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Estimating the extent of salinity

Richard George, Bunbury and Rod Short, Pinjarra

Salinity measurement, monitoring and equipment

Salt is a natural component of land, water and ecological systems in Western Australia. Large areas of naturally saline land were present before settlement ('primary salinity'). Clearing for agriculture has, however, greatly increased recharge, raised groundwater and increased the extent of salinity ('secondary, dryland salinity'). Irrigation has also increased the problem of salinity ('secondary, irrigation salinity').

Landholders and scientists in Western Australia have been estimating, since 1955, the extent and location of saline land in agricultural areas, and predicting likely impacts and the rate of spread. Authorities have published three estimates of the current and future extent of salinity in recent years: Australian Bureau of Statistics (1955-1993), Department of Agriculture hydrologists' estimate (1994), and the National Land and Water Resources Audit (2001). The Land Monitor project has provided a new way to estimate salinity from areas with consistently low productivity.

What is dryland salinity?

'Salinity' can be defined in many ways. The basis of most definitions is when salt concentrations in soil are high enough to cause damage to plants, animals and man-made structures (for instance, roads, buildings and railways). In Western Australia, secondary salinity is associated with shallow watertables. The severity of salinity on plants varies with plant species, salt concentration in the root zone, interactions with waterlogging and soil type, and management.

Soil salinity measurements can be related to plant growth, and this can be used to define classes of salinity. The classes accepted across Australia are:

Estimates of salinity

ABS Census

The Australian Bureau of Statistics agricultural census was done every five years between 1955 and 1993. The ABS census asked landholders to estimate the area of 'all salt affected land on your holding which has previously been used for crop and pasture.' The Journal of Agriculture reported the census results as 73,476 hectares in 1955 rising to 529,000 hectares in 1993. The 1993 result is 3.1 per cent of 'saline land' in the agricultural area of the State. Researchers considered this estimate to be too low, especially where there were salt and waterlogging interactions.

Hydrologists' estimates (1994)

In 1994, the Department of Agriculture's regional hydrologists estimated that 9 per cent of agricultural soil had high soil salinity and shallow watertables. They estimated that, by 2020, up to 3.3 million hectares (17 per cent) was likely to have reduced yields caused by this problem. With current agriculture, salinity and shallow watertables could eventually affect as much as 6.1 million hectares (31 per cent) (see Table 1).

Table 1. Estimated areas affected by secondary salinity* to some extent in 1994, 2010-2020 (depending on rainfall) and potential (after Ferdowsian et al. 1996)

*For agriculture, salinity is considered severe when the yield of the preferred crop or pasture is reduced by more than 50% and moderate when the yields are reduced by between 10 and 50%.

**Includes some non-cleared land which is prone to salinisation in highly-cleared catchments.

National Land and Water Resources Audit (2001)

The National Land and Water Resource Audit (NLWRA) identified dryland salinity as one of seven major themes for an audit of the nation's land, water, vegetation and natural resources. 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 5 metres and rising'. Researchers in the Audit used 'the risk of shallow watertables' as an indicator of 'the risk of salinity'. They recognised that not all shallow watertables would result in salinity and reduced yields.

Key results from groundwater bore data in the south-west agricultural region are:

• Groundwater trends are dominated by rising or stable trends. No systems have significant falling trends;
• About 16 per cent of the region had potential for salinity in 2000 due to shallow watertables;
• 20 per cent of the SW region has the potential for salinity in 2020 due to shallow watertables;
• Around 6.5 million hectares of agricultural land will be subject to shallow watertables and salinity by 2050 (33 per cent of the region);
• Based on expert Department opinion and hydrological modelling, predicted land use change between 2000 and 2020 was not sufficient to modify groundwater trends and to impact on the estimates of extent;
• Shallow groundwaters may potentially affect approximately 30,000 kilometres of road and rail networks and at least 27 major rural towns;
• Surface water resources in the south-west of the State are likely to become more saline; and
• The impacts on biodiversity are only just starting to be measured, but it is estimated that at least 1500 plants species will be reduced in some way, and 450 likely to become extinct. Salinisation is also likely to reduce animal species by 30 per cent in affected areas.

Land Monitor Project estimates

The Land Monitor project has delivered another means to estimate the area of dryland salinity. This process uses land surface information and satellite images to define areas of the landscape that have a consistently low productivity' that may be caused by salinity. Farmers and scientists check areas 'on-the-ground', and extrapolate to other areas. Using this data and a set of rules, areas with potential for a shallow watertable can be mapped. Advantages of the Land Monitor approach are that the process is 'electronic' and therefore able to be updated and 'trained', and it reports its accuracy levels. The biggest advantage of Land Monitor is that it reports at paddock and catchment scale, suitable for use by land managers.

Summary

Prior to 1994, salinity statistics were based on ABS estimates, the last of which suggested the area of secondary salinity in WA was 3.1 per cent (1993).

In 1994, Hydrologists concluded the area of salt affected land was about 1.8 million hectares (9.4 per cent), could increase to 3.2 million hectares by 2020, (17.1 per cent) and potentially reach 6.1 million hectares (31.8 per cent) at equilibrium.

The NLWRA (Short and McConnell, 2000) analysed groundwater data. The process relied on a survey of the existing groundwater data sets and a classification of the landscape to distribute estimates. High risk was defined as areas with water tables 'less than 2 metres' and '2 to 5 metres and rising'. It provided a regional scale estimate.

In the entire south-west agricultural region (26.7 million hectares), the area of land at high risk of 'a shallow watertable' was 4.4 million hectares (16 per cent). The estimated area at risk in 2020 was 5.2 million hectares (20 per cent) and in 2050 8.8 million hectares (33 per cent).

In the cleared agricultural land (19.7 million hectares of land without perennial vegetation), the estimated area at risk of shallow water tables was 3.6 million hectares (18 per cent) and 6.5 million hectares (33 per cent) in 2050.

It is now clear that salinity threatens major infrastructure, land, water and ecology. Separate definitions of salinity are required for each affected land use (such as infrastructure, soil, water, and ecological systems) so that appropriate targets and actions can be planned.

References and further information

Ferdowsian, R, George, R, Lewis, R, McFarlane, D, Short, R and Speed, R (1996). The extent of dryland salinity in Western Australia. In: Proc. 4th National Workshop on the Productive Use and Rehabilitation of Saline Lands, Albany March 1996, pp 89-88.

George, R.J., (1990). The 1989 Saltland Survey. Journal of Agriculture, Western Australia. 4th Series, 31 (4):159-166. Department of Agriculture, Western Australia

Short, R.J, and McConnell, C. (2001). Extent and Impacts of Dryland Salinity (945KB pdf). National Land and Water Resources Audit. Resource Management Technical Report 202, Department of Agriculture, Western Australia.

Short, R.J, and McConnell, C. (2001). National audit on Dryland salinity (32KB pdf) Farmnote 40/2001, Department of Agriculture, Western Australia.

Tille, P.J., Mathwin, T.W. and George, R.J., (2001). The South-West Hydrological Information Package. Bulletin 4488, 284 pp. Department of Agriculture, Western Australia.

Page last reviewed November 2006

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