Journal of the Department of Agriculture, Western Australia, Series 4 Volume 42 Number 1 Journal of Agriculture, 2000/2001 Article 5 2001 WA soil erosion under investigation George Richard Dr [email protected] Follow this and additional works at: http://researchlibrary.agric.wa.gov.au/journal_agriculture4 Part of the Environmental Monitoring Commons, Natural Resources and Conservation Commons, and the Soil Science Commons Recommended Citation Richard, George Dr (2001) "WA soil erosion under investigation," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 42 : No. 1 , Article 5. Available at: http://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol42/iss1/5 This article is brought to you for free and open access by Research Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Research Library. 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We recommend you also search for more recent information on DAFWA’s research library website, DAFWA’s main website (https://www.agric.wa.gov.au) and other appropriate websites and sources. Information in, or referred to in, documents on DAFWA’s research library website is not tailored to the circumstances of individual farms, people or businesses, and does not constitute legal, business, scientific, agricultural or farm management advice. We recommend before making any significant decisions, you obtain advice from appropriate professionals who have taken into account your individual circumstances and objectives. The Chief Executive Officer of the Department of Agriculture and Food and the State of Western Australia and their employees and agents (collectively and individually referred to below as DAFWA) accept no liability whatsoever, by reason of negligence or otherwise, arising from any use or release of information in, or referred to in, this document, or any error, inaccuracy or omission in the information. A survey of soil erosion at 70 sites throughout Western Australia, recently completed as a part of a National Reconnaissance Survey, has shown that erosion is occurring at an unsustainable rate. While still to be compiled into a national publication, Dr Richard George' reports on the Western Australian results, and discusses options for management and further investigation. Soil loss, or sheet erosion through wind and water, represents a significant threat to the sustainability of land not subject to salinisation. Accordingly, an Australia-wide assessment of soil loss was initiated in 1990 by the National Soil Conservation Program (now Natural Heritage Trust). Each Australian State and Territory used a standard sampling method to compare the relative abundance of the radioactive tracer, Caesium-137, with uneroded and others sites to assess soil loss. Samples were collected at each site and sent for analyses to the Australian Nuclear Science and Technology Organisation (ANSTO) laboratory in Lucas Heights, New South Wales. Results for Western Australia indicated that: • • • • about 10 per cent of the sites had soil losses in excess of 20 tonnes per hectare per year, or approximately 15 to 20 millimetres per decade; about 30 per cent of the sites had soil losses of 5 - 20 tonnes per hectare per year, or more than 3 to 5 millimetres per decade; about 40 per cent of the sites had soil losses of 1 to 5 tonnes per hectare per year; about 25 per cent of the sites had negligible soil loss (less than 1 tonne per hectare per year). The highest rates of cumulative soil loss occurred where the following land uses were prevalent: • grazed pastoral areas in the Kimberley and West Pilbara-Gascoyne regions; J O U R N A L OF A G R I C U L T U R E • sloping gravelly soils in the South West woolbelt (500 to 700 millimetres rainfall); Intense summer storms wheatbelt slopes used for crop - pasture rotations; removing nutrient rich can cause significant erosion and damage, • sediment and organic matter. Erosion rates in • high rainfall sites used for intensive horticulture (row cropping). the woolbelt can be very high when associated with Lowest soil losses were associated with permanent pasture land on low slopes, provided they had not been subject to wind erosion, and many pasture-based systems in the woolbelt and wheatbelt regions also had lower soil loss rates. In addition, pastoral areas in the Kalgoorlie and Eucla areas appeared less eroded, although there were too few samples to conclude why soil loss rates were lower in these particular areas. cultivation. 1 - Senior Research Scientist, the Department of Agriculture, Bunbury 15 Vol 42 2000/01 Table 1. Locations of Western Australian sites sampled for ,37Cs. Locations Land use Kimberley Rangelands Annual rainfall (mm/year) Number of sites 500-700 9 West Pilbara-Gascoyne Rangelands 4 <300 Southern rangelands Rangelands 6 <300 Wheatbelt Crop pasture 27 300-500 Woolbelt Pasture 7 500-700 South West Pasture - horticulti re 17 700-1400 Table 2. Summary of net soil loss rates in relation to land use. Net soil loss (t/ha/yr) Pasture & cropping sites Horticulture (potato) sites Annual pasture sites Rangeland sites % 30-49.9 2 0 0 0 2.9 20-29.9 1 0 0 4 7.1 10-19.9 6 1 1 4 17.1 5-9.9 3 1 0 3 10 1-4.9 16 3 3 5 38.6 <1 7 1 6 3 24.3 % 50 8.6 14.3 27.1 100 Methodology The Caesium technique was considered the best indicator of soil loss. Caesium-137 (137Cs) is a radioisotope