Understanding salinity management
What do we mean by salinity management?
Salinity can be managed by prevention, treating the cause, ameliorating the symptoms, living with it, or a combination of these. It is important to specify objectives when evaluating the appropriateness of management options.
Prevention and protection:
- native vegetation retention particularly in areas of recharge on lands with a salinity risk;
- adequate resource assessment to identify areas at risk; and
- water-balance modelling to assess the impact of any proposed land use change.
Treatment of cause:
- recharge management to prevent or reduce the rate of rise of groundwater and thus, the area of land affected by salinity and the delivery of salt to water courses, wetlands and storages; and
- interception of fresh water to reduce the rate of rise of groundwater and the delivery of salt to land and water courses, wetlands and storages.
Amelioration of symptoms:
- interception and storage of salt, and reduction of groundwater level to reduce impacts of salt on assets such as water resources, infrastructure and biodiversity; or
- managing saline discharge, adapting to more saline land and water conditions and developing production systems for these saline lands and waters.
Living with salt:
- alternative use of saline land and water resources; and
- optimisation of the use of non-saline resources.
Understanding salinity
To understand salinity across the Australian landscape and through time, we need to understand how groundwater systems respond to changing recharge, and how the excess water that results from increased recharge is distributed.
Groundwater systems are not identical across all Australian landscapes and their contribution to dryland salinity also differs (Coram 1998, Coram et al. 2000, Figures 17 & 18). Lack of knowledge on these systems limited our ability to take a national view of salinity management and the effectiveness of options has been limited. Although management solutions have been identified for a few intensively studied catchments, it has not been feasible in terms of time or money to undertake similar resource intensive investigations for every catchment. This lack of information has been complicated by inappropriate extrapolation of known causes to unstudied catchments, and limited availability of information to non-specialists.
The Audit has supported the development and application of a catchment classification approach that categorises Australia's groundwater flow systems. The classification (Coram 1998) is based on recharge and flow behaviour, and uses measures such as length of flow paths through aquifers, aquifer permeability and driving pressure gradients for groundwater flow. It identifies groundwater flow systems where particular management activities will lead to similar responses and provides a framework for action. The broad distribution of groundwater flow systems in Australia (Figure 17) has been mapped using attributes such as elevation, landscape form, and geology.
Groundwater flow systems slowly responding to changes in water balance and land use
Figure 17.Distribution of groundwater flow systems across Australia.
Substantiation of the flow systems
A questionnaire-based review of the groundwater flow systems approach was undertaken to test its validity. The opinions of local hydrogeologists working in the field of dryland salinity across Australia were sought to assess how accurately the groundwater flow systems approach characterised the processes driving dryland salinity at 102 sites, within constraints of existing knowledge and information. The results of the review have shown the conceptual understanding underpinning the framework to be applicable at both national and catchment scales. The groundwater flow systems framework was widely regarded to be a good description of the groundwater processes contributing to dryland salinity. The level of agreement between the local hydrogeologists' understanding and the national map differed between the groundwater flow system types, with almost universal agreement regarding the regional systems, and general agreement on most of the local and intermediate systems. More detail is provided in the report on the substantiation of the groundwater flow systems; available on the Audit's Australian Natural Resources Atlas.
As in any classification system, there are limitations. More detailed groundwater flow system maps are needed for catchment scale planning using catchment scale data (geology, geomorphology, slope) to accurately locate the groundwater flow systems contributing to dryland salinity. Limited availability of appropriate scale data for geology, elevation and regolith limits the development of more accurate maps of the groundwater flow systems. The concepts of the systems can still be applied using existing data.
- At a national scale, the framework provides an excellent overview of salinity provinces and processes and it is useful for strategic policy development. The same principles applied at local and regional scales provide a sound basis for salinity planning within catchments.
The framework provides a basis for:
- defining management options; and
- ensuring that investment is targeted, actions are appropriate, and outcomes are measurable.
Groundwater flow systems
An assessment of the 12 specific types of groundwater flow systems contributing to dryland salinity across Australia has shown that:
- groundwater processes in the deeply weathered landscapes of Western Australia are similar to those in the landscapes of the Eyre Peninsula in South Australia and the Dundas Tablelands in western Victoria;
- groundwater processes in the sedimentary deposits of the Murray Darling Basin are similar to those in the Perth and Bremer Basins in Western Australia;
- clear similarities exist between the groundwater processes underlying salinity on the northern and western foot slopes of the Great Dividing Range in both Victoria and New South Wales.
Groundwater flow systems can be classified as local, intermediate or regional on their spatial extent and influence. The extent of the system has implications for its responsiveness to change in water balance and therefore influences the types of management options that are more appropriate for modifying the water balance.
