Dryland salinity – extent and impact in the South West of Western Australia

Page last updated: Thursday, 22 December 2022 - 9:24am

More than 1 million hectares of previously productive land in South West Western Australia (SW WA) is severely affected by dryland salinity, and about 0.75 million hectares is moderately affected.. Dryland salinity is a major cause of land degradation and remains a threat to 2.8–4.5 million hectares of highly productive, low-lying or valley soils.

The Department of Primary Industries and Regional Development has calculated the lost value of agricultural production from dryland salinity in SW WA to be at least $519 million per year since 2009–10.

Estimating the extent of dryland salinity

Landholders and scientists estimated the extent and location of dryland salinity in several ways, including estimating or measuring the actual area of salt scald (no plant growth), measuring areas of severely reduced plant growth, and calculating the area with shallow saline watertables (from bore data).

Monitoring groundwater levels provides the information needed to estimate dryland salinity expansion, and when that may occur. Groundwater trends 2007–12 in the agricultural areas showed where the risk from groundwater was rising, falling or stable. This is expected to change with climate change and intensive rainfalls.

Estimates of current and projected extent of salinity in WA have been published by the Australian Bureau of Statistics, Department of Agriculture (now Department of Primary Industries and Regional Development), National Land and Water Resources Audit, the Land Monitor project and the Salinity Investment Framework.

Impact

We estimated the value of lost agricultural production from land affected by dryland salinity in SW WA to be at least $344 million per year in the period 2003–04 to 2008–09, and $519 million per year in the period 2009–10 to 2013–14. However, dryland salinity does not affect all farms equally: farms on flat valley floors are most affected. Also, this estimate does not take into account the benefits of investing more – dollars and labour – on the non-saline land.

Dryland salinity remains a potential threat to 2.8–4.5 million hectares of productive agricultural land. The long-term extent of salinity may take from decades to centuries to develop, especially in areas where recharge is episodic, clearing was staggered, the area cleared is small, or where watertables are deep.

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Dryland salinity extent estimates (from most recent)

Land Monitor update (2009–2018)

The Land Monitor salinity mapping process was updated in 2022 using the latest remote sensing data and improved computational resources to provide a revised estimate of the extent of salt affected land (Caccetta et al. 2022).

Severely salt-affected land in the dryland agricultural area of the SW of WA was mapped between two time periods 2009–2011 and 2016–2019 and compared to the last estimate from 2000. The revision classified over 1.08 million hectares of land as severely salt affected (Table 1). However, independent ground-truthing data from 70 sites in the grainbelt indicated that the update and previous estimates did not identify and map 40.0% of the salt affected land (omission error) and incorrectly mapped 3.1% as salt affected (commission error). Severely salt affected land was mapped more accurately (75%), while less severely affected land was not mapped as accurately (23%).

For the year 2018 a statistically robust method of validation was done – based on the ground-truthing information – to determine the likely extent accounting for omission errors.  This resulted in an estimated extent of salinity of 1.75 million hectares ± 344,000 hectares at the 95% confidence level (Table 1). The independent ground truth mapping and the bias adjusted estimate were in agreement and provide confirmation of the adjusted extent. This result confirms the underestimation of previous surveys. It is not possible to apply the adjustment to previous Land Monitor assessments, as historic validation data is not available for the earlier time periods.

From tha latest Land Monitor estimates and data on groundwater bores, it is clear that salinisation is still occurring in the south west of WA.

Salinity Investment Framework (2000)

Developing a Salinity Investment Framework (SIF) was commissioned by the former State Salinity Council in 2000 to guide public investment in salinity management initiatives at state, regional and catchment levels.

The Department of Agriculture conducted the SIF analysis at a state scale using soil-landscape zones to assess the impact of salinity on private and public land, and infrastructure (towns, roads, rail). The analysis revised the Land Monitor mapping results, hydrologic data and models, provided judgement (impact and potential for adopting options) and economic analyses.

Results from revised Land Monitor data indicate about 1 million hectares of land in the south-west region was affected by salinity in 1998 (Table 1). Of this, 821,000ha is agricultural and 226,000ha is public land. The SIF analysis also estimated about 250 km of highways and main roads and 3850 km of local and unclassified roads were affected by salinity (Sparks et al. 2006, George et al. 2005).

Land Monitor (1988–2000)

The Land Monitor project used sequences of satellite (Landsat TM) imagery, a high resolution digital elevation model (DEM) and other spatial data to map areas of severely salt-affected land based on areas of consistently low productivity.

The project mapped severely salt-affected land over most of the dryland agricultural area of WA for 2 time periods: typically within 1988–92 and 1996–2000. The estimate of severely salt-affected agricultural land was about 860,000ha (4.6%) in 1989 and about 960,000ha (5.1%) in 1996 (Table 1). These estimates indicate the area of severely salt-affected agricultural land increased by 100,000ha over the 7 years or about 14,000ha per year (McFarlane et al. 2004).

This process underestimates salinity in high rainfall areas because much saline land carries permanent cover. It may also overestimate salinity in drier areas where consistently low productivity occurs for reasons other than salinity (Furby et al. 2010).

Land Monitor data was the most comprehensive, high resolution mapping of the extent of salinity currently available for the dryland agricultural region of WA (Caccetta et al. 2010). See Land Monitor update (2009–2018) for an update.

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National Land and Water Resources Audit (2000)

The National Land and Water Resource Audit (NLWRA) identified dryland salinity as one of 7 major themes for an audit of Australia's land, water, vegetation and natural resources. The NLWRA reported areas at risk of shallow watertables as an indicator of the 'risk of salinity', based on regional-scaled soil-landscape system mapping (Short & McConnell 2001).

