Monitoring the white death – soil salinityNew technology is being used to help monitor the extent of dryland salinity threatening large areas of Australia's agricultural zone.
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Back to basics You will get more from this topic if you have mastered the basics of electromagnetic radiation – this link will take you to an annotated list of sites with helpful background information. Key text
The term 'dryland salinity' strikes fear into the hearts of many Australian farmers. Some call it the white death because it conjures up images of lifeless, shining deserts studded with dead trees. Fears of the 'white death' seem justified. Dryland salinity currently affects more than 5 million hectares of land, mostly in southern Australia and causes damage totalling $270 million each year. What is dryland salinity? There are two kinds of soil salinity: dryland salinity (occurring on land not subject to irrigation) and irrigated land salinity. Both describe areas where soils contain high levels of salt. Usually, plants and soil organisms are killed or their productivity is severely limited on affected lands. 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 problem occurs when this salt is brought to the soil surface by rising water tables (Box 1: Salinisation – causes and prevention). Where does the salt come from? The salt that sits deep in the soil profile may have several sources. In Western Australia, 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 a time scale of 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. Monitoring the problem In the past, farmers estimated the extent of salinisation on their properties in response to questionnaires issued by the Australian Bureau of Statistics. This method is thought to have underestimated the extent of salinisation, partly because the definition and recognition of salinisation varies between farmers. Nor do such methods provide maps of where the salinity is, where it is spreading to, or the rate at which it is spreading. In recent years, scientists have developed new techniques for monitoring salinity. Most involve what is known as remote sensing. This is the collection of data using devices fitted to an aeroplane, satellite or some other craft located above the Earth's surface. Such technology can be used to gather a range of information related to salinity. Often remote sensing involves cameras that can record electromagnetic radiation – particularly visible light and infrared light – reflected from the Earth's surface. Electromagnetic reflections – sensing the differences When the sun's rays – made up of electromagnetic radiation of many different wavelengths – strike plants, water bodies, soils and other features on the Earth's surface, some wavelengths are absorbed by molecules in these features and some are reflected. Different features on the Earth's surface will absorb and reflect different parts of the electromagnetic spectrum depending on their chemical make-up. In this way, different parts of the electromagnetic spectrum provide information about the Earth's surface that may be useful for the detection of salinisation. Monitoring by aeroplane Australian scientists have tested a number of techniques to collect and analyse electromagnetic information. For example, colour infrared film can be used to take photographs from aeroplanes. Different colours (corresponding to different wavelengths within the infrared band) will show vegetation under varying levels of stress, which can then be related to the degree of salinity. Dark-green vegetation produces a bright red image, light-green foliage a pink image, barren saline soil a white image, salt-stressed vegetation a reddish-brown image. If such photographs are taken of the same area over different years, changes in the pattern of salinisation can be monitored. Similarly, video cameras can be used from aeroplanes to collect information in the visible band of the spectrum. The videos show salinity patterns and the way these change over time. Another airborne electromagnetic technique makes use of the fact that electrical conductivity increases with increasing salinity. It involves an aeroplane flying low over the ground. Mounted on board is an electromagnetic transmitter and trailing behind on a cable is a receiver. The transmitter sends out pulses of electromagnetic radiation. When these hit the ground, they induce electrical currents to flow in conductive areas. The decay of these currents produces a magnetic field which is recorded by the receiver trailing behind the aircraft. The recording is then analysed to determine the conductivity of the ground. Monitoring by satellite Increasingly, scientists are also using satellite images to analyse salinity patterns across large areas. Most images are supplied by a series of scientific satellites known as Landsat. These orbit the Earth, recording information about the electromagnetic radiation reflected by the Earth's surface. In Landsat satellites, an instrument called a Thematic Mapper makes regular observations in bands ranging from the visible to the thermal on each area of the Earth's surface, sending the information back to Earth. Many scientists consider that data produced in this way can be used effectively for the detection and monitoring of salinity, and experimental results support this view (Box 2: Mapping salinity). Predicting and preventing the advance of salinity The knowledge gained from the new monitoring techniques, along with that generated by decades of painstaking field research, is offering many insights to the causes of salinisation. Importantly, this is aiding scientists in the development of methods to predict sites most at risk of salinisation so that preventative measures such as tree-planting can be taken (Box 1). Armed with the information such methods will provide, a coordinated community response could succeed in combatting the white death, before it eats out our agricultural heart.
Ground water rising Ground water is, as the name implies, water in the ground. Usually, somewhere below the surface of the soil, the soil is saturated with water. It is not quite like an underground lake – the water is most commonly held within the soil profile rather than in some vast underground cavern. The top surface of the ground water layer is called the water table. Ground water recharge is the amount of water being added to the ground water. If this is higher than discharge, which is the amount of water lost from the ground water, then the water table rises. As it does, the water dissolves salt held 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 evaporates, leaving the salt behind. Why is the ground water rising? In our quest to prepare Australian soils for agriculture, we cleared trees by the billion. Yet trees played a crucial role in maintaining the water balance in our ancient soil profiles. It was our success in clearing trees 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 water table to rise. The problem is made worse if the irrigation water itself is also saline.) Trees help control ground water levels in two ways: by decreasing recharge and by increasing discharge.
