Dryland Salinity

What has caused the dryland salinity problem?

The clearing of native vegetation has been a key element of rural development in Australia since the early 1800s. However in the last few decades it has become clear that this has produced major changes in the water cycle, causing groundwater levels to rise rapidly. Under the dominant forms of native vegetation in the Murray-Darling Basin the leakage to groundwater past the root zone in an average year is about 1 to 2 mm per year. Under most forms of modern agriculture and grazing the leakage is much greater. This leakage has been causing groundwater tables to rise in many areas; as the water rises, it brings with it natural salt stored in the soil.

The salt mobilisation process across all the major river valleys in the Murray-Darling Basin is on a very large scale; this has been described in the Salinity Audit. In 1999, the Salinity Audit predicted that in the next 100 years the annual movement of salt in the landscape will increase two to three times. The salt load exported to and through rivers will double. Average river salinities will rise significantly, exceeding the critical thresholds for domestic and irrigation water supplies, and for the riverine environment, for prolonged periods of time in the Macquarie, Namoi, Lachlan, Castlereagh and Bogan Rivers of New South Wales and the Condamine- Balonne, Border and Warrego Rivers of Queensland.

Are our current farming systems helping to address salinity?

An understanding of the limits of the effectiveness of current farming systems to control the leakage of rain to the groundwater table is essential if we are going to develop successful strategies to address salinity. In a recent review of field studies, the CSIRO concluded that leakage rates under current farming systems, even when implemented at best practice level, generally far exceed the leakage rates of native vegetation (see The effectiveness of Current Farming Systems in the Control of Dryland Salinity CSIRO 1999). In summary, the CSIRO review reached the following conclusions.

Grazing systems

In high rainfall zones (greater than 600 mm) neither perennial nor annual pastures are capable of limiting leakage rates as much as trees. A high proportion of trees is the only option for salinity control.
In medium rainfall zones (400-600 mm) perennial pasture systems may significantly reduce leakage.
In low rainfall zones (less than 400 mm) the use of lucerne can reduce leakage rates to levels which are similar to those under Mallee vegetation.

Cropping systems

In some areas of the Southern Basin where rainfall is 250-500 mm, the removal of long fallow rotations and the inclusion of lucerne in rotations (phased cropping) has reduced leakage to 5 per cent of rainfall and eliminated recharge.

For agroforestry systems

In general, trees in agroforestry formats can halve leakage rates but not mimic native vegetation.

For plantations

In medium and low rainfall zones (less than 700 mm) the leakage underneath the plantation is close to zero.

In conclusion, the management interventions in dryland catchments over the past 20 years have yet to demonstrate a level of salinity control such that future costs and impacts will be reduced below those predicted in the Murray-Darling Basin Salinity Audit. Clearly, this is a major challenge for future salinity management in the Basin.

In Basin-wide terms, monitoring and studies of the capacity of farming systems to effectively minimise recharge and control salinity indicate that:

For irrigation areas, a combination of works and improved farming systems under strategies allowing for tradeable pollution rights and accountable land and water management plans supported by public investment, can achieve a level of control.
For the wheat-sheep zone there are options which may achieve a level of salinity management that is close to or mimics native vegetation.
For the winter and summer rainfall grazing zones, with more than 600 mm per year, there are no options for salinity control except large-scale tree plantings.
For the rangelands, the current land uses dependent on native vegetation are in hydrologic equilibrium and there are no significant threats of off-site impacts.

The Commission is developing a Basin Salinity Management Strategy (2001) to help address the problems associated with dryland salinity.

The Commission has also funded projects through its Basin Salinity Management Strategy (and previously the Strategic Investigations and Education (SI&E) Program) on tools for improved management of dryland salinity in the Murray-Darling Basin. The Tools will include details about regional information and planning sessions, professional and community networks, and access to the latest technical and planning information. For more information, see the Salinity Page or the Tools webpage.