The definition of dryland salinity was taken to be 'the area where watertables were mapped at a regional scale to be within 2 metres of the land surface and showing no trend, or from 2 to 5m and rising'. The NLWRA estimated that about 3.55 million hectares (16%) of the south-west agricultural region was at risk of being salt-affected in 2000 (Table 1). However, the NLWRA methodology overestimated the area actually affected because not all of the area with a shallow watertable would be affected by salinity.

Department of Agriculture (1994)

In 1994, the Department of Agriculture estimated the area of salt-affected land in the south-west agricultural area (Ferdowsian et al. 1996). The estimate was based on extrapolations from catchment-scale, aerial photograph interpretations, landholder-mapped estimates, ground-based terrain conductivity surveys and satellite remote sensing estimates of salt-affected areas. The area of salt-affected land in 1994 was estimated to be about 1.8 million hectares or 9.4% of the area cleared for agriculture (Table 1).

Australian Bureau of Statistics (1955–2002)

Since 1955, the Australian Bureau of Statistics (ABS) agricultural census has asked landholders to estimate the 'area of salt-affected land which was previously used for crops and pasture'. These surveys, conducted between 1955 and 2002, are the longest running estimates of the area of salt-affected land. They indicate that the area of salt-affected land has grown from about 73,000 to 933,000 hectares (Table 1). However, only 4 surveys included all agricultural shires and the same question in comparable portions of the survey form.

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Table 1 Estimates of the area of salt-affected land in the south-west of WA (adapted from Raper et al.) – from oldest to most recent
Date Area (ha) Source

Methods

1955

   73,000

ABS agricultural census

Landholder estimate
1962   124,000

ABS agricultural census

Landholder estimate

1979   167,000

ABS agricultural census

Landholder estimate

1984   255,000

ABS agricultural census

Landholder estimate

1989    446,000

ABS agricultural census

Landholder estimate

1989    859,000 Land Monitor project

Remotely-sensed data (Landsat TM and DEM) classification

1993    529,000

ABS agricultural census

Landholder estimate

1994 1,804,000

Conference paper

(Ferdowsian et al. 1996)

Department of Agriculture estimates based on available data and information
1996    958,000 Land Monitor project Remotely sensed data (Landsat TM and DEM) classification
2000 3,553,000 National Land and Water Resource Audit Groundwater depth and trend analysis
2002    933,000

ABS Agricultural Census

Landholder estimate

2002 1,047,000

Salinity Investment Framework: Phase I

Revised remotely-sensed data classification from Land Monitor
2010 1,085,000 Land Monitor update Remotely sensed data (Landsat TM and DEM) classification
2018      
2018 1,748,000
+/- 344,000
Land Monitor update bias adjusted Bias adjusted estimate based on the validation of Land Monitor 2018 data

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For more information

See the Report card on sustainable natural resource use in agriculture Chapter 2.7 Dryland salinity.

References

Australian Bureau of Statistics 2002, Salinity on Australian farms, Bulletin 4615, ABS, Canberra.

Caccetta D, Dunne G, Georg, R & McFarlane D (2010) ‘A methodology to estimate the future extent of dryland salinity in the southwest of Western Australia’, Journal Environmental Quality, vol. 39, pp. 26–34.

Caccetta PA, Simons J, Furby S, Wright N, and George R (2022) Mapping salt-affected land in the South-West of Western Australia using satellite remote sensing, CSIRO Report Number EP2022-0724, CSIRO, Australia, accessed 12 September 2022.

Ferdowsian, R, George, R, Lewis, F, McFarlane, D, Short, R & Speed, R 1996, ‘The extent of dryland salinity in Western Australia’, Proceedings of the 4th National Conference and Workshop on the Productive Use and Rehabilitation of Saline Lands, National Program on Productive Use and Rehabilitation of Saline Lands, Albany, Western Australia, pp. 89–97.

Furby, SL, Caccetta, PA & Wallace, JF 2010, ‘Salinity monitoring in Western Australia using remotely sensed and other spatial data’, Journal Environmental Quality, vol. 39, pp. 16–25.

George, RJ, Kingwell, R, Hill-Tonkin, J & Nulsen, R 2005, 'Salinity investment framework: agricultural land and infrastructure', Resource management technical report 270, Department of Agriculture, Western Australia, viewed 18 December 2019, https://researchlibrary.agric.wa.gov.au/rmtr/253/

McFarlane, DJ, George, RJ & Caccetta, PA 2004, ‘The extent and potential area of salt-affected land in Western Australia estimated using remote sensing and Digital Terrain Models’, Engineering Salinity Solutions: Proceedings of 1st National Salinity Engineering Conference, Institution of Engineers, Perth, Western Australia, pp. 55–60.

Raper, R, Speed, R, Simons, J, Kendle, A, Blake, A, Ryder, A, Smith, R, Stainer, G & Bourke, L 2014, 'Groundwater trend analysis for south-west Western Australia 2007–2012', Resource management technical report 388, Department of Agriculture and Food, Western Australia, viewed 18 December 2019, https://researchlibrary.agric.wa.gov.au/rmtr/374/

Short, R & McConnell, C 2001, 'Extent and impacts of dryland salinity', Resource management technical report 202, Department of Agriculture, Western Australia, viewed 18 December 2019, https://researchlibrary.agric.wa.gov.au/cgi/viewcontent.cgi?article=1352&context=rmtr

Sparks, T, George, R, Wallace, K, Pannell, D, Burnside, D & Stelfox, L 2006, Salinity Investment Framework Phase II, Western Australian Department of Water, Salinity and land use impacts series, Report No. SLUI 34, Perth.

Contact information

Richard George
+61 (0)8 9780 6296
John Simons
+61 (0)8 9083 1128
Russell Speed
+61 (0)8 9956 8561
Paul Raper
+61 (0)8 9780 6295