As the groundwater system fills and eventually reaches a new equilibrium, the amount of water entering the landscape as recharge and the amount of water leaving as discharge is balanced. However there is a time lag between when changes in land use or improvement in water balance occurs and evidence of a response. It will take decades to reverse the water rise in most groundwater systems (see figure below).
Re-establishing the water balance requires farming systems with similar water use to that of deep-rooted native vegetation. Designing and implementing such farming systems is a major challenge.
Recharge processes are generally faster than discharge processes. If it takes 30 to 50 years for our fastest groundwater system to fill with water, then it is reasonable to expect that it might take at least 30 to 50 years for it to empty back to where it was. If the system takes 100 years or more to fill, we can again expect at least a similar amount of time to establish the original equilibrium. This is an important issue for management as the degree of recharge reduction and the time taken have important consequences on land use options during any adjustment period, and the degree of change sought. Beneficial effects of land use options may well occur before the system has returned to an equilibrium.
As more water moves through an aquifer, more salt is mobilised. Very long periods of time are needed for catchment salt stores to be reduced to the point where the amount entering the system equals the amount leaving the system, that is, to achieve a salt balance. The net amount of salt that exits a catchment via stream flow indicates the time it will take for the catchment to flush its store of salt, when compared with the total mass of salt stored in that catchment. In some of the more responsive groundwater flow systems, the net output of salt may take about 150 years to flush from the system. In larger catchments (e.g. the Murray groundwater basin), it may take as much as 15 000 years. This means that although management may lower the watertable and allow productive use of land, there may be ongoing salt inflow to streams via groundwater.
This makes managing stream salinity very difficult. It is very important to prevent the interception of groundwater with salt stores in regions where we still have this opportunity.
The substantial lag times for catchments to come back into water balance and change salt mobilisation mean that it is inevitable that dryland salinity will be a feature of many Australian landscapes for some time. This is true even with widespread adoption of innovative land uses that manage to turn off the recharge tap and re-establish water balance. Ultimately the decisions on the measures to be taken will be influenced by the value of the threatened assets, the capacity to manipulate the environmental processes, the economic feasibility and social acceptance of the proposed actions.
What is the scale of the groundwater systems contributing to dryland salinity, and how can they be changed?
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, see maps below). 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.
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 (see map below) has been mapped using attributes such as elevation, landscape form, and geology.
Groundwater flow systems
An assessment of the 12 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 (see map below). 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.
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.
- 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.
Australian Dryland Salinity Assessment 2000 has focused on developing an understanding of how the major groundwater systems across Australia function and, from this basis, an analysis of management options to control dryland salinity.
The improved understanding of groundwater processes and types provides information on:
- the extent of land use change and recharge reduction required to halt, and maybe reverse, the spread of rising watertables and salinised lands.
- the lag times between adopting recharge reduction or interception of saline groundwater and consequent responses in groundwater levels, area of land salinised and/or salt delivery to streams.
Local groundwater flow systems
Australia has close to 25 million hectares of local groundwater flow systems. Approximately 3% of these are considered to be at risk of developing some dryland salinity. These systems are commonly deeply weathered, low permeability systems that are already almost full and occur in cleared areas of temperate Australia.
These areas are likely to exhibit a lag of three to ten years or more between changes in the water balance and the initial occurrence of salinity. For these systems there is a probable lag of several decades before hydrogeological balance is reached. Consistent with these relatively small systems, changes in land use to effect significant reduction in recharge are needed on a local scale for each system.
Based on a conservative assumption that changes are required over half of each catchment area, approximately 12 million hectares in temperate Australia could require treatment to reduce recharge and restore hydrogeological balance. If these treatments are undertaken the area of salinised land will reduce fairly rapidly, probably within 10 to 15 years and in some cases much less time. However, low discharge capacity of many of these systems means that it is likely to be decades before salt delivery to water resources is significantly reduced. Biophysical options are appropriate for these systems. Recognising that lag times to improve land will be much less than those to improve water resources, application of recharge management will depend on whether the main objective is land or water rehabilitation.
Intermediate groundwater flow systems
Australia has around 40 million hectares of intermediate groundwater flow systems. Approximately 5% of these systems are considered to have a high risk of developing dryland salinity. They are mostly (75%) deeply weathered, low permeability systems. They are already close to full and occur in cleared areas of temperate Australia. These systems also include some high permeability, buried river channel systems.
