Much of Australia's landscape is naturally saline – think of the great salt lakes in our interior. Many of our agricultural lands also contain vast reservoirs of salt, but normally these are held deep within the soil profile where they don't affect plant growth. The salt that sits deep in the soil profile may have several sources. In Western Australia, for example, the main source is believed to be the ocean – salt is carried inland by the prevailing winds and deposited on the land in rainfall and dust. Over millions of years, this process has deposited large amounts of salt in what is now the West Australian wheatbelt. Some salt in the soil profile may date back even further, to when the parent rocks themselves were formed. These rocks release salts as they weather. Other possible sources of salt are ancient drainage basins or inland seas that evaporated during arid periods, leaving behind salt deposits that still remain today. Groundwater, recharge and discharge Soil salinity occurs when the salt in the soil profile is brought to the surface by rising watertables. �To understand this process, you need to know about groundwater. Groundwater is, as the name implies, water in the ground. Usually, somewhere below the surface of the soil, the soil is saturated with water. The top surface of the groundwater layer is called the watertable. Water that drains through the soil profile and reaches the watertable is known as recharge; water leaving the groundwater – perhaps through uptake by tree roots, or when it flows into a river system – is called discharge. In the past, native woodlands and forests were able to keep the salt sitting deep in the soil profile at bay – recharge and discharge were more-or-less in balance; the native vegetation used up most of the rain that fell, and some species were also able to 'drink' from the groundwater in times of drought. Since little water made its way through the soil profile, the salt stayed where it was – dispersed and quite harmless. When does the salt become a problem? Surprisingly in such a dry continent as Australia, salt becomes a problem when there is too much water. When European farmers arrived in Australia about 200 years ago, they began to fell large areas of forest and woodland to make way for agriculture and grazing – a practice that continues in some areas today. But clearing the native vegetation has an unintended consequence. The annual crops and pastures that replace the native vegetation cannot use all the rain that falls; they only grow for part of the year, and their shallow roots cannot absorb water deep below the soil surface. Thus, groundwater recharge increases and the watertable rises. As it does, it dissolves the salt lying dormant in the soil profile and the salt becomes more and more concentrated as the water moves upwards. If the salty water keeps rising, it eventually reaches the surface and subsurface layers of the soil. The water then evaporates, leaving the salt behind. It was our success in clearing native vegetation that has led to the development of dryland salinity. (Irrigated-land salinity is caused by a similar effect – the application of excess water to land causes the watertable to rise. The problem is made worse if the irrigation water itself is also saline.)
The productivity of crops and pastures, as well as the health of other vegetation, declines as the saline watertable reaches their root zones. At low points in the landscape the white scars of surface salt start to appear, an ominous warning to farmers that not far below their land lurks a dreadful beast. High concentrations of salt reduce plant growth in two ways. First, salt is hydrophilic: that is, it attracts water. So, when it is present in soil at sufficiently high concentrations, salt makes it more difficult for plants to absorb water. Second, because many plants can't exclude salt from the water they take up, or expel it, the concentration of salt increases in their cells and eventually causes their death. Salinisation of Australia
In 2001 an estimated 2.5 million hectares of land had become salinised since the introduction of European farming methods. At first glance this may not sound very serious: Australia covers an area of 768 million hectares, so salinisation has claimed less than one-hundredth of one per cent of the country's surface area. Unfortunately, though, much of the land lost to salinisation was valuable farmland – parts of the West Australian wheatbelt, the crop and pasture zones of the Murray-Darling Basin, and some once highly productive irrigated lands. Scientists are predicting that salinisation may cause the withdrawal of up to 14 million hectares of land from agriculture and pasture within the next 50 years (and affect a total of 17 million hectares of land). It will therefore have serious consequences for our local, regional and national economies and the livelihoods of thousands of farming families. Threat to water quality Salinity also poses a serious threat to Australia's water resources. As salinity spreads, it contaminates rivers, lakes, reservoirs and groundwater supplies. Southern Australia, where salinity is most prevalent, is also chronically short of fresh water and cannot afford to lose what it has to salinity. But stream salinity in the lower reaches of the Murray River – Australia's most important freshwater resource – already exceeds 800 electrical conductivity (EC) units (the World Health Organization's recommended limit for safe drinking water) for about 35 days a year and is expected to increase. The potential impacts of rising stream salinity are severe. Hundreds of thousands of people rely on the Murray River for their drinking water; Adelaide, for example, draws about 40 per cent of its water supply from the Murray in a normal year and up to 90 per cent during a drought. The 1999 Salinity Audit conducted by the Murray-Darling Basin Ministerial Council predicted that by 2020 Murray River water at the town of Morgan (upstream of where Adelaide draws its water) will exceed 800 EC units for nearly 150 days a year. Salt can be removed from water but at a considerable cost; rising salinity in the Murray therefore poses a very real threat to the economy of Adelaide and other South Australian towns. Threat to native species
The impacts of salinity are not confined to economics or agriculture. The Murray-Darling Basin and the West Australian wheatbelt are already largely cleared of their original vegetation. Many of the original plant and animal species found in these regions are therefore already scarce and in many cases threatened with extinction. Salinisation might deliver the final blow to many such species. According to the 2000 National Land and Water Resources Audit, the water quality of 80 wetlands across Australia is either affected or threatened by dryland salinity. In Western Australia, the audit estimated that salinisation threatens up to 450 plant species with extinction. The salinisation of streams, rivers and lakes is also likely to cause the degradation and extinction of aquatic biota, although this has not been well studied. Grinding down the salt problem For more than a decade, scientists have been working with land managers, Landcare groups, government and industry to learn more about salinity and how it can be dealt with. As the research effort has increased it has become clear that the problem is highly complex and that, in many cases, stemming the rise of watertables and then lowering them may take many decades – if it can be done at all. Since removing trees from the landscape caused the problem in the first place, in areas not yet affected by salinity but thought to be at risk, smart land managers are putting a halt to land-clearing (although clearing does continue in some places, despite repeated warnings by scientists). In areas already affected by salinity, replanting trees seems an obvious way to solve it. Planting native species, with their ability to grow (and use water) all year round, and to perhaps use water from the watertable in times of drought, is certainly one strategy available to land managers and when done on an adequate scale might often be successful. But it also seems clear that for the worst salinity problems small-scale plantings – such as along fence-lines, or on just one farm – are unlikely to have much impact. To understand why, we need to dig a little deeper – down to the groundwater (Box 1: Groundwater systems). Managing saline lands Given that tree-planting to reduce recharge may not result in lower watertables within a reasonable timeframe, land managers have to consider a range of other measures as well, such as:
Such efforts are more likely to succeed if they are based on a profound understanding of all the processes of salinisation – from the paddock to the region.
Long-term studies using these techniques can assess the effectiveness of a land management strategy. For example, long-term monitoring is needed to determine how long it takes a groundwater system to respond to different management interventions. In the early days of European settlement, no-one realised that land-clearing and other land-use practices would unleash a monster; now we face a huge challenge to bring it back under control. Rigorous science combined with strong community and government support offers the best hope of doing so – before a big chunk of the Australian landscape is devastated. Box Related Nova topics: |
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