- Local groundwater flow systems respond rapidly to increased groundwater recharge. Watertables rise rapidly and saline discharge typically occurs within 30 to 50 years of clearing of native vegetation for agricultural development. These systems can also respond relatively rapidly to salinity management practices, and afford opportunities to mitigate salinity at a farm scale. Examples are:
Kamarooka catchment, Victoria (local groundwater flow system in weathered fractured rock)
Great Southern, Western Australia (local groundwater flow system in deeply weathered rock)
- Intermediate groundwater flow systems have a greater storage capacity and generally higher permeability than local systems. They take longer to `fill' following increased recharge. Increased discharge typically occurs within 50 to 100 years of clearing of native vegetation for agriculture. The extent and responsiveness of these groundwater systems present much greater challenges for dryland salinity management than local groundwater flow systems. Examples are:
Upper Billabong Creek, New South Wales (local and intermediate groundwater flow systems in fractured rocks in connection to regional flow system in alluvial aquifers)
Wanilla catchment, South Australia (local to intermediate groundwater flow system in deeply weathered rock)
- Regional groundwater flow systems have a high storage capacity and permeability. They take much longer to develop increased groundwater discharge than local or intermediate flow systems - probably more than 100 years after clearing the native vegetation. The full extent of change may take thousands of years. The scale of regional systems is such that farm-based catchment management options are ineffective in re-establishing an acceptable water balance. These systems will require widespread community action and major land use change to secure improvements to water balance. An example is:
Lake Warden, Western Australia (regional groundwater flow system in alluvial sediments)
Local, intermediate and regional groundwater flow systems are distributed across Australia (Figure 18). In some areas flow systems may be superimposed or physically linked. Each system has a unique combination of attributes, but each in turn is composed of different landscapes with a degree of variability.
Figure 18.Distribution of local, intermediate and regional groundwater flow systems across Australia.
The hydrogeological and topographical features associated with the groundwater flow systems provide a basis for evaluating the appropriateness of salinity management options.
The capacity of a given groundwater flow system to respond to changes in land use is driven mainly by its ability to move groundwater and is defined by:
- the groundwater gradient (water flows from a higher to a lower position in the landscape); and
- permeability of the material through which the groundwater flows (gravel, sand, clay).
If both gradient and permeability are high, the time it takes a groundwater system to respond to changes in land use is likely to be fast (a decade or so); if both are low, the response time is likely to be slow (hundreds of years). Low permeability local groundwater flow systems experiencing significant groundwater elevation within the catchment respond poorly to recharge management (alone) as a salinity management measure. This is the more general condition found throughout Australia, and the position established through the application of groundwater modelling in the Audit case studies.
Groundwater flow systems have much slower response times to changes in land use than is widely recognised. Once those changes are initiated, it takes a long time to reach a balance. Even if we manage to reduce recharge, it will take time for the excess water to flow out from the system once the groundwater system is full.
In summary:
- Local flow systems have a relatively small capacity to store the additional recharge and so respond relatively rapidly to changes in land use; in many cases, they also have a relatively small discharge capacity through which to drain the excess water.
- In contrast, regional flow systems have a very large capacity to fill and subsequently respond very slowly to changes in land use, they will also take a long time to empty of excess water. Intermediate flow systems behaviour falls between local and regional systems.
Groundwater flow systems and impact on rivers
Selection of options for managing each groundwater flow system type needs to consider the way salt is mobilised out of the landscape. Salt may be mobilised as wash off from the land surface by water running into streams, as lateral sub-surface seepage or as groundwater seeping directly into streams and rivers as base flow.
- Local flow systems are more likely to lead to landscapes where river salt is sourced mainly from wash off. Minimising wash off is achieved by minimising recharge or improving the water balance and therefore the amount of salt-laden groundwater that discharges onto the land.
- Intermediate and regional systems often have a higher level of groundwater discharging directly to streams. Intercepting groundwater can prove beneficial if the key objective is to keep salt-laden water from the stream. Knowledge of the groundwater flow systems and their variability is essential to pinpoint likely areas of high discharge and identify management options. With this knowledge, plans can be developed to intercept groundwater by engineering means such as pumping.
Figure 19.Groundwater flow systems respond to changes in water supply through changes in vegetation types.
A framework for action
The groundwater flow system is a useful framework to support cost-effective action in salinity management across catchments and regions. When fully implemented the framework should be able to be used to:
- apply and transfer knowledge of well-studied catchments to unstudied catchments;
- communicate that knowledge to management agencies and communities, who need to understand salinity and management options;
- identify key attributes to be monitored;
- develop cost-effective systems for monitoring; and
- prioritise investment areas for salinity management.
Table of Contents for the Australian Dryland Salinity Assessment 2000
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