Salinity – the awakening monster from the deepThis topic is sponsored by the CRC for Landscape Environments and Mineral Exploration. Salinity is making more and more of our land unusable and our water undrinkable.
Key text
Key textMuch 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 1. Groundwater systemsAustralian groundwater systems are not all alike. Recently they have been categorised into three main groups: local, intermediate and regional, each of which is further subdivided into four on the basis of their underlying geology.Local groundwater systems Local groundwater system catchments usually occur on a horizontal scale of about 1-3 kilometres and tend to occur in hilly terrain. Because of their relatively small size and the often low permeability of the underlying geology, these systems rapidly ‘fill up’ with water after land-clearing, and salinity and waterlogging may occur within a few decades. Such systems also respond quite rapidly to efforts to reduce recharge; well-designed and strategically located tree plantations, for example, could start to lower the watertable within a few years. Unfortunately, the worst salinity problems usually occur in intermediate and regional systems, where tree-planting is less likely to solve the problem. Intermediate groundwater systems Intermediate groundwater systems occur on a horizontal scale of 10-15 kilometres. These systems give rise to some of the more spectacular scenes of salinisation in Western Australia. Large-scale tree-planting – over much of the catchment – would usually be required in such systems to significantly reduce recharge. Regional groundwater systems Regional groundwater systems occur on a scale of 50 kilometres or more, usually in flat terrain. Limiting recharge is difficult because of the large surface area and also because regional groundwater systems often occur in semi-arid areas where tree-planting may not be economically viable. Moreover, efforts to lower watertables in intermediate and regional-scale groundwater systems is made more difficult by their generally low discharge capacities: in other words, since the terrain is often quite flat, the flow of groundwater to low points in the catchment and subsequent drainage out of the system can be very slow. So even if recharge is prevented, the watertable may take decades – and even hundreds of years – to return to previous levels. Related sites
Activities
Further readingAustralasian Science July 2003, page 27 Lifting the lid on an ancient curse (by Julian Cribb) Describes new technologies that pinpoint the presence of salt under the landscape.
March 2001, pages 22-23 Balancing biodiversity with land clearing (by Greg Siepen and Clive McAlpine) Discusses ways for farmers to protect natural resources.
Ecos No. 125, 2005, page 33 A fish farming salinity solution (by Steve Davidson) Looks at the feasibility of inland aquaculture in salt-affected areas.
No. 121, 2004, page 33 Salt water wheat from wild barley Looks at the development of a salt and waterlogging tolerant cereal.
No. 118, 2004, page 33 Bacteria help wattles 're-green' Australia Reports on the inoculation of wattle seedlings with bacteria to help them establish and grow.
No. 117, 2003, pages 8-9 One golden pond (by Wendy Pyper) Describes a solar pond that generates heat energy and helps restore salinity-affected pastures.
No. 115, 2003, pages 23-36 The Wentworth view (by Wendy Pyper) Presents the views of the Wentworth Group – Australian scientists with a vision for land-use change.
No. 111, 2002, pages 18-25 Reinventing agriculture (by Steve Davidson) Looks at options for re-vegetation and new ways of farming that mimic the hydrology of pre-existing natural ecosystems.
No. 102, 2000, page 7 New agriculture needed to combat salinity (by Wendy Pyper)
Focus on Salt Provides articles on salinity research and development projects.
Issues March 2007, pages 31-35 The salinity threat (by Ken Lawrie) Looks at salinity monitoring, sources of salt, indicators of salinity, climate change and salinity, and dealing with salinity.
New Scientist 19 December 2005 Dead Sea fungus's secret of survival may help crops (by Kurt Kleiner) Reports on the discovery of a fungus that thrives in the Dead Sea and the possible use of its genes in making plants salt tolerant.
28 September 2002, page 17 Supercrop thrives on saline soil (by Andy Coghlan) Explains that saltwort could be a useful crop for saline soils.
