State of the Environment Report 2007
Saline landscapes are a natural feature of low rainfall parts of WA to which many flora and fauna species have adapted over time. Salt is normally stored deep in the soil below the root zone of native vegetation. Problems arise when unaffected land becomes salty due to widespread land clearing and replacement of native vegetation with annual crops and pastures that use less water. This can have deleterious impacts for native ecosystems that are not accustomed to persistent elevated salinity.
Shallow rooted annual crops and pastures and bare soil typically use much less water compared to native vegetation. Excess water not used by vegetation seeps into the soil, past the root zone, and contributes to groundwater stores. This increase in storage in areas from which native vegetation has been removed occurs at a much faster rate than would otherwise occur naturally. Over time, continued recharge causes groundwater watertables to rise, bringing with them salt stored deep in the soil. For this reason it may take several decades for land salinisation to become noticeable on the land surface following clearing of native vegetation. This process is called 'dryland salinisation' and now affects a significant area of agricultural soils in the South West (Government of Western Australia, 2000). Local geology, topography and groundwater aquifer characteristics also play an important role in the susceptibility of land to salinisation.
Land salinisation can also result from irrigation of agricultural land. 'Irrigation salinity' is typically caused by recharge of groundwater from excessive irrigation and leakage from irrigation channels, causing watertables to rise and bringing salt to the surface. In both dryland and irrigation salinity, saline groundwater near the soil surface kills native vegetation, reduces biodiversity, results in loss of agricultural productivity, and damages infrastructure.
The State Salinity Strategy lists specific goals for managing the impact of land salinisation in the South West agricultural zone (Government of Western Australia, 2000). Objectives relevant to the land resource include to:
The National Land and Water Resources Audit estimated that about 75% of Australia's dryland salinisation problem is in WA, the worst affected area being the South West agricultural zone (Commonwealth of Australia, 2001a). The extent of salt-affected land has historically been difficult to ascertain. Modelling methods, expert assessments and farmer surveys have produced estimates of area affected and possible trends that often disagree. Recent analysis of Land Monitor data, using a combination of satellite imagery and ground truthing, has proven to be a more reliable means of generating accurate information (McFarlane et al., 2004). In 1989, over 859 300 hectares (ha) were impacted by dryland salinity, representing 2.6% of local government land and 4.6% of agricultural land. By 1996, nearly 957 600 ha had been affected, representing 2.9% of local government land and 5.1% of agricultural land. The increase in salt-affected land over this seven-year period is an increase of 14 000 hectares per year. A continuation of this rate would suggest that about 1 112 000 ha of South West land is likely to be affected by dryland salinisation in 2007.
![Figure L1.1: Extent of salt-affected land in 2000 and the area at risk of developing high water tables. [Data source: Department of Land Information – Land Monitor [ver. 2006], Analysis: EPA; Presentation: EPA.]](/site/files/images/Figure-L1.1_Large.jpg)
Most salt-affected land is located in low-lying valley floors, adjacent to Wheatbelt waterways and wetlands (Figure L1.1). The extent of salt-affected land varies among local government areas, with many waterways and wetlands in the eastern Wheatbelt being naturally saline. However, the extent of dryland salinisation in some local government areas is significant, some being more than 10% affected in 1996 (Table L1.1). Eight of the top 10 local governments with a dryland salinity problem are situated in north-west parts of the Avon Wheatbelt, where average annual rainfall is low (300-450 mm per year). The extent of salt-affected land continues to worsen in many areas. Seventeen local governments recorded increases in affected land of more than 0.5% between 1989 and 1996. While this rate of growth appears small, land salinisation is a slow-developing problem and has cumulative effects over time.
Recent estimates indicate that up to 5.4 million hectares of land in the South West is potentially at risk of salinisation (McFarlane et al., 2004; Department of Environment, 2003). About 80% of this is agricultural land, but it also includes important areas of native vegetation, wetlands and infrastructure. Estimates of future land salinisation should be treated with some caution because areas at risk may not become saline due to factors such as climate change or changes in land use or water balances. Even local hydrogeology and soil types may affect how susceptible a particular area of land is to salinisation.
Land at future risk of dryland salinisation represents an expansion of existing salt-affected areas (Figure L1.1). As watertables continue to rise, land situated higher in valleys becomes saline. As much as 43% of land in some local government areas is potentially at risk (Table L1.2). Seven of 10 local government districts most at risk of dryland salinisation (in terms of area of land affected) are located in the mid to north-west Wheatbelt areas of the Avon River Basin. Most of these districts are characterised by a flat landscape, low rainfall (<350 mm per annum) and poor drainage.
In some areas, such as the Swan and Scott coastal plains, a salinity hazard is present despite little evidence of land salinisation. These areas are particularly prone to irrigation salinity, having poorly drained soils that are subject to waterlogging. About 40% of the Swan Coastal Plain (a stretch from Gingin to Dunsborough) is at risk of increased salinity and there is already some evidence of salt scalds appearing (Government of Western Australia, 2000). Irrigation salinity is also a possible threat to the Ord River Irrigation Area with groundwater salinity levels being elevated in some areas.
Historical widespread clearing of perennial native vegetation and its replacement with low water-using annual crops and pastures has contributed to altered hydrological regimes in the South West agricultural zone. Although most broadscale clearing ceased many decades ago, clearing controls and legislation are now in place to protect native vegetation remnants. Unfortunately, excessive overclearing of native vegetation occurred in some areas. For example, 93% of native vegetation in the Avon-Wheatbelt bioregion has been cleared, resulting in significant land salinisation problems and loss of biodiversity.
Surprisingly, loss or degradation of vegetation cover is still continuing in the Wheatbelt, as shown by changes apparent from Land Monitor data (see 'Loss or degradation of native vegetation'). Temporary loss or degradation of vegetation cover in eastern areas (rangelands) may be attributed to broad scale fires, with much of this bushland likely to regenerate over time. In coastal and Wheatbelt areas, vegetation loss or degradation is likely to be caused by clearing of native vegetation for agriculture or urban development, harvesting of plantations, land salinisation or Phytophthora-induced dieback. Further investigation and on-ground truthing of satellite imagery data is required to explain why loss or degradation of vegetation is still occurring. Irrespective of the cause, the continued loss of perennial vegetation cover is not desirable and further exacerbates salinisation in susceptible areas.
Although local improvements have been noted, regional groundwater modelling across the South West agricultural zone suggests a long-term trend of mostly rising or stable groundwaters and none falling significantly (see 'Altered water regimes'). Under these conditions dryland salinisation can be expected to worsen. About 16% of land (4.65 million hectares) in this region already has shallow watertables (Department of Agriculture, 2001). Assuming groundwater trends remain constant, 20% (6.37 million hectares) of the region is at high risk of shallow watertables by 2020, and 33% (13.66 million hectares) by 2050 (Short & McConnell, 2001). These figures were calculated on the basis that watertables were less than 2 m from the soil surface or between 2-5 m and rising. It should be noted that not all land with shallow watertables will be affected by salinisation.
There is recent evidence to suggest that the drying climate in the South West has lowered local groundwater levels in some places. A decrease in winter rainfall and general lack of 'wet winters' are likely to have slowed the rate of land salinisation, especially for central and eastern Wheatbelt areas. Some areas cleared a long time ago appear to be reaching groundwater equilibrium, while the trend of rising groundwater continues in recently cleared areas, such as high rainfall parts of the Wheatbelt and south coast (Indian Ocean Climate Initiative, 2005).
State Salinity Strategy: was released in 2000 and focused on developing a partnership-based approach, prioritisation tools, improved protection and conservation mechanisms, promoting further research and development, and ways to help the farming community move to more sustainable production systems. In response to the strategy, the State Government formed the Natural Resource Management Council and a stronger strategic role for regional natural resource management groups in managing salinity.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP): are two Commonwealth Government programs that aim to ensure environmental (on-ground) improvements occur via a targeted strategic approach at the regional level. Natural resource management groups in the South West, South Coast, Northern Agricultural, Avon and Swan regions have recognised dryland salinisation as a threat to natural resources, and have projects to manage salt-affected land and protect valued natural assets at risk. Irrigation salinity has been recognised as a threat in the Ord River catchment by the Rangelands regional group.
Engineering Evaluation Initiative: is a State Government project to mitigate land salinisation by investigating a range of engineering options including deep drains, groundwater pumping, diversion and surface water management. It is also investigating options for safe saline water disposal and regional drainage planning.
Monitoring: Land Monitor produces satellite images that accurately map the extent of salt-affected land and its change over time and changes in areas of forest and perennial wood vegetation. Groundwater monitoring by the Department of Agriculture and Food is undertaken across the South West agricultural zone to assist in modelling and management of salt-affected and at-risk areas.
Salinity Investment Framework: is a prioritisation tool developed to help guide investment of funds towards projects that have the best chance of protecting high value assets from salinity. This prioritisation method has since been applied to broaden natural resource management investment to consider other environmental issues.
Remnant vegetation protection and management: The revised Environmental Protection Act 1986 now protects all native vegetation and replaces former Regulations under the Soil and Land Conservation Act 1945. Clearing of remnant vegetation is prohibited unless a permit is granted or an exemption applies. Clearing controls still apply in some recovery catchments where total clearing bans are in place. Significant areas of remnant vegetation are protected in conservation reserves managed by Department of Environment and Conservation.
Revegetation: Combined with other management efforts, revegetation of cleared areas has been pivotal in reducing the threat of land salinisation and lowering salinity in waterways. There are currently no reliable estimates of land revegetated with perennial woody vegetation. However, specific estimates are available in the Denmark and Collie river catchments, where dedicated restoration work has been undertaken to recover salt-affected water supplies. Between 1990 and 2002, nearly 50% of previously cleared areas in the upper Denmark River catchment were converted to plantations. In the Collie River catchment, 27% of previously cleared areas had been converted to plantations by 2000. Recent surveys (Department of Agriculture, 2006) indicate that 40% of farmers in the Wheatbelt planted non-irrigated perennial pastures and 32% planted saltland pasture species. Twenty per cent of farmers across the South West planted trees for commercial production (e.g. oil mallees, pines and bluegums) and 60-70% planted trees and shrubs. Between 60-80% of farmers acted to preserve or enhance areas of conservation value.
Commercial revegetation projects: Innovative tree farming programs, such as Infinitree, have been established in recent years, partnering the Forest Products Commission with farmers and investors to create commercial tree plantations (largely blue gums, sandalwood, and pine) on farm properties. In addition the Department of Environment and Conservation, in conjunction with the Collaborative Research Centre for Plant Based Management of Dryland Salinity, is developing new commercial industries based on native plants such as oil mallee.
Research: Murdoch University is investigating Wheatbelt deep drains and whether they are an effective strategy for alleviating salinisation in the Wheatbelt. The CSIRO Water for a Healthy Country program is undertaking research evaluating Wheatbelt drainage and developing models to help assess drainage plans. Recent biodiversity surveys of the South West agricultural zone will also provide important baseline information for future research and monitoring.
Recovery catchment programs: focus on stabilisation and improvement of important natural resource assets impacted by salinity. For drinking water resources, action is currently being taken in the Collie, Denmark, Warren, Kent and Helena catchments to reduce salinity levels in runoff to water supplies. There are also six natural diversity recovery catchments (Toolibin Lake, Muir-Unicup, Lake Warden, Lake Bryde, Buntine-Marchagee and Drummond) focused on the conservation and recovery of wetlands and other salinity-threatened assets. The Rural Towns Program provides 38 rural communities with necessary tools to fight townsite salinity. Monitoring bores in these towns have enabled improved modelling of salinity spread and the development of town-specific salinity management strategies.
Collaborative Research Centre for Plant Based Management of Dryland Salinity: The centre is at the forefront of research in understanding ecosystem functions and their responses to salinisation, in developing and trialling various land use systems and vegetation options for sustainable farming, and in best productive use for salt affected land. The centre is soon to become the Future Farm Industries Cooperative Research Centre with a focus on research into novel dryland farming methods in a changing climate, and industry opportunities based on perennial plants.
Land salinisation results in a rapid and catastrophic collapse of terrestrial and aquatic ecosystems. Biodiversity has already declined in the South West and some 450 plant and 400 animal species are at risk of global or regional extinction as a result of salinisation and other hydrological changes. Previous estimates indicate that up to 80% of bushland on farms and 50% of bushland on public lands in agricultural areas will eventually be affected (George et al., 1995). Salinisation is likely to reduce animal species by 30% in affected areas; a 50% reduction for waterbirds is anticipated due to salinity-induced loss of habitat (Government of Western Australia, 2000). Ecosystems situated low in the landscape, especially those associated with wetlands and waterways, are most at risk. Loss of vegetation to land salinisation also results in a loss of carbon storage, effectively contributing to increased greenhouse gases.
Land salinisation already results in significant impacts on agricultural productivity, such as reduced pasture, crop growth and shelter for livestock. Estimated annual economic costs of dryland salinity across Australia are $664 million, based on best-guess estimates and not including the costs of strategies to combat salinity impacts on biodiversity (Commonwealth of Australia, 2001a). The true economic cost of dryland salinisation, taking into account off-site costs, is not known but is conceivably in the billions of dollars (Land and Water Resources Research and Development Corporation, 1999). Farmers in affected areas, already under difficult circumstances, are faced with having to modify their farming practices and investigate alternative land use options. However, land use changes will generally not have any significant impact on the effects of salinisation for at least 20 years (Commonwealth of Australia, 2001a; Short & McConnell, 2001). Damage from flood flows may also increase significantly as the proportion of land affected by shallow watertables increases (Bowman & Ruprecht, 2000; George et al., 1999).
Twenty South West towns are currently affected by dryland salinity and about 15 000 km of rail and road networks are currently at risk, with shorter infrastructure lifespans expected. Impacts on road and rail infrastructure are expected to double in the next 50 years (Commonwealth of Australia, 2001a). Social ramifications of land salinisation can also be significant. Apart from lost farm income, land owners may have to spend more time and money combating lost agricultural productivity and damaged infrastructure. Adaptation and changes to traditional production systems may be required and families unable to cope with the uncertainty and change may leave farming. This is already occurring in some Wheatbelt towns where falling population has led to withdrawal of services and closure of businesses in a cycle leading to further rural community decline. Salinised landscapes may also reduce recreational, cultural and spiritual values and affect the tourism industry.
3.2 Continue to implement the recommendations of the State Salinity Strategy, recognising it as a component of the proposed State Soil Protection Policy. Although significant progress has been made, more focus should be given to integrating engineering and plant based solutions to cater for a variety of farm and catchment characteristics, and promoting alternative farming systems in low to moderate rainfall areas.
3.3 Continue the Land Monitor program to provide five-yearly updates of the extent of land salinisation in the South West. Land Monitor remains one of the few tools to accurately measure regional trends in land salinisation and to evaluate the effectiveness of actions.
See also 'Salinisation of inland waters':
