain growing areas of southern and central Queensland, the overall area under no tillage is about 50% of the cropped area (Thomas et al. 2006).
The below-average rainfalls over the past four years would have reduced the overall incidence of soil erosion in cropping areas. Many cropped areas are vulnerable to erosion, however, as droughts have resulted in large areas under bare fallow conditions because crops have not been planted. The use of wider row spacings to reduce the vulnerability of crops to moisture stress also results in lower cover levels.
No data are kept on the area of land treated with contour banks and strip cropping. It is likely, however, that the prolonged drought conditions have led farmers to become complacent about the need for these measures. As a result, the rate of expansion of these practices would have decreased. Contour bank maintenance is likely to be neglected and there is anecdotal evidence that some farmers on the Darling Downs floodplain have abandoned the practice of strip cropping.
Urban development and construction of infrastructure
Urban expansion arising from rapid population increases also exposes more land to erosion during the development works associated with the construction of transport infrastructure and building projects.
Response
While some erosion problems are relatively easy to control, great difficulties are met in controlling erosion that has resulted in the loss of all of the topsoil or where large gullies have been carved into unproductive soils. For this reason, prevention is far better than cure. In cropping lands, landholders have widely accepted that retaining stubble on the soil surface will both improve crop yields and protect the soil from erosion. In grazing lands, there is also widespread recognition that more conservative stocking rates and maintenance of higher levels of groundcover will improve productivity and minimise erosion. The adoption of this strategy during extended drought conditions provides many challenges for graziers, however.
Landholders in rural areas generally have a keen interest in preventing soil erosion because it affects the future productivity of their land. This incentive does not apply to people responsible for construction activities in urban areas. Local governments are dealing with water-sensitive urban design, water quality and erosion and sediment control through development approvals.
Education can demonstrate the need to implement erosion and sediment control practices in order to improve water quality in downstream areas, but such activities add to the costs of a project, and many project managers would prefer to take the risk that heavy rainfall and subsequent erosion will not occur during the construction phase. Local authorities can impose fines for non-adoption of erosion control practices, but in many cases these do not provide a sufficient deterrent.
Procedures aimed at assisting practitioners with the implementation of erosion and sediment control measures are being produced for both rural and urban areas. The publication Soil Conservation Measures-a Design Manual for Queensland (DNRM 2004), aimed at rural areas, is available on the Department of Natural Resources and Water website. An update of the publication Soil Erosion and Sediment Control-Engineering Guidelines for Queensland Construction Sites (IEA 1996) is being prepared for urban areas.
Soil Condition Hazard Mapping project
The Soil Condition Hazard Mapping project conducted by the Department of Natural Resources and Water will analyse information about soils, topography and climate as well as land use and management to determine the location and potential severity of erosion and other land degradation issues at the catchment and regional scale. The information produced will be used by regional bodies, agencies and industry groups to assist with target setting and strategic decision making in the control and management of soil erosion and other land degradation issues. The project will be linked to a range of water quality catchment modelling activities, notably the Great Barrier Reef Catchment Modelling Project.
A number of initiatives have been introduced to deal with a range of environmental issues including erosion by water.
They include the:
- Reef Water Quality Protection plan;
- Property Management Systems Initiative (Department of Primary Industries and Fisheries);
- Grazing Land Management program (Department of Primary Industries and Fisheries);
- Farm Management Systems Initiative ( Queensland Farmers Federation); and
- OnePlan initiative.
Programs of regional Natural Resource Management bodies
Regional Natural Resource Management (NRM) bodies coordinate the views of regional communities in relation to environmental issues and are responsible for preparing regional natural resource management plans. The plans contain targets for managing the condition of natural resources and include a number of initiatives aimed at the control of erosion by water. They receive funding under programs such as the National Action Plan for Salinity and Water Quality (NAPSWQ) and the Natural Heritage Trust (NHT) extension programs.
Some regional bodies have employed staff with a specific role in providing assistance to landholders with the implementation of soil conservation practices.
References
DNR 1997, Salinity Management Handbook, Department of Natural Resources, Brisbane.
DNRM 2004, Soil Conservation Measures-a Design Manual for Queensland, Department of Natural Resources and Mines, Brisbane, viewed 29 January 2007, www.nrw.qld.gov.au/land/management/erosion/index.html#design.
IEA 1996, Soil Erosion and Sediment Control-Engineering Guidelines for Queensland Construction Sites, Institute of Engineers Australia (Qld Division).
Thomas, G.A., Titmarsh, G.W., Freebairn, D.M. and Radford, B.J. 2006, No tillage and conservation farming practices in grain growing areas of Queensland-a review of 40 years of development, Tamworth No-till Conference, 2005.
Wockner, G.H. and Freebairn, D.M. 1991, 'Water balance and erosion studies on the eastern Darling Downs-an update', Australian Journal of Soil and Water Conservation 4: 41-47.
Wrigley, T. 2005, Canegrowers Public Environment Report 2005, Canegrowers, Brisbane.
Return to State of the Environment Queensland 2007 content page
Last reviewed 16 May 2011
Last updated 4 September 2007