4 August 2001, page 13 Greening the badlands (by Catherine Zandonella) Describes genetically modified salt-tolerant crops that will grow on saline soils.
Salt Magazine Reports on success stories from people tackling dryland salinity.
Useful sitesCSIRO, Australia
Salinity – an introduction (Department of Agriculture and Food, Western Australia)
Explains what salinity is and its impact. Focuses on the Western Australian situation.
Australian Dryland Salinity Assessment 2000 (Australian Natural Resources Atlas) This report from the National Land and Water Resources Audit provides
a variety of information (eg, the situation in individual States, case
studies, management options and a number of fact sheets). Dryland salinity
in Australia is a summary of the Assessment.
Australian Broadcasting Corporation
Brochures and factsheets (Australian National Action Plan for Salinity and Water Quality)
The National Action Plan for Salinity and Water Quality is a strategy for tackling salinity and for improving water quality in some of Australia’s worst affected areas. Factsheets include 'Australia's salinity problem' and 'Frequently asked questions'.
Science overcoming salinity: Coordinating and extending the science to address the nation’s salinity problem (Parliament of Australia) Report of the House of Representatives Standing Committee on Science
and Innovation into how Australia is combating salinity. The full report
and each of the eight individual chapters are available as PDF files.
Information from each State
Glossarydryland salinity and irrigation salinity. There are two kinds of soil salinity resulting from human activities: dryland salinity (occurring on land not subject to irrigation) and irrigated-land salinity. Both occur when rising watertables dissolve natural salts in the soil and bring them to the surface.electrical conductivity (EC) units. The measure of a solution's ability to conduct electricity. EC units are used to express salinity levels in soil and water. When salt is dissolved in water the conductivity increases, so the more salt, the higher the EC value. Another salinity measurement is the total dissolved solids (TDS). For more information see Measuring the salinity of water (Department of Primary Industries, Victoria). electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA). gamma rays. The shortest wavelength of electromagnetic radiation. For more information see Gamma waves (NASA,USA). groundwater. Water stored naturally below the land surface in a saturated zone of the soil. The top of this groundwater is called the watertable. For more information see Interacting sub-systems of the Earth that together produce a unique biome – What is groundwater? (University of New South Wales Groundwater Centre, Australia). parent rock. The original rock from which a soil has come. For example, sandstones are often the parent rocks for sandy soils. Except where there is extensive weathering, the composition of the mineral fraction of the soil generally indicates the nature of the parent rock underneath. Layers of soil and subsoil lie on top of the bedrock. recharge or discharge.The recharge rate is the rate at which an aquifer is replenished or topped up with water (inflow). The other important variable for groundwater management is the discharge rate, or the rate at which water is taken out of the system (outflow). In some cases aquifers can discharge naturally to rivers and springs and so the water is not being removed from the system. The two variables determine the water balance, which is part of the larger water cycle involving the journey of water as it falls from the sky, onto land or sea or aquifer, and back again. remote sensing. The act of obtaining information about an object from a distance. Although that distance can be small or large, remote sensing usually means gathering data from some distance above the Earth's surface (eg, aerial photography and satellite remote sensing). For more information see About remote sensing (Geoscience Australia) and An introduction to remote sensing (CSIRO Mathematical and Information Sciences, Australia). soil profile. Where soil has been cut through vertically, such as along a roadside embankment, you may see that it has various layers of different textures and shades. This is called the soil profile. The top layer, called the A horizon, contains most of the plant roots, it is where most biological activity occurs and where organic matter accumulates. Water washes clay particles down out of this horizon. The next layer – the B horizon – is where the clay particles and soluble substances washed down from above tend to accumulate. Below that is the C horizon, or parent rock. The type of parent rock can affect the fertility and structure of the soil that develops above it. watertable. The top surface of the groundwater.
External sites are not endorsed by the Australian Academy of Science. Posted August 2003. The Australian Foundation for Science is also a supporter of Nova.
This topic is sponsored by the CRC for Landscape Environments and Mineral Exploration.
|