These areas are likely to exhibit a lag of several decades or more between changes in water balance and the initial occurrence of salinity, and 50 years or more before hydrogeological balance is reached. To rehabilitate land and waters, changes in land use / water balance are needed over a significant proportion of each catchment. Based on a conservative assumption that land use changes to reduce recharge are required over half of each catchment this would amount to an area of approximately 20 million hectares. In most intermediate flow systems, the low discharge capacity means that it is likely to be decades before the effects of such changes become evident on land. In the higher permeability flow systems the effects of changes in land use are likely to become evident within a shorter period, possibly with similar response times to the local systems.
Regional groundwater flow systems
Australia has around 45 million hectares of regional groundwater flow systems. Approximately 6% of this land is considered to be at high risk of salinity in the next 100 years. These systems are characterised by broad plains and deep sedimentary sequences. They are likely to exhibit a lag of over 100 years between changes in water balance and the initial occurrence of salinity, and probably over 1000 years before hydrogeological balance is reached. Consistent with these extensive systems and very slow response times, it is likely to be many decades before the effects of recharge management become evident in groundwater levels. Improvements in salt loads to streams as a result of recharge management may not be detected within our natural resources management planning horizon of 50 years.
The ways forward in salinity management are many and varied depending on objectives and the local biophysical environment. A combination of approaches that considers both individual farm and whole catchment factors is likely to be the best option.
Reducing the recharge
The fundamental step in managing salinity should be to address the cause of the problem by reducing recharge. Local groundwater systems might be addressed at the individual farm scale or at least at the catchment scale. Regional systems, however, will be far more difficult to manage, because landholders with the recharge problem will rarely experience the discharge symptom and also because the time lag between intervention and meaningful response might span generations.
Native vegetation minimised recharge prior to clearance for white settlement, so the retention and enhancement of remnant vegetation should be the highest priority. Remnant vegetation, to function effectively, must be actively managed as it will deteriorate unless protected from grazing, weeds and rabbits. The conservation of remnant vegetation brings added benefits such as biodiversity enhancement and carbon sequestration. However the individual land managers share only a small fraction of these benefits, so that public investment in these assets is essential.
The very presence of dryland salinity reflects the fact that little remnant vegetation is to be found on agricultural land in SA. Revegetation using local native plants is a useful alternative for which the necessary establishment and management skills are well developed in South Australia. Once again, public investment will be necessary where private benefits are very limited.
The amount of revegetation required to arrest recharge will vary from catchment to catchment depending on the hydrogeology, soils and climate. In some instances, the loss of production from the revegetated land will exceed any gains from discharge reduction, and therefore would be impossible to justify economically. Even where this is not the case, it will be essential to target non-commercial revegetation for maximum impact and to optimise other potential benefits such as shade and shelter.
Commercial farm forestry offers an opportunity to intercept recharge in a cost effective manner. Although this has been successful in WA and Victoria with blue gum plantations, the options for low to medium rainfall regions (< 500mm/annum) are limited and in some cases still at the development stage. For this reason, and given the long time interval between establishment and harvest, it is important that government invest in the development of these alternatives.
TOPCROP programs are developing the opportunities to improve the water use efficiency of annual crops and deliver productivity gains to farmers, however the most optimistic improvements to annual cropping systems will be unlikely to reduce recharge by more than about five percent. It is probable that significant impacts on recharge will result only from development of new cropping systems that include a considerable component of deep-rooted perennial species. Fodder crops, phase farming and alley farming have already attracted some interest, but the results so far indicate that there is still much to do before these become commercially attractive options. Public investment in developing these new systems will be essential.
Perennial pastures provide some recharge control that can be further enhanced under sound management. The Sustainable Grazing Systems program has developed management guidelines (e.g., rotational grazing, liming acid soils) to optimise pasture growth and persistence, which should in turn minimise recharge. Native grasses should provide better recharge control than annual exotics in unimproved pastures. Lucerne is an excellent option for recharge control, with healthy stands performing as effectively as native vegetation. Whilst the commercial value of lucerne is an incentive for farmers, the opportunities for its use are limited by soil properties and climate.
In some catchments, discharge sites will also act as recharge sites. Saltbush has been shown to reduce recharge in some of these situations, lower the watertable locally and provide a more hospitable site for other salt tolerant species.
Significant recharge can occur in urban environments, particularly if water is imported as occurs in many SA towns. Leaky septic tanks or sewerage settlement ponds and inefficient garden watering systems can contribute to the development of groundwater mounds and associated dryland salinity.
