What are groundwater flows 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. The broad distribution of groundwater flow systems in Australia has been mapped using attributes such as elevation, landscape form and geology. The classification groups groundwater systems with similar recharge and flow behaviour, and other 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.
Case studies were implemented in catchments in southern Australia (see map below) as part of an evaluation of the groundwater flow systems and a catchment water balance approach to identify:
- areas of the catchment where changes in recharge will most affect catchment salinity;
- how much recharge reduction would be required to reduce salinity by a given percentage in an area of salt-affected land;
- land use and farming system options for reducing recharge enough to manage salinity;
- information for an economic analysis of the costs, benefits and viability of the options for change;
- constraints to achieving required change.
Location of Audit case study areas:
Groundwater flow system types of the Audit case studies.
|Catchment||Groundwater flow system|
|Kamarooka, Victoria||A local flow system in variably weathered fractured rock. Groundwater discharge at break of slope|
|Lake Warden, Western Australia||A local and regional groundwater flow system in alluvial sediments and deeply weathered rocks|
|Upper Billabong, New South Wales||A local and intermediate groundwater flow systems in variably weathered fractured rocks in connection to regional flow system in alluvial aquifers|
|Wanilla, South Australia||A local to intermediate flow system in deeply weathered rock. Groundwater discharge at break of slope and valley floors|
The case study region was:
|Great Southern, Western Australia||a local and intermediate flow systems in deeply weathered rocks|
The catchments were selected on the basis of their salinity status and information availability, and because they represent the most salt-affected land types in Australia. The case studies build on the conceptual development and systems understanding gained through the National Dryland Salinity Program Phase 1 focus catchments-in particular the Liverpool Plains (New South Wales) and Loddon-Campaspe catchments (Victoria).
Results from the case study catchments confirm that the level of intervention required is far greater than generally understood:
- In many areas, we are unlikely to be able to prevent worsening dryland salinity using our current farming systems.
- We need to adopt radical changes to farming approaches (in terms of increased water use, adaptability to variable rainfall regimes) if we are to slow the advance of salinity.
- Even if we could make the massive land use changes required, in many cases groundwater systems will take a long time to respond.
These results can be used to rate the likely effectiveness of different land use options in similar catchment settings (or groundwater flow system types).
Two modelling approaches were undertaken:
- Farm scale modelling to assess the relative effectiveness of different farming systems to manage recharge and use of water.
- Groundwater modelling to simulate the major processes driving the recharge and dischage of water in the catchment.
Modelling of water flows through the soil layer and through groundwater was undertaken in this study, based on the available information on soil and groundwater characteristics. The results of the modelling were compared with the behaviour of the catchment, and used to predict the likely extent of salinisation under different land uses, and thus the feasibility of managing salinity.
Farm-scale modelling is useful to rank different land use options for their effectiveness in controlling excess water in catchments. Despite a perception that water balance modelling at this scale is data intensive, the minimum data requirements can be met in most catchments. To ensure high levels of accuracy in this farm-scale modelling, improved availability of data on soil-water for most agricultural regions characteristics is required.
The Flowtube catchment-scale groundwater model (Dawes et al. 2000) proved to be suitable for estimating recharge and predicting the broad-scale impacts of recharge reduction on land salinity (provided data are available for bore hydrographs, borelogs, land use and topography). This modelling approach has also been applied to several case studies in Western Australia as part of the evaluation of the long-term effects of salinity management options (Campbell et al. 2000). This approach has broad application for estimating the magnitude of recharge control that must be achieved to reduce groundwater levels, and the timeframes involved in land use options that either slow, halt or reverse dryland salinity. To do this well we need groundwater data from strategic locations in catchments. The data must reflect sites and aquifers where changes in groundwater volumes are most evident and for time periods that are appropriate for responsiveness of the groundwater flow systems. Data need to be coupled with information about stream flows and salt loads, local climatic trends, and land use changes.
The case studies indicate the appropriateness of this tool to broadly predict the impact of land use change on dryland salinity. The case studies also identified that we lack data for making the best use of the predictive powers of this groundwater modelling tool. Its widespread use will require considerable effort to establish databases of aquifer characteristics and groundwater level trends across all major flow systems, particularly in those known to be at risk of dryland salinity.
The case studies also emphasised the difficulties in reconciling with confidence outputs from the crop and pasture water balance models with those from the groundwater models. If we are to make more detailed predictions about the effectiveness of individual farming systems on dryland salinity in systems, we need to invest in integrating the groundwater modelling tools with farming systems modelling tools. To achieve this we also require detailed data about the land-systems characteristics of those groundwater flow systems. In most catchments at risk of dryland salinity, this information is not yet available.
The following case studies present estimates of the percentage of each catchment subjected to dryland salinity or high water tables resulting from changes in recharge through time. The reductions considered were:
- no change; and
- 50, 75 and 90% reductions.
Any reduction in area at risk is based on the assumption that salinity caused as a result of high water table is completely reversible when the water level drops, and that any salt can be flushed from the system. In reality, there may be areas which do not recover due to changes in surface soil properties, or because drastic recharge reductions do not allow for leaching of salt from the near surface.
Case studies overview report: Groundwater and Farming Systems Water Balance Modelling by Jane Coram, Warren Bond, Warrick Dawes, Mirko Stauffacher.
Link to the Map Maker to make a map using this information.
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