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Commonwealth Scientific and Industrial Research Organisation


Dryland salinity is a pervasive form of land degradation that has resulted from the clearing of about 17 M ha of native vegetation and the introduction of predominately cereal and pasture-based farming systems in the South-West of Western Australia. The change in water balance caused by clearing deep rooted endemic woodlands increased recharge and resulted in rising groundwater levels. After a lag period, the regolith began filling and groundwater approached the soil surface, evaporating and depositing stored salts in the rootzone of salt sensitive crops. Groundwater levels also rise and affect areas of remnant native vegetation, streams, wet-lands and rural and urban infrastructure (Salinity Strategy, 2000). The area affected was first reported by farmers in the 1955 Australian Bureau of Statistics report as having affected 73,000ha of previously arable land. A further six surveys reported farmers as having estimated the area to have grown to 933,000ha (Trewin, 2002). The area and rate of growth reflected the changed extent and the awareness of farmers as knowledge and extension systems evolved. To provide an objective and ongoing means to map the extent of dryland salinity, a systematic approach to the acquisition and processing of Landsat satellite images was developed as part of the Land Monitor Project. Analysing scenes from 1988 - 1992, and 1996 -2000, two estimates of extent were derived as 860,000ha and 960,000ha (Furby et al 2010, McFarlane et al 2004). Using these estimates and two dates McFarlane et al (2004) extrapolated a rate of increase of 14,000 ha per year. The Land Monitor mapping incurs measurement errors of salt-land omission (areas of salt-land, not mapped as salt-land) and salt-land commission (areas not saline, mapped as salt-land). As previously reported (Furby et al, 2010), the Land Monitor satellite mapping had a tendency to underestimate the extent of salinity in the wheatbelt, particularly moderately affected classes. That is, when Land Monitor reported an area as being saline, it was mapped as saline with a high accuracy. However, the moderate affected areas were either underestimated or not detected. Since the publication of the estimates in about 2000, there had been no update to the change in area affected. This gap – nearly 20 years – was noted by the Office of the Auditor General (OAG) in a review of Dryland Salinity undertaken in 2018 (OAG 2018). The OAG requested the State to establish regular monitoring and reporting of the spread, impact, and cost of dryland salinity. This Report updates the estimate of the extent of dryland salinity. This Report specifies the revised methodology used in the Land Monitor approach and provides an accuracy assessment. Statistical analysis of the Land Monitor results has enabled an updated estimate of the extent of dryland salinity for the South-West of Western Australia. It is not intended that this Report discusses details of what is causing changes in extent.

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dryland salinity, Land Monitor, satellite remote sensing, south-west Western Australia


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