You are here: ANRA home » Natural resource topics » Dryland Salinity
This web site is no longer being updated
In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Case study: application of groundwater flow systems across geographic scales
Australian groundwater flow systems have been classified as local, intermediate or regional. Each system has a unique combination of attributes, but each in turn is composed of different landscapes with a degree of variability. At the national scale, the variability within each groundwater flow system is minor compared with the differences between groundwater flow systems. However, at finer scales, this variability becomes important, and so it is necessary to undertake more detailed analysis to ensure that the framework and concepts developed at the national level can be applied at the catchment scale.
In one sense classifying landscapes into their groundwater flow systems is an approach to standardising all of our knowledge of those landscapes. This enables us to use the groundwater flow system classification principles to define discrete catchments/regions that have common groundwater and salinity processes, common responses to land management, and a common range of salinity management options. The principles as established are based upon local knowledge and as such are more easily applied at the local level, than they are at the national level.
This finer scale of detailed analysis has already been accomplished in some catchments (see examples given below).
The Macquarie Catchment, Central New South Wales
The Macquarie Catchment is a large catchment in Central West New South Wales that drains to the north-west into the Darling