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Salinity explained | LC0082 |
Peter Dixon & Helen Anderson (Hamilton) and David Cummings (Melbourne)
December, 1999 |  |
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Salinity refers to the movement and concentration of salt, in landscapes. Both soil and natural waters can become saline. Hence salinity can be described as either soil salinity or water salinity.
Salt in the Australian landscape
Throughout Australia, there are large areas of naturally saline soils. Australia's arid climate and internal drainage system have produced a huge number and great variety of salt lakes. These lakes provide a history of salt in the landscape.
- A number of our main rivers drain to internal lakes such as Lake Eyre in South Australia, where over time salts have been concentrated. These naturally saline lakes are called salinas. Areas such as these are regarded as Primary Salinity.
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Sources of salt
Retreating seas
Many areas, such as the Mallee, were once covered by an inland sea. When the sea retreated about 10 million years ago, the sediments it left behind contain large quantities of salt. Many soils in the Mallee region have been derived from these materials.
Rain
Salt from the sea is carried inland by strong winds, and falls in rain. As you would expect, salts in rainfall are higher near the coast, decreasing inland. Deposition of windborne salts can range from 30g/m2/y close to the coast down to 5g/m2/y further inland (CSIRO, 1980).
Rocks
Salts are present in rocks and are released by weathering. Many rock types including marine sediments, granites and rhyolites contain high levels of sodium and potassium, which may be mobilised after weathering.
An old environmental problem
Cyclical climatic changes which have recurred over south eastern Australia, from wet to dry to wet, have not only influenced the landscape but its inhabitants as well. Aboriginal occupants of inland Australia suffered the effects of major climate-induced changes in the landscape 36,000 years ago. In those times, families hunted and camped on the shores of lakes in northern Victoria and western NSW. They caught fish and harvested mussels from the lake. As the climate warmed, the lakes shrank the waters increased in salt concentration and fish numbers fell. Eventually the lakes dried, winds blew clay and salt from the lake floors and people were forced to abandon their homelands. The previous lush lake environments had become harsh drylands with saline basins.
Upsetting the balance
Victoria's present salinity problems have resulted largely from human activities, which, in the brief period since European settlement, have modified the natural distribution of salt in the landscape. The reason for most salinity problems of soil and water has been a rise in the level of saline water tables, bringing salt to the surface.
Australia's first white settlers were graziers and made little impact on Victoria’s tree cover. The first major impact came with the discovery of gold in the 1850’s and 60’s when miners stripped the vegetation from mine areas and cleared the nearby forests for timber and fuel. When the gold ran out many miners turned to farming.
This coincided with the opening up of Crown Land for selection. Selectors were required to clear and fence their land to maintain their tenure. Unlike the large grazing properties of earlier decades, their farms were small. Large areas of Stringybark and Box-ironbark forests disappeared – the Red Gum forests of Gippsland were reduced to small pockets along roadsides and creeks.
The next major impact came in the early 1900’s with the Closer Settlement and Soldier Settlement schemes. After the Second World War when there was a great demand for timber for housing and construction projects. Over 300 sawmills opened to meet the demand for timber. Removal of the deep-rooted native trees and grasses and the development of irrigation projects have allowed more water to move into the soil, raising the groundwater level. Saline areas resulting from mans intervention in the landscape are referred to as secondary salinity.
Water balance before clearing
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Water balance after clearing
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Figure 1: Dryland Salting (Diagram from Murray Darling Basin Commission, 1999)
The Water Balance
During and after rainfall, water moves into the soil. This water is stored in the soil profile. Most is used by plants, however a proportion seeps down past the root zone into the groundwater system. In many cases this will cause groundwater levels to rise, mobilising salts. The upper surface of the groundwater is called the watertable.
When a saline water table rises to within one or two metres of the ground’s surface, water moves to the surface by capillary action, bringing salt with it. The water evaporates leaving the salt in the surface layers of the soil. Over time the soil becomes saline, limiting the growth of vegetation.
As salinity levels increase salt sensitive plant species die out leaving the soil prone to erosion. This is further exacerbated by declining soil structure caused by the high levels of salt. Irrigation is just like increasing rainfall. In irrigation areas, where saline ground water is present, sub-surface drainage and salt disposal are needed to prevent the otherwise inevitable rise in saline water table and accumulation of surface salt. If the saline water table rises to within two metre of ground level, waterlogging and salinisation occur causing substantial productivity losses. (See Figure 2).
The extent of the problem
Problems with salinity occur predominantly throughout northern and western Victoria and are considered to be increasing in severity (The most concentrated problems occur in the Murray-Goulburn Irrigation District, where a shallow water table threatens about 385,000 hectares of productive land. This salt-prone area includes 72% of the Kerang Region and 22% of the Shepparton Region. Secondary salting occurs throughout Victoria, with 120,000 hectares identified to date.
The effects of salinity are broad:
- reduction in the productive capacity of affected land,
- degradation of the environment and wildlife habitats,
- loss of water quality for stock and domestic water supplies
- production losses causing social and psychological hardship as well as economic hardship.
- damage to roads
- damage to water-using household equipment.
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Managing the problem - a catchment approach
Rarely can one land manager solve a salinity problem. While sometimes causes and effects can be seen within a single farm; mostly the cause and effects cross property boundaries. Everyone in a catchment is affected by the agricultural activities of others in it. Therefore effective salinity management requires a total catchment approach.
Figure 2: Irrigation salting
The long-term solution to salinity, is to restore the balance of inputs and outputs of the soil-water system, by controlling the process of groundwater recharge. Treatment of salt affected land is an important component of salinity management.
In dryland areas, large scale establishment of deep rooted plants such as trees and lucerne will be required to restore the water balance. In high rainfall areas plantation forestry may be one means of achieving the scale of tree planting required. Strategic tree planting as a part of a whole farm plan or catchment plan will also provide some benefit to recharge management. Deep-rooted perennial pastures are more likely to be effective in lower rainfall than in higher rainfall areas.
In irrigation regions control techniques may include more efficient irrigation, drainage and reuse systems, groundwater pumping and re-layout of land to facilitate surface drainage and water application.
For salt affected areas appropriate management can reduce further degradation of the site and enhance their productivity. Utilization of salt tolerant pasture species and grazing management may substantially increase productivity.
Control of salinity in the Murray-Darling basin depends on joint action by all States in the Basin. Victoria is already committed to a major program of salinity control, coordinating the efforts of landholders and community groups with salinity control activities across catchments large and small. The success of the program depends as much on community support as on government resources. We all, therefore, need to be alerted to the salinity issue and to our opportunity for input and involvement.
Further information
Allan, M.J. (1994) An assessment of secondary dryland salinity in Victoria. CLPR Technical Report No. 14. CNR.
CSIRO (1980) Salinity: man's oldest environmental problem. CSIRO Information sheet no. 1-32.
MDBC (1999) The Salinity Audit of the Murray Darling Basin. Murray darling Basin Commission, Canberra
Walker G, Gilfedder M and Williams J (1999) Effectiveness of current farming systems in the control of dryland salinity
This note replaces note number SC0026.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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