Indicator: Land Degradation
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All areas burnt in the 2001 and 2003 bushfires as well as all streams emanating from burnt areas have been degraded, or have the potential to be degraded, by erosion. The full impacts need to be assessed. A comprehensive study should be undertaken of the post-fire erosion that has occurred plus a risk assessment for future erosion that may result from high rainfall and the continued movement of sediment already mobilised.
Areas not burnt have been assessed as static or improving. Accessible spatial databases on land degradation still need to be developed. A proposal for ‘Acquiring, Monitoring and Managing Natural Resource Management under the National Action Plan for Salinity and Water Quality’ had not been finalised at the end of the reporting period.
The precise condition of much of ACT land is not known, as baseline data are not available. A comprehensive soil and water data collection process and land capability study, funded through the Decade of Landcare Program, identified some landuse issues and gave some detail of land condition. However, the data are in an inaccessible format and there is now a need to review these data and undertake further studies to give accurate baseline data.
Before the 2003 bushfires there was very little active gully erosion in the ACT ( Soil Conservation Priority Plan for the ACT , 2000).
However, between 18 January 2003 and 11 June 2003 a series of events occurred that triggered a new erosion phase within the Canberra region. The January fires bared the soils of a large part of the region, turning much of the surface to powder. Storms followed over the next five months, the first on 8–9 February, resulting in massive erosion of a well-conditioned landscape.
Five of the nine major river catchments are affected by some form of land degradation. This includes all or almost all of the Cotter, Gudgenby– Naas and Paddys River catchments, and parts of the Molonglo and Murrumbidgee River catchments – some 70% of the ACT .
In Corin Dam catchment (upper Cotter River catchment) the erosion was studied in detail (ActewAGL 2003; Wasson et al. 2003). When the results are compared to erosion forms and sediment yields for the 19th century and recently (1944 to 1991) in the Canberra region, it is clear that these bushfires and subsequent high rainfall have had a significant impact (see box on erosion facts).
Two post-fire storms that generated erosion over an area of about 18 square kilometres yielded from 1314 to 2354 tonnes of sediment per square kilometre. This compares with the 1000 tonnes per square kilometre estimated as being the peak of the main erosion phase of the 19th century.
At Corin Dam, the post-fire input of inorganic sediment is equivalent to 17 years at the average annual rate, while organic matter input is equivalent to 27 years' input.
Other erosion across the region, from Burrinjuck to Bredbo, including one site on the eastern slopes of Mt Tennent, is of a similar severity to the Corin dam erosion. Storms sheet-eroded slopes and deposited sediment up to two metres deep at the bottom of the mountain and in drainage lines. These deposits have subsequently eroded and are in or near the Gudgenby River.
Sheet erosion was the dominant sediment source post-fire for Corin Dam and, although not calculated, is estimated to be well in excess of 50% of the total sedimentation. This compares with 7% before the fires.
Sheet erosion occurred across the landscape, moving material from landforms ranging from steep sideslopes to river flats. Some gully erosion occurred and formerly stable creeks have begun to deepen and widen. The flow-on effect of sediment entering the main channel, the Cotter River, has already initiated changes at one site. Stream bank and stream bed erosion of the river and tributary creeks will almost certainly continue.
The full impact of these events has not been assessed across the whole of the affected area.
Some sheet erosion is also apparent on hills in rural areas, due to overgrazing. However it is currently being managed by trying to persuade lessees to use better grazing practices. There is also scope to make use of Land Management Agreements and monitoring to ensure the problem does not become too severe.
The precise condition of much ACT land is not known, as baseline data for soil acidity and salinity are not available. A comprehensive soil and water data collection process and land capability study, funded through the Decade of Landcare Program, identified some landuse issues and gave some detail of land condition. However, the data are in an inaccessible format and there is now a need to review these data and undertake further studies to give an accurate baseline.
Soils in the ACT are inherently acidic; and somewhat ‘patchy’ soil acidity data demonstrates the existence of strongly acid (pH 5.0–5.5) and very strongly acid (pH 4.5–5.0) soil in some areas of the ACT. Agricultural practices involving superphosphate application and improved pasture tend to keep the soil in this state. Some rural lessees use sewage ash and lime to make the soil more alkaline. Further survey work is needed to obtain a clear picture to determine whether soil acidity is a problem in the ACT.
It is generally accepted that salinity is not a major problem in the ACT. In the upper Murrumbidgee River catchment, serious dryland salinity is limited to about 5% of the Yass River catchment (Starr, Wasson and Caitcheon 1999). There are some saline spots in rural lease and urban areas in the ACT but these are not significant.
Given the extent of natural regeneration of trees and shrubs in rural areas of the ACT, an expansion of dryland salinity is extremely unlikely. Native vegetation dynamics should be assessed and mapped for rural and other areas in the ACT at least every 20 years, as regeneration of cleared land can occur in that time.
There is a need for more integration of management at the catchment level that will protect the values associated with water supply catchment, aquatic habitat and nature conservation of the Upper Cotter River, the Murrumbidgee and Molonglo River valleys and their tributaries.
In areas managed by ACT Forests, the fires had a significant impact on recommendations in the Soil Conservation Priority Plan for the ACT, and the Code of Forest Practices is being amended to manage the changed environment caused by the fires. An environmental management plan is being prepared to help ACT Forests manage the cleanup operations.
