Salinity - Risk and Hazard - South Australia
South Australia
Where is there a dryland salinity risk or hazard and how was this determined?
Estimate of salt-affected lands and risks was based on field survey at scale of 1:100 000. Projection for 2050 was based on extrapolation of field survey and groundwater trend data from representative catchments across the agricultural regions. The South Australian estimates of current extent cannot be compared directly to other States as they are better estimates of affected land than exist for the other States. The figures for 2050 are considered comparable to other State 2050 projections.
For the purposes of this study, the most recently available aerial photography (at a scale of 1:40 000) was used where possible to delineate the actual affected areas. A GIS coverage was created of the mapped current areas (both primary and secondary) affected for the year 2000 and is available in the Australian Natural Resources Atlas. An important contribution to this process was made by the PIRSA "Catchments back in balance" project which mapped salinised areas during the formulation of salinity management plans in various catchments throughout the State.
Despite the difficulty in determining when historical salinity occurred, an attempt was made to distinguish possible pre-European or primary salinity from secondary or human-induced salinity. Primary salinity has, and probably never will be considered for agricultural purposes, and therefore will not be included in the cost estimates of the impact on agricultural production. However, these areas will have an ongoing impact on infrastructure costs and were included in those estimates. Lagoons and wetlands which have, or may become salinised have been delineated and excluded from areas of secondary salinity because again, they are not used for agricultural production. Areas under tidal influence along the coast were also excluded from the mapping coverage.
The table below presents a summary of current areas which has been derived from the GIS coverage and professional judgement given some limitations in the available aerial photography.
| Region | Primary | Secondary | Total |
|---|---|---|---|
| Upper South East | 22 500 | 250 500 | 272 000 |
| Murray Basin | 16 700 | 19 800 | 36 500 |
| Eyre Peninsula | 35 200 | 20 400 | 55 600 |
| Kangaroo Island | 500 | 5 600 | 6 100 |
| Mid North | 300 | 14 800 | 15 100 |
| Yorke Peninsula | 8 000 | 13 900 | 21 900 |
| Mt Lofty Ranges | 1 200 | 1 200 | |
| Total | 84 000 | 326 000 | 410 000 |
How can dryland salinity risk change over time?
Estimate of salt-affected lands and risks was based on field survey at scale of 1:100 000. Projection for 2050 was based on extrapolation of field survey and groundwater trend data from representative catchments across the agricultural regions. The South Australian estimates of current extent cannot be compared directly to other States as they are better estimates of affected land than exist for the other States. The figures for 2050 are considered comparable to other State 2050 projections.
In determining areas at risk to dryland salinisation, there are two fundamental questions that need to be considered.
- In a region where dryland salinisation has already occurred, is it possible that there will be new areas affected? In other words, given similar geology and climate throughout the region, if salinisation is going to occur, would it have already happened by now?
- Are areas currently affected going to expand? In other words, is it already as bad as it's going to get, and has a new equilibrium already been reached?
Before European settlement, a natural hydrological balance existed between recharge and discharge. Land clearing lead to increased recharge which inevitably resulted in increased discharge in order to restore this balance. Eventually, a new equilibrium will be reached when the increased recharge is balanced by increased discharge, through evaporation from salinised areas and increased groundwater discharge. When this occurs, the areas affected by dryland salinity will stabilize. This stability of the system is dependent on length of time since clearing has occurred. Based on local experience, it is felt that on average for local flow systems, a post clearing period of 30-40 years is required prior to a new equilibrium being established.
When considering areas at risk, it must be remembered that about 80% of the agricultural regions in SA are almost completely devoid of native vegetation, with considerable areas cleared over 100 years ago.
In areas with regional flow systems such as the Murray Basin, where groundwater level trends were established, the areas at risk to dryland salinisation were assessed on the basis of where the rising groundwater would rise within 1 - 2 m of the land surface. Because the regional watertable elevation contours are generally well known, it is a relatively simple exercise to extrapolate the rising trend and determine where and when the watertable will intersect the ground surface. Accurate topographic maps or digital terrain models are required for the best results using this method, which was successfully used in the Upper South East using GIS modelling. Unfortunately, most areas of SA do not have sufficient data to do this type of modelling.
GIS coverages have been prepared showing the areas at risk in 2020 and 2050 for the Murray Basin and are available in the Australian Natural Resources Atlas. Broad areas with very low topographic gradients have more land affected for a given watertable rise, than steeper undulating country. Also, the watertable rise will be restricted close to discharge areas with a fixed water level eg the River Murray, the Coorong and Lakes Alexandrina and Albert. Similarly, any rise will also be limited in areas of significant salinisation because much of the recharge to the shallow aquifers will be lost by evaporative discharge.
Elsewhere in SA, fractured rock aquifers predominate with local flow systems. There are few watertable contour maps for these aquifers and even fewer detailed digital terrain models. Consequently, the trend extrapolation method cannot be used in these regions. A combination of anecdotal evidence and professional judgement has been used to determine areas at risk. Because the actual areas at risk are quite small compared to the scale of the mapping coverage, no attempt was made to map them. Instead, estimates of the percentage increase from the current extent were made to determine the risk areas.
The areas at risk assuming a non-intervention scenario are presented in the table below. These figures exclude areas of primary salinity. The extrapolated areas were derived assuming an approach to an equilibrium of salinised land and not an indefinite linear rising trend in watertables.
| Region | 2000 | 2020 | 2050 |
|---|---|---|---|
| Upper South East | 250 500 | 324 000 | 409 500 |
| Murray Basin | 19 800 | 29 600 | 34 000 |
| Eyre Peninsula | 20 400 | 24 000 | 27 000 |
| Kangaroo Island | 5 600 | 6 500 | 8 000 |
| Mid North | 14 800 | 18 000 | 21 000 |
| Yorke Peninsula | 13 900 | 17 500 | 20 000 |
| Mt Lofty Ranges | 1 200 | 1 400 | 1 500 |
| Total | 326 000 | 421 000 | 521 000 |
Further information
- South Australia Dryland Salinity Assessment 2000 report
- Australian Dryland Salinity Assessment 2000 report
- National Technical Overview Report of the State-based dryland salinity assessments
- Australian Groundwater Flow Systems Report
- Primary Industries and Resources South Australia
- National Dryland Salinity Program
- National Action Plan for Salinity and Water Quality
South Australian Dryland Salinity Assessment 2000
Link to Map maker to make a map using this information.
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