that was released during atmospheric nuclear weapons tests in the mid-1950s, and the fallout from these tests has been used as a tracer of soil movement. The 137Cs isotope, which has a half-life of 30.5 years, is strongly fixed to the soil and is lost when soils are eroded. By comparing the amount of isotope in an agricultural soil with nearby areas that had not been disturbed since the 1950s, it was possible to estimate how much soil had been lost in the past 50 years. Of the 70 sites surveyed, 50 per cent had been used for annual pasture-crop rotations, more than 25 per cent were rangelands located in the West Pilbara, Gascoyne, Kimberley and the State's southern regions, and the remainder were annual pastures or under potato-vegetable-cropping rotations (see Table 1). The usefulness of the 137Cs technique is often debated as it is strongly influenced by site history and land use. However, the level of erosion estimated using this technique appeared robust in the many areas sampled, so long as care was taken during the survey and when interpreting site-specific results. For example, soil loss rates were dependent on the relative loss between sites chosen as reference sites, the sampling techniques used, 16 and any recent history of soil disturbance (eg. date of clearing and cultivation). McFarlane et al (1992), Loughran et al (1993), and Harper and Gilkes (1994) had also conducted previous studies of soil erosion using the 137Cs isotope in Western Australia. All reports from these surveys indicated that the 137 Cs technique was appropriate as a tool to assess variations in soil loss, so long as the field methods and interpretations were supported by sound field evidence. In particular, the estimates of erosion are dependent on the accuracy of the reference value. At some sites, especially those cleared after 1955, this assumption may not always be valid. For this most recent survey, the method proved useful in providing a long term or cumulative measure of soil loss, but could not determine when the losses had occurred. It was therefore possible that some sites were heavily eroded, but had subsequently been stabilised. Further analyses and more thorough sampling will be required before the 137Cs technique can be used as a quantitative or predictive tool. Sites sampled Over 70 sites were sampled throughout Western Australia. At each site, between 4 and 20 cores (average of 10) were taken on a hillslope (catena), taking note of soil conditions, land use and site history. Sample cores were extracted from a depth of 20 centimetres (maximum 40 centimetres), and were sieved and weighed before being sent for analysis to the Australian Nuclear Science and Technology Organisation in New South Wales. Net soil loss was calculated by comparing the field value to samples taken from a reference site and other local sites. The field value was also compared to soil loss rates derived for both cultivated and pasture land uses using predictive equations from elsewhere in Australia (see McFarlane et al, 1992, 2000). Discussion Impact The impact of soil loss is dependent on land use, timing of the loss, soil characteristics (eg. depth and nutrition) and soil formation rates. In eastern Australia, soil loss rates of less than 0.5 tonnes per hectare per year have been regarded as equivalent to 'natural rates' of erosion in pasture cropping systems. Rates of 1.2 tonnes per hectare per year are considered J O U R N A L OF A G R I C U L T U R E Vol 42 2000/01 acceptable in areas where there are higher rates of soil formation, such as might be expected in the wetter and northern rangelands. The United States Department of Agriculture considers 5 tonnes per hectare per year as a tolerable soil loss rate. In higher rainfall areas the combination of steep slopes, intensive gazing and heavy rainfall at the break of the season creates a high risk of erosion. Soils in this area are In the South West of Western Australia, where soil formation is likely to be the lowest in Australia, perhaps in the order of one metre every million years, soil loss at almost any rate is unlikely to be sustainable. With results from this study indicating loss rates of typically more than 5 tonnes per hectare per year or approximately 3 to 5 millimetres per decade, and extreme rates of erosion equivalent to a loss of 35 to 50 millimetres per decade, mismanagement of erosion can have a serious effect on agriculture. With such low soil formation rates and the extremely low nutrition status typical of soils in Western Australia, erosion can lead to the loss of any inputs of agriculture used over the last 50 years in building up these soils. In addition, soil erosion has offsite impacts. For example, eroding soils can carry nutrients to streams and cause eutrophication, while the sediment deposition from storms may result in direct damage to infrastructure and the environment. In other areas, soil loss may impact in different ways. For example, sediment loads into Lake Argyle from rangelands have been recorded at over 24 Megatonnes per year, and potentially threaten to shorten the design life of Lake Argyle. In other areas, soil loss may remove usually well covered by winter and longer term rates of erosion can be minimised. finer textured soils, such as clays and silts, which may lead to the loss