Stream salinity

In a country as dry as Australia, any threat to its water resources is extremely serious. The rising level of salt in streams, rivers, lakes and reservoirs presents one of the main threats to our water supplies. Land degradation, as in stream salinity, is one of the most direct ways, which influence city dwellers.

Before clearing of land for agriculture began, most of the rivers and creeks in Western Australia were predominantly thought to be fresh. Only 48 per cent now remain fresh, while 35 per cent have become so saline that they are undrinkable and the remaining 17 per cent are of marginal quality, needing catchment management to prevent further deterioration. Now there is virtually no area in the south-west with an annual rainfall of less than 900 mm where a creekline dam can supply potable water. The salinities of major rivers continue to increase, because of the slow response of groundwater systems affected by earlier clearing.

What causes stream salinity?

Salinisation of water and soil are brought about by the same process of soil salt accumulation as explained in the section, 'Insalting the earth'.

Measurements of salt content in rainfall have established that annual salt deposition via rainfall ranges from more than 200 kg/ha (about 18 kg on a suburban block) near the west coast, to 18 kg/ha at inland sites such as Merredin. The native vegetation uses the rainfall, but leaves the salt behind in the soil.

Where rainfall is high enough, water passes through the soil to leach out salt, thus preventing its accumulation. In low rainfall areas, however, large amounts of salt accumulate in the soil. Even in these areas, a balance is eventually reached between the amount of salt entering the soil and the amount that leaves it.

After clearing the deep-rooted perennial native vegetation, farmers plant shallow-rooted annual crops and pastures. This type of vegetation uses less water therefore, more water reaches the water table. Rising water tables bring stored salts to the soil surface, often as seepage. Surface water flows transport the salt to streams and rivers.

What can be we do about stream salinity?

Since rising water tables are the major cause of stream salinity, its remedy lies in keeping forest cover, especially in the salt-sensitive areas or changing our farming systems to use high water using plants. Clearing bans have been applied progressively from the 1950s to protect major water catchments in the south-west of the State.

On private land, legislation now makes it an offence to clear or destroy native vegetation greater than one hectare without first notifying the Commissioner of Soil and Land Conservation, Agriculture WA. In 1973, intensive logging operations associated with the growing woodchip industry were excluded from the increasingly salt-sensitive north-eastern sector of the Woodchip License Area. Bauxite miners have had to show that their operations would not adversely affect salinity before they could proceed with mining in the salt-susceptible eastern zone of the forested Darling Plateau.

The revegetation of purchased farmland with selected 'high-water-using' eucalypts by the Water Authority of Western Australia, (for example in the Collie River catchment) is reversing the process. A joint Government/private enterprise scheme has been set up to establish Tasmanian bluegum plantations and shelterbelts in a sharefarming venture with cooperating landholders. Grown in ten-year rotations for paper pulp production, these plantations can also reduce stream salinity.

Albany in Western Australia has a developed a world first where the town's sewerage and waste water is used in a Farm Forestry Project growing trees. This has the effect of recycling nutrients and protecting the coast from pollution.

Where catchments have been partly cleared for agricultural development, landholders should be encouraged to maintain and regenerate remnant vegetation and protect streamlines from further degradation. By encouraging the establishment of tree plantations in rainfall zones below 900 mm per annum, where salt storage is greatest, water tables could be reduced, thus reducing saline discharge to streams. Farmers could use more fodder trees, particularly on the less-productive soil, and plant more deep-rooting crops to increase water use, thus reducing recharge to the water table.

It's important that catchment communities get together to address the issues of salinity and planning for healthy catchments.

Davies-Ward, Edwina and Finlayson, Rob, (Ed.)(1997), Environment Western Australia 1997 Draft State of the Environment Report for Western Australia

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