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Issues ConditionThreatening Processes |
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At a glance 
The issue The loss of topsoil and nutrients due to erosion reduces productivity, and may also reduce the soil's ability to store water for plants to draw upon between rainfall events. Off-site impacts created by sheet erosion of soil and its deposition away from the erosion site, may include crop and pasture damage, water-quality deterioration and stream, dam, lake and reservoir sedimentation. Damage to aquatic habitats is likely to follow. Sheet erosion may also result in removal of seeds or seedlings. The main mechanisms of soil loss identified in Tasmania are sheet, rill, gully, tunnel and streambank erosion. - Sheet erosion by water is the removal of a fairly uniform layer of top soil by raindrop splash and run-off during storms.
- Numerous small channels gouged out by run-off are termed rill erosion.
- Gully erosion occurs when rills expand into channels deeper than 30 cm.
- Tunnel erosion is the removal of sub-surface soil by water while the surface soil remains relatively intact. This may produce long cavities beneath the soil surface (i.e. tunnels), which can enlarge until the surface soil is no longer supported and collapses to form an open gully. (Both rill and tunnel erosion can lead to gully erosion if left unchecked.)
- Streambank erosion results from stream flow eroding banks or rapid loss of flow causing the banks to slump.
This 'At a glance' section provides an overview of soil erosion. More detailed information and references are available in the Soil Erosion Issue Report. Three indicators are included in this Issue Report: Area Affected By Water Erosion, Potential for Water Erosion, and Percent of Land Managers Using Agreed Best Practice By Land Use and/Or Catchment. Recommendations are also provided outlining possible responses to these problems (see Strategic Land Resource Management, Productivity of Soils and Information Needs for Land Management). Related issue reports are presented in other chapters, such as the Inland Waters and Wetlands chapter. For example, the removal of riparian vegetation may cause rivers to become susceptible to streambank erosion. Favourable news - On private land in Tasmania, sheet and rill erosion hazard is not appreciable or only occurs at a moderate level. Private land gully erosion hazard is generally not appreciable or only occurs at a minor level. Tunnel erosion is the most limited form of land degradation in Tasmania, estimated to be affecting 5% of private land.
- Over 80 farmers in the low rainfall agricultural areas of Tasmania-the Midlands, Fingal and Derwent valleys-have protected their north facing slopes from erosion through the Natural Heritage Trust funded 'north facing slopes project'. The project aims to encourage improved grazing management of sheep on highly erodible north facing slopes. The project has fenced about 465 north facing slopes, protecting 11,000 ha on 150 properties from soil erosion, and has used 550 km of fencing.
- A set of soil management guidelines, published by the DPIWE, has been formulated. Soil Management - A Guide for Tasmanian Farmers (Hamlett 2002) provides a blueprint for farmers wishing to adopt and improve soil management practices.
- An erosion control technique has been developed for use in annual cropping systems on sloping land. This mulched rip lines technique has been used to protect 400 ha in north-west Tasmania in 2001 and 600 ha in 2002. It has been shown to stop soil erosion in autumn and winter-down crops.
Unfavourable news - Research on farms in north-west Tasmania shows that on steeper cropping land (i.e. over 18 degrees slope) significant reduction in yield (8-15%) occurred as a result of water erosion (Cotching et al. 2002). These were not apparent on lower slope eroded areas.
- Studies on river condition indicators in north-west Tasmania provide markers of the extent of erosion, and its in-stream consequences. Simms and Cotching (2000) found that erosion from intensively cropped catchments in north-west Tasmania is resulting in off-site environmental degradation such as high stream turbidity levels and suspended sediment loads. Measurements were made at both catchment and paddock scales with the highest turbidity values recorded in run-off from fallowed paddocks.
- Severe to extreme sheet and rill erosion occurs in the vegetable cropping areas of the northern coast (e.g. between Devonport and Wynyard), and on the heavily grazed drier hills of the Southern Midlands and Derwent Valley. The shallow soils on dolerite hills are of specific concern, as the north-facing slopes are denuded of vegetation making them susceptible to sheet and rill erosion. The main areas of severe gully erosion are south of Launceston, in the Southern Midlands and east of Hobart near Buckland. The severe (i.e. deep tunnels) areas of tunnel erosion are largely confined to the south-east of the State in the areas around Kempton, Forcett and Cygnet, and in the Fingal Valley.
- Erosion risk occurs broadly across both public and private lands in Tasmania. The south-west and the Central Plateau, as well as the intensively farmed areas in the north-west Midlands, are shown as areas susceptible to erosion. Further analysis of vegetation vigour with slope would assist in identifying susceptible areas.
Uncertain news - The only systematic Statewide survey of water erosion (gully, tunnel, and sheet and rill) was conducted in 1992, which was part of the soil and land degradation assessment coordinated by Grice (1995). These assessments were conducted on private land only, and the survey did not include data on the area affected by streambank erosion. These surveys were limited to the identification of hazard rather than actual occurrence of various forms of erosion. There has been no repeat of the 1992 survey.
- Since this 1992 Statewide survey was undertaken, there has been an increase in intensive agriculture accompanied by more intensive irrigation. There is uncertainty about the consequences of intensification for the distribution and severity of soil erosion given the absence of systematic surveys.
- There is no current information on the economic costs in lost productivity due to erosion in Tasmania. This information is important in encouraging investment, management responses, and initiatives. It would also allow the costs of action to be compared with costs already being incurred.
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