of soil nutrients and the lowering of potential yields, or during wind events, result in direct crop damage due to sandblasting. Management Management of soil loss depends on land use and site conditions. In cropping areas the use of grade banks, contour systems, no-tillage, and stubble retention is likely to result in lowered rates of soil loss. Pastured areas, when mixed with cropping, are especially vulnerable to erosion in dry years and careful grazing management is required. Results There were 23 sites sampled within the pasture-cropping rotation and rangelands sites that had net soil losses between 5 and 49.9 tonnes per hectare per year, and there were two potato-vegetable-pasture rotation sites and one annual pasture site with the same order of erosion. In the soil loss category of 1 to 4.9 tonnes per hectare per year, there were between 3 and 16 occurrences in each land use group, the greatest number being under a pasture-cropping rotation. Approximately 25 per cent of sites had soil losses of less thanl tonne per hectare per year. Two sites had net soil losses in the highest category (30 to 49.9 tonnes per hectare per year), both with pasture-cropping rotations. Little ,37Cs remained on either transect. The Kimberley (13.5 tonnes per hectare per year) and West Pilbara-Gascoyne regions (14.5 tonnes per hectare per year) had the highest average net soil losses. Land uses in the southern rangelands typically had low rates of net soil loss (1 tonne per hectare per year). The horticulture-pasture belt (4.6 tonnes per hectare per year) had relatively high erosion (particularly associated with potato cropping), which was consistent with other independent studies carried out in the same area. The average soil loss in the crop-pasture region (wheatbelt) was 6.6 tonnes per hectare per year, while in the woolbelt (pasture-crop), it was 9.8 tonnes per hectare per year. Many of the sites in this region were known to have experienced serious sheet erosion, caused by heavy grazing and summer storms. Analysis of the impact of removing all sites cleared after 1955 (21 sites) from the database did not significantly alter the magnitude of soil loss, or change the relative erosion ranking of the regional land uses. J O U R N A L OF A G R I C U L T U R E 17 Vol 42 2000/01 Acknowledgements The author thanks Department of Agriculture colleagues Russell Speed and Dan Carter for dieir significant contribution to the survey, including report editing. The author is also grateful to the landholders of Western Australia and colleagues in the Department of Agriculture for their help, including Ian Watson, Paul Findlater, Geoff Elliott, John Simons, Jan-Paul van Moort, Rob Hetherington, Peter Tille, Don Bennett and Geoff Russell. This survey has not been a definitive study of the erosion status of Western Australian soils, although the indications from the range of erosion rates encountered show there is cause for concern in the management of our soil resources. Erosion from upland areas accumulates in lower slopes that are also prone to salinity. Saline areas are unstable without plant cover and in turn allow sediment to be moved to streamlines and river pools. Mr Greg Elliot (ANSTO) managed the sample analysis process. Dr Bob Loughran (Department of Geography, University of Newcastle, New South Wales) and Dr Don McFarlane (now with Water and Rivers Commission, Perth), managed the project and were senior authors of the final report - A National Reconnaissance Survey of Soil Erosion of Australia, Western Australia. Assessing soil erosion in the State's South West. References In areas such as the Swan Coastal Plain, wind breaks and crop residue management may result in lower erosion rates, while in the rangelands, livestock and feral animal management may produce similar outcomes. Conclusions For a truer indication of the State's erosion status, the survey should be expanded to take into account the natural variation of land use, landscape and erosion rates. This was not possible with the resources available at the time of the survey. Also, analyses of erosion, independent of the 137Cs technique, should be employed before firm conclusions are drawn. The results of the National Reconnaissance Survey indicate that land management requires continued attention, both in terms of research and extension of methods that are currently available for erosion control. 18 J O U R N A L OF Harper, R.J. and Gilkes, R.J. (1994). Evaluation of the ™Cs technique for estimating wind erosion losses for some sandy Western Australian soils. Australian Journal of Soil Research, 32: 1369-1387. Loughran, R.J., McFarlane, D.J., Campbell, B.L. and Shepherd, R. (1993). The distribution of caesium-137 in rangeland soils at three sites in Western Australia. Rangeland Journal, 15 (1): 24-38. McFarlane, D.J., Loughran, R.J. and Campbell, B.L. (1992). Soil erosion of agricultural land in Western Australia estimated by caesium-137. Australian Journal of Soil Research 29: 533-546. McFarlane, D.J., George, R.J., Loughran, R.J., Elliott, G.L., Ryder, A.T., Bennett, D. and Tille, P.J. (2000). A National Reconnaissance Survey of Soil Erosion of Australia, Western Australia, ISBN 0725909919, University of Newcastle, NSW, 2308, 264. AGRICULTURE Vol 42 2000/01
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