What is the problem?
Western Australia has over 70 per cent of Australia’s reported dryland salinity (see table in Dryland Salinity – An Introduction). An estimated 1.8 million hectares of farmland are already (1996) salt-affected to some extent, and this area could double in the next 15 to 25 years, and then double again before reaching equilibrium.
Due to Western Australia’s landforms and climate, the extent and rate of expansion of secondary salinity are particularly severe compared to other parts of Australia (see Dryland Salinity – An Introduction for more information and definitions). Land managers, the community and government have started action to manage the problem, but further decisive action is needed to halt the spread.
Before European settlement (pre 1829), salt was visible in the landscape, generally in salt lakes. These receive stream flow during rainfall events, and filled with water which then evaporated leaving salt behind (between 20 and 200 kilograms of salt per hectare per annum are dropped in rainfall over the south-west of Western Australia). This is referred to as ‘primary salinity’.
With the removal of native vegetation for agricultural development, recharge to the groundwater increased, leading to rising watertables. This rising water dissolved salt stored in the soil, and often reaches the surface as discharge. This secondary salinity due to clearing is the salinity threat being dealt with in Western Australia
How big is the problem?
Clearing in WA
Of the 25 million hectares of land in south-western Australia previously covered by native perennial vegetation, almost 18 million hectares have been cleared.
Land Use and vegetation cover in the south-west of Western Australia (Beeston et al. 1994)
Land use Area(millions of ha) % of areawithin theagricultural region
Agricultural region 25.3 100
Area of private land 20.8 82
Cleared land 18.0 71
Private remnant vegetation 2.8 11
Public land 4.5 18
Clearing and salinity – the link
Recognition of the link between clearing and development of salinity in Australia is not new.
Subsequent scientific investigations, especially over the past two decades, have confirmed and elaborated Wood’s explanations.
One experimental catchment was deliberately cleared in 1976 to study the processes leading to salinity. Watertables rose dramatically after clearing, rising nearly 17 metres in 11 years to reach the surface (on the lower slope). In contrast, watertables fell in nearby native forest due to low annual rainfall in the period.
‘Lag’ from clearing to equilibrium ranges from as little as ten years in the high rainfall areas (greater than 800 mm) to more than 200 years in the low rainfall areas (less than 300 mm).
Estimated areas affected by salinity in Western Australia, and potential at equilibrium (Government of Western Australia 1996a)
Hydrologic region Area cleared*(ha) 1994 salt(ha) 2010/20 salt(ha) Potential salt(ha)
South CoastSouth-westSwan-AvonNorthern 4,078,9603,310,0007,590,5004,251,900 395,400273,800759,000376,250 687,580595,5001,290,400722,800 977,180820,0003,036,2001,275,600
Total 19,231,360 1,804,450 3,296,280 6,108,980
* Includes some uncleared but alienated land which is also subject to salinity in highly cleared catchments.
Causes of dryland salinity
Changes in water balance cause dryland salinity. Tree clearing, reduced plant growth and thus reduced use of water in the soil increase the amount of water feeding into groundwater. This raises the groundwater and mobilises salts stored in Australia’s soils.
Understanding how groundwater responds to changes in land use and water balance is the key to managing dryland salinity. Not all groundwater systems are the same. Increases in groundwater levels and therefore dryland salinity do not happen overnight. There are long responses and lag times-often 100 years or more.
Impacts of Dryland salinity as estimated by the Audit
Dryland salinity affects land and water resources on site, e.g. at the farm scale, elsewhere in the catchment, or outside the catchment (downstream). It is more pervasive than other forms of degradation but is closely linked to them (e.g. causing soil erosion, nutrient build-up in streams which sometimes promotes algal blooms, as well as the loss of plants from the river / creek edge leading to riverbank erosion and loss of wildlife habitat)
On farms, salinity reduces production, income and the capital value of land. Salinity damages infrastructure, salinises water storage, causes loss of farm flora and fauna and loss of shelter and shade. These effects are magnified at the regional level. Salinity can, and is having a substantial impact on public resources such as water supplies, roads, buildings and biodiversity.
Using information provided by State and Territory water management agencies, the Australian Water Resources Assessment 2000 describes surface water and groundwater allocation to all users, including for the first time, the environment.
+ 26% of Australia’s surface water management areas were assessed to be at a high level of development and approaching or beyond sustainable extraction limits.
+ 34% of Australia’s groundwater management units were assessed to be at a high level of development and approaching or beyond sustainable extraction limits.
Australia has a wide range of soil, vegetation and climate types that affect the natural nutrient levels in water. State and Territory water quality guidelines take account of this natural variation, the pattern of land use within a catchment and the water values requiring protection.
This assessment shows that nutrients are a major water quality issue, particularly in the more intensively developed river basins in the North-East Coast, Murray Darling, South-East Coast, and South-West Coast Drainage Divisions.
Rivers basins assessed to have nutrient levels within the guidelines generally have more extensive vegetation cover and are less intensively developed. These include river basins of north Queensland, north-eastern Victoria and south-western Western Australia.
Australia’s ancient weathered landscapes and some surface waters are naturally salty. River salinity is compounded by the outbreak of dryland salinity which has mobilised salt stores in the soil (see Fast Facts 21).
This Water Resources Assessment reveals salinity is a significant water quality issue in much of temperate southern Australia and affects river basins in most of the South-West Coast, the southern South-East Coast and southern Murray-Darling Drainage Divisions.
The availability of data and the intensity of monitoring limit the comprehensiveness of this assessment.
This image is the product of radar and other data and is a salinity prediction map. Purple represents areas with a high potential for salinity, red: medium to high, beige: low. (Pic: University of South Australia)
Salinity can be categorised in a number of different ways:
2000 Map+ Western Australia has over 70 per cent of Australia’s reported dryland salinity. Due to the state’s landforms and climate, the extent and rate of expansion of secondary salinity are particularly severe compared to other parts of Australia. + The largest area at risk from dryland salinity is in the agricultural zone of southwest Western Australia where 4 million hectares are currently at risk from rising groundwaters. + Salinity has steadily degraded most of the inland waters in the south west of Western Australia. Over half the state’s divertible water is already saline, brackish or of marginal quality. + In 1982, 264 000 hectares of Western Australia was affected by salt. In 1990, nearly 443 000 hectares of once productive agricultural land was affected by salinity. By 1996, nearly 1 804 000 hectares was salt-affected. The 1990 figures have been estimated as representing a loss of $44.2 million in gross agricultural production. Losses of fresh water supplies, habitats and amenities are also substantial.
New South Wales
2000 Map+ NSW has only 5% of the land currently affected by dryland salinity in Australia, however it has almost 50% of the potentially affected land. + Known areas of major concern in NSW in relation to dryland salinity are: o A north-south belt near Canberra taking in the Yass River Valley o The south-western part of the Lachlan River Catchment o East of Dubbo in the Macquarie River Catchment+ Irrigation salinity is estimated to affect 15% of irrigated land. About 70-80% of irrigated land in NSW is threatened by rising watertables and associated salinity problems. + The areas at greatest risk from irrigation salinity are in the: o Murrumbidgee River Irrigation Areas in the vicinity of Griffith, including the Mirrool, Yanco and Colleambally Irrigation Areas o Murray Irrigation Areas around Deniliquin, including the Berriquin, Deniboota and Wakool Irrigation Districts o Jemalong Irrigation District, west of Forbes