Salinity - Groundwater Flow
Australia
Groundwater Flow Systems
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. 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 (see map below) has been mapped using attributes such as elevation, landscape form, and geology.
Click here to download and view map in.pdf format (2.9 MB)
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.
- 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.
- 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:
Local, intermediate and regional groundwater flow systems are distributed across Australia. 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.
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.
The groundwater flow system fact sheets
The following fact sheets describe each hydrogeological province in terms of the biophysical and landscape context in which salinity occurs, the attributes that determine groundwater responsiveness, and the processes that operate in the landscape to affect salinity. The likely efficacy of salinity management options has been rated. The ratings are based on a set of mainly quantitative criteria and expert judgement (see table below).
The evaluation process
Listing the attributes of each salinity/groundwater flow system provides a common basis for defining hydrogeological performance in terms readily appreciated by the Australian salinity and groundwater specialists, and a common basis for considering the responsiveness of each system. Specifying the biophysical and landscape determinants of each system has allowed the experience and knowledge gained over many decades of salinity research to be considered, in addition to more recent hydrogeological modelling, particularly that achieved within the case studies of the Audit. In this sense the fact sheets represent the outcome of an 'expert' decision making process.
Fact sheets: version 1
The information in each fact sheet illustrates our knowledge and understanding of the general salinity and groundwater processes that prevail in Australian groundwater flow systems. The discussion of management options presented under each of the groundwater flow systems is intentionally generic. Readers should use these comments as a starting point for the consideration of options at a catchment level and refine with more detailed local information. It is anticipated that over time this material will be revised as new information becomes available.
| Attributes | Rating | Meaning/value |
|---|---|---|
| Scale (of groundwater processes) | Local | Groundwater flows over distances less than 5 km within the confines of sub-catchments |
| Intermediate | Groundwater flow over distances of 5 to 30 km and may occur across sub-catchment boundaries | |
| Regional | Groundwater flow occurs over distances exceeding 50 km at the scale of river basins | |
| Aquifer transmissivity | Low | Less that 2 m2/day |
| (ability to transmit groundwater | Moderate | 2 m2/day to 100 m2/day |
| through the aquifer) | High | Greater than 100 m2/day |
| Groundwater salinity | Low | Less than 2000 mg/l |
| Moderate | Ranging from 2000 mg/l to 10 000 mg/l | |
| High | Greater than 10 000 mg/l | |
| Catchment size | Small | Less than 10 km2 |
| Moderate | Ranging from 10 km2 to 500 km2 | |
| Large | Greater than 500 km2 | |
| Annual rainfall | Low | Less than 400 mm |
| Moderate | Ranging from 400 mm to 800 mm | |
| High | Greater than 800 mm | |
| Salinity rating | S1 | Loss of production |
| S2 | Saline land covered with salt-tolerant volunteer species | |
| S3 | Barren saline soils, typically eroded with exposed sub-soils | |
| Responsiveness to land | Low | Salinity benefits accrue over timeframes that management exceed 50 years |
| Moderate | Salinity benefits accrue over timeframes ranging from 30 to 50 years | |
| High | Salinity benefits accrue over timeframes less than 30 years |
| Fact Sheet Name | Technical Classification Name | |
|---|---|---|
| Fact sheet 1. | Local flow systems in deeply weathered rocks | - Local flow systems in Precambrian rocks |
| Fact sheet 2. | Intermediate flow systems within sedimentary sequences infilling large valleys | - Intermediate and local flow systems in Cainozoic sediments |
| Fact sheet 3. | Local flow systems in fractured rocks | - Local flow systems in Paleozoic rocks or Mesozoic intrusives |
| Fact sheet 4. | Local flow systems in deeply weathered fractured rocks | - Local flow systems in Paleozoic rocks or Mesozoic intrusives |
| Fact sheet 5. | Local flow systems associated with colluvial fans | - Local flow systems in Paleozoic rocks or Mesozoic intrusives |
| Fact sheet 6. | Intermediate flow systems in fractured rock aquifers | - Intermediate and local flow systems in Palaeozoic rocks or Mesozoic intrusives |
| Fact sheet 7. | Local flow systems in fine grained unconsolidated sediments | - Local flow systems in Cainozoic volcanics or Mesozoic sediments / volcanics |
| Fact sheet 8. | Regional flow systems in alluvial aquifers | - Regional and local flow systems in Cainozoic sediments - Regional flow systems in Cainozoic sediments |
| Fact sheet 9. | Regional flow systems within unconfined sediments | - Regional flow systems in Cainozoic sediments - Regional and local flow systems in Cainozoic marine sediments |
| Fact sheet 10. | Local flow systems associated with sand dunes | - Local flow systems in Cainozoic sediments |
| Fact sheet 11. | Regional and intermediate flow systems within fractured basaltic rocks | - Regional and local flow systems in Cainozoic volcanics or Mesozoic sediments/volcanics - Regional flow systems in Cainozoic and Mesozoic volcanic plains/plateaus - Regional flow systems in Cainozoic volcanics or Mesozoic sediments/volcanics - Regional flow system Great Artesian Basin recharge beds |
| Fact sheet 12. | Intermediate and local flow systems in fractured basaltic rocks and layered sedimentary rocks | - Intermediate and local flow systems in Cainozoic volcanics or Mesozoic sediments - Intermediate flow systems in Cainozoic volcanics or Mesozoic sediments/volcanics |
The Australian Groundwater Flow Systems Contributing to Dryland Salinity technical report uses hydro-geological names for the groundwater flow systems. The table above provides a translation of the plain English names used in Australian Dryland Salinity Assessment 2000 report and on this web site for readers of the technical report.
distribution of groundwater flow systems
Australian Dryland Salinity Report
Link to the Map Maker to make a map using this information.
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