Trees: Weapons against salt? If salinisation is caused by the removal of trees from the landscape, it seems logical that putting them back will solve the problem. Farmers throughout the country, including those in zones most affected by salinisation, have embarked on a massive tree-planting campaign, giving hope that the rural landscape will recover from its many ailments, including salinisation. The ability of trees to reduce salinisation is still not fully known, although they have been shown to lower water tables in some areas. Australian scientists continue to investigate the potential of trees to reclaim saline areas and to prevent currently unaffected land from becoming salty. In the meantime, programs such as the Joint Venture Agroforestry Program have produced guidelines to ensure that any trees planted have the maximum positive benefit. For example, healthy, highly productive trees will be more effective than less productive trees in lowering water tables because they will use more water. Some farmers may be tempted to plant trees on the salty areas, but unless these are specially adapted to saline soils they may not grow well and therefore not play much of a role in solving the salt problem. In some situations, trees planted higher up in the catchment, in areas of high recharge, may be more effective. And, in general, the more trees planted, the more impact they will have on water table levels. Other weapons against salt Tree-planting is just one of many strategies that show promise in the fight against salinity. For example, deep-rooted perennial crops such as lucerne lower water tables and may often be a viable alternative to trees. In places where soils are likely to remain saline for some time, salt-tolerant species such as saltbush – which can be eaten by sheep – have had some success. And scientists and agriculturalists are working to enhance the salt tolerance of other plant species through breeding programs. Innovative farmers are experimenting with other possible solutions. For example, driven by the knowledge that salt is a potentially valuable product, some farmers are pumping their saline ground water into evaporation ponds. The salt harvested from these ponds can be sold as a raw material in the production of important chemicals such as sodium carbonate and sodium hydroxide, or as table salt. Related sites
Scientists may peer at satellite images or process them using high-powered computers, but the only way to assess their accuracy is to go out into the field and measure the salinity at ground level. A recent study by scientists at CSIRO Mathematical and Information Sciences tested a remote sensing technique in three study areas in Western Australia. They analysed a series of Landsat images, which they combined with information on contours, the location of roads and farm boundaries, and farm management histories. They then compared the results of these analyses with the locations of known salt-affected and changing sites, as supplied by farmers, field officers from Agriculture Western Australia and from previous salinity mapping exercises. Results were very encouraging. At one study site, salt-affected land was mapped remotely at an accuracy of almost 100 per cent. Accuracy was lower at other sites, but refinement of the techniques will continue to improve results. Predicting where salinisation will occur next Scientists have shown that a number of factors determine the vulnerability of sites to salinisation. These include:
Combining information on these and other factors could allow the prediction of sites vulnerable to the saline menace. This is where a geographic information system (GIS) can play a role. GIS is a computer application that involves the storage, analysis, retrieval and display of data that are described in terms of their geographic location. The most familiar type of spatial data is a map – GIS is really a way of storing map information electronically. A GIS has a number of advantages over old-style maps, though. One is that because the data are stored electronically they can be analysed readily by computer. In the case of salinity, scientists can use data on rainfall, topography, soil type – indeed, any spatial information that is available electronically – to first determine the combinations most susceptible to salinisation, and then to predict similar regions that may be at risk. Much information is already in a form that can be used in a GIS, and more is being added continually – including that produced by Landsat. As the databases and prediction techniques improve, farmers and land management agencies will be better placed to wage an assault on salt. Related sites
Australasian 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.
January/February 2000, pages 23-24 Audit predicts grim salinity future (by Stephen Luntz)
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. 114, 2003, page 8 Mining tool retrained to seek acids and salts (by Wendy Pyper) Explains how a tool developed for minerals exploration and mining may soon be used against acid mine drainage and salinity.
No. 111, 2002, pages 18-25 Reinventing agriculture (by Steve Davidson) Looks at options for revegetation 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)
No. 96, 1998, pages 9-28 Dealing with dryland salinity (by Bryony Bennett) A special issue with eight articles dealing with dryland salinity, including:
No. 88, 1996, pages 22-27 And how is the Earth today? (by Bryony Bennett) Describes an Australian sensor that monitors the temperature of the Earth from a remote-sensing satellite.
Focus on Perennials June 2008, pages 4-5 Climate change reduces salinity risk (by Jill Griffiths) Claims that dryer conditions have slowed the effect of salinity on agriculture.
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.
Soils and their conservation (University of Adelaide, Australia) This site includes general information about soils. Click on 'What are soils good for?' and then 'Issues'
to find out more about erosion, salinity, acidity, pollution and fertility
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.
Salinity – an introduction (Department of Agriculture and Food, Western Australia)
Explains what salinity is and its impact. Focuses on the Western Australian situation.
Salinity (Department of Sustainability and Environment, Victoria)
Contains information on how to recognise dryland salinity, including salinity indicator plants, and a guide to salt-sensitive plants.
The salinity problem (Bureau of Rural Sciences, Australia)
Provides an introduction to the salinity problem. From here you can link to more specific information such as 'Salinity processes' and 'Dryland salinisation'.
The salinity audit of the Murray-Darling Basin (Murray-Darling Basin Commission)
Both the full report and a summary of the salinity audit are available as PDF files. Also of interest is Water and land salinity, which gives a good coverage of these problems in the Murray-Darling Basin.
Remote sensing and monitoring (CSIRO Mathematical and Information Sciences)
Explains what remote sensing is and how it is done. Includes case studies and projects relating to monitoring and predicting salinity.
CRC for Plant-based Management of Dryland Salinity
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). 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. 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, is the one where most roots are, where most biological activity occurs and where organic matter accumulates. Water washes clay particles down out of this horizon. In the next layer – the B horizon – 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.
External sites are not endorsed by the Australian Academy of Science. Posted November 1998. The Australian Foundation for Science is also a supporter of Nova. This topic is sponsored by the Land Monitor Project and the Australian Government's National Innovation Awareness Strategy.
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