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Salinity |
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Indigenous plant communities have roots that use a lot of water. In a balanced situation rain falls, is taken up by the roots and evaporates from the leaves into the air again (transpiration). With the arrival of Europeans in Australia, large expanses of land were cleared for farming. Deep rooted native plants were replaced with shallow rooted annual crops and pasture. These introduced plants used less water, so that unused rain and irrigation water filtered down into the groundwater. Groundwater is water held deep underground in the soil and rock pores. The cause - Recharge areas A recharge area is where water enters the groundwater system. These are usually higher areas such as sandy rises, rocky ridges and outcrops. As the water moves through the soil layers it dissolves the stored salts, transporting them to the ground water. When vegetation is removed the groundwater level becomes raised bringing salinity with it. The effect - Discharge areas A discharge area is where groundwater reaches the soil surface on lower slopes and the base of valleys. If the groundwater is salty it causes damage to soil structure and excess water leads to waterlogged soil. These low lying areas tend to be the most productive in terms of farming and are of economic importance. What are the problems caused by salinity? Salinity can set off a whole chain reaction of problems: • Salty and waterlogged soil can slow and eventually prevent plant growth. • Along waterways and wetlands salinity can mean a loss of native vegetation, loss of habitat for wildlife and reduced biodiversity. • For farmers salinity results in reduced crop and livestock yields, and restricts the type of crops and pastures that can be grown. • Eventually salinity can lead to loss of vegetation. This leaves exposed patches of ground known as salt scalds. Erosion then occurs where the exposed top soil is carried away by rain and wind. • This eroded soil causes siltation of creeks, rivers, wetlands and eventually affects marine environments. Silt flooding out of estuaries has the power to ultimately kill marine ecosystems such as the Great Barrier Reef. • The combination of salt and siltation results in poor water quality in creeks, rivers and wetlands affecting wildlife, and humans who use this water for irrigation, industry, drinking and recreation. • Where salinity has affected natural bush areas tourists no longer want to visit, resulting in loss of local business. What is being done to combat salinity? Recharge and discharge areas may be great distances apart. So landholders need to cooperate regionally in order to successfully reduce water entering recharge areas. Remote sensing from aeroplanes and satellites is used to identify and map recharge and discharge areas in a region. This information allows for the development of strategic solutions to salinity. To prevent salinity the basic idea is to use water where it falls. Recharge areas need to be replanted with native trees, shrubs and groundcovers to create root zones which spread throughout the soil at a range of depths so maximising rainfall uptake. Remaining areas of natural bush need protection. Farmers on discharge areas need to replace shallow rooted annual plants with deep rooted perennial crops and pastures to keep the subsoil dry and the watertable low. Planting with salt tolerant trees, fencing off sensitive sites, sensible irrigation and careful grazing are all ways to prevent salinity and erosion. In the Murray Darling River region harvesting salt from saline groundwater is another way used to tackle salinity. This apricot coloured salt is sold as gourmet salt flakes for cooking and seasoning.
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