Managing discharge sites
Discharge sites are the visible aspect of dryland salinity. They are recognisable by salt or saline water lying on the surface, the presence of indicator plants, and the damage inflicted on native vegetation, crops and structures (e.g., fences, buildings, roads, etc). Salinity can also be responsible for the development of soil sodicity with the associated increased risk of erosion, and for significant off-site effects on surface water quality and native vegetation.
Simply protecting saline ground from livestock and traffic often allows native salt tolerant plants to colonise and protect the ground, reduce evaporation, lower the watertable locally and improve the visual appearance.
Recognition that recharge reduction comes at a cost, which may be impossible to justify in some cases, has stimulated further investigation of the productive use of saline land. Many plants are naturally adapted to saline conditions and some have commercial value, enabling landholders to turn adversity into opportunity.
Saltland varies from region to region depending on salinity levels, waterlogging, soil properties and climate. Not only might these variations affect the establishment of salt tolerant plants, they might also limit their persistence. These variations must be mapped regionally and the implications understood for the appropriate treatments to be implemented.
Treatment of saline sites with non-local salt tolerant plants has the potential to expose native vegetation to weed threats. This risk should generally be assessed at the local level and appropriate monitoring and safeguard procedures established. Sites of particular environmental significance should be identified at the regional level and risk management procedures developed with local communities.
Managing surplus water
Engineering options will sometimes be required to complement recharge reduction and plant based management of discharge. These are generally costly and may require ongoing maintenance, however they can be justified for protection or restoration of high value assets such as town infrastructure, areas of environmental significance and highly productive agricultural land. It is essential that all engineering solutions are implemented as part of catchment plans and take into account inevitable off-site impacts. Clear general guidelines are needed to assist catchment groups in this regard.
Surface water drains, whilst often dealing with fresh water, can be a very effective means of reducing inundation of low lying areas which might otherwise cause plant death due to waterlogging. Drainage may therefore assist in both reducing recharge and enhancing safe discharge. There is evidence to suggest that surface drainage on saline sites, normally subject to seasonal inundation, may deny the normal flushing process that prevents salt accumulation in the root zone.
If the surface water is saline, as will generally be the case on discharge sites, much greater care must be taken with drainage. Soils might be sodic and therefore highly erodible and in some instances acid sulfate soils might be disturbed with serious off-site consequences.
Groundwater drains may be deep open drains or subterranean drains with the specific aim of lowering the local watertable. The effectiveness of these systems is very dependent on the soil properties, an inverse relationship often existing between soil permeability and stability. Furthermore, every drop of saline groundwater drained will be delivered further down the catchment with consequences that must be anticipated and managed. Given these constraints, it is essential that groundwater drainage only proceed after careful evaluation of site responsiveness and as part of a whole catchment plan.
Groundwater pumping is usually so costly that it is applicable only where it offers protection to high value assets. The cost of pumping can sometimes be offset if the water is harvested (e.g., for aquaculture or for salt extraction).
Productive uses of saline land and water
The productive uses of saline land and water include: halophytic vegetation and salt-tolerant grasses for stock fodder; salt-tolerant trees and horticulture; saline aquaculture; and nature conservation areas for biodiversity protection, greenhouse credits and recreational values. Chemical extraction and desalinisation of water are further options, more likely to be used to defray some of the costs of protecting high value assets.
These measures are limited to discharge areas, and their suitability is determined by the quantity and quality of groundwater, and varies between groundwater flow systems. The demand for saltland production systems will become more widespread as the extent of salinity increases.
- Different groundwater flow systems require substantially different combinations of management options.
- Holistic approaches to salinity that take account of the groundwater flow systems and the objectives of management are essential.
- In most instances there will be no single solution to the problem, and the combination of approaches will differ across temperate Australia.
- Adaptive management and innovation have a significant role to play in maintaining productivity and profitability. Strategic investment in management measures and mitigation may provide options to live with the current and rising levels of salinity.
- Intervention needs to be driven by asset protection plans for infrastructure, biodiversity, productive soils, water resources and combinations of these assets, with realistic targets set in terms of the level of salinity management that is feasible.
- Australian Dryland Salinity Assessment 2000 report
- National Technical Overview Report of the State-based dryland salinity assessments
- Dryland Salinity Evaluation and Monitoring Report
- Australian Groundwater Flow Systems Technical Report
- National Dryland Salinity Program
- National Action Plan for Salinity and Water Quality
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