Land Management Agreements tied to a leasing arrangement impose management conditions that include protection of land condition. However, in areas such as Gungahlin (Ginninderra and Sullivans Creeks catchments), Majura Valley (Woolshed Creek–Molonglo River catchments) and the Naas Valley (Gudgenby–Naas catchment) where land may be needed for development, lessees do not have security of tenure and leases include a 90-day withdrawal clause. In the event of withdrawal, lessees are compensated for capital improvements only and this does not include expenditure on issues such as weed control. There is scope to explore incentives for further expenditure to encourage those aspects of good land management. These areas, therefore, lack the additional protection afforded by Land Management Agreements and protection relies on the relevant legislation and policies.
The Soil Conservation Priority Plan for the ACT (2000) identified the most active erosion sites at the time and recommendations were made for rehabilitation. Other priorities for soil conservation were also identified, including the need to address erosion from logging and management tracks (with particular reference to recreational vehicles), assessing maintenance requirements of older earthworks, weed control in stream channels and developing a set of criteria for filling of valley floor gullies.
Detailed studies were carried out by various Landcare Groups and others to provide specific management direction for soil conservation and associated river bank and vegetation management. Groups included Reedy Creek (Starr & Houghton 1999), Lanyon Bowl (Starr 1999), Naas (Starr 2000), Paddys River (Starr 2000), Southern Catchment Group (Starr 2002) and ACT Department of Health, Youth and Community Services (Starr 2003).
Before January 2003, 519 hectares of degraded and/or cleared land underwent revegetatation through the Rural Conservation Fund (468 hectares) and the Vegetation Investment Project (51 hectares). Most of these sites were located in the Murrumbidgee River catchment, with minor areas in the Molonglo and Ginninderra catchments as (see Figure 1 in the pdf version).
Figure 1: Map of revegetation sites prior to 2003
Much of that work was undone by the fires. In immediate recovery work after the fires $22,455 was allocated to nine affected rural properties for replacement of fences, tubestock (plants grown in tubes until they are large enough to be planted to stabilise river banks) and direct seeding.
In March 2002, ACT Forests completed an environmental audit of its roads following recommendations in the Soil Conservation Priority Plan for the ACT. This audit highlighted some issues. Most issues, such as closing roads, improving drainage off roads, erecting sediment traps and redirecting runoff from roads near creeks and rivers, were historic and would be rectified over time.
To minimise erosion, the ACT Forests Code of Forest Practicesdiscourages snig tracks (tracks formed by snigging, or dragging logs along the ground from the point of felling) on steep slopes. If for some reason these tracks are allowed, the tracks are barred once harvesting in that area is completed. The use of such tracks is not allowed in wet conditions.
Following the bushfires, it will be important for both catchment management and biodiversity to develop and resource long-term research and monitoring programs into post-fire recovery of ecosystems. Such research should be of at least 30 years duration and should also be part of a joint program with other States affected by fire.
During 2003 a proposal, Acquiring, monitoring and Managing Natural Resource Management Data , is expected to be presented for funding consideration under the National Action Plan for Salinity and Water Quality.This project involves acquiring data where gaps exist, establishing monitoring programs and establishing an integrated natural resource management information system with a focus on rural land.
ACTEW Corporation 2000, Googong catchment, the state of drainage networks and trends in catchment condition since 1944 , prepared by Barry Starr Pty Ltd.
Starr, B (principal author) Wasson, R, Caitcheon, G (principal editors) 1999, Soil erosion, phosphorus and dryland salinity in the upper Murrumbidgee, past change and current findings , Murrumbidgee Catchment Management Committee.
Starr, B, Houghton, C 1999, Reedy Creek management plan , Reedy Creek Landcare Group.
Starr, B 1999, A review of Lanyon Bowl soil erosion and sedimentation and management recommendations , Lanyon Bowl Landcare Group.
Starr, B 2000, Soil Conservation Priority Plan for the ACT , prepared for and on behalf of ACT Urban Services, ACT Government (a priority plan for Decade of Landcare funding).
Starr, B 2000, The nature of the Naas, Naas Landcare Group .
Starr, B 2000, Paddys River catchment – heeding the lessons of the past , Paddys River Landcare Group.
Starr, B 2000, A reconnaissance of trends in the condition of streams in the Australian Capital Territory , ACT Urban Services.
Starr, B 2001, Budgeting for 'bidgee banks , prepared by Barry Starr Pty Ltd.
Starr, B 2002, Lower Yarralumla Creek - from urban drain to community asset , Environment ACT.
Starr, B 2003, Cotter catchment, fire and storm - an analysis of the impact of the January 2003 wildfire and following rainfall events on the stability of upper Cotter catchment and management recommendations , ActewAGL.
Starr, B 2003, Birrigai Outdoor Education Centre - Post bushfire erosion assessment and management recommendations , ACT Department of Education, Youth and Community Services.
Sydney Catchment Authority 2002, Landscape trend analysis of the upper Shoalhaven River catchment, with a focus on Sydney Catchment Authority lands , prepared by Barry Starr Pty Ltd.
Wasson R, Mazari R, Starr B and Clifton G 1998, 'The recent history of erosion and sedimentation on the southern tablelands of southeastern Australia: implications for soil conservation', Geomorphology 24:291–81.
Wasson, R, Croke, B, McCulloch, M, Mueller, N, Olley, J, Starr, B, Wade, A, White, I and Whiteway, T 2003, Sediment, particulate and dissolved carbon, iron, and manganese input to Corin reservoir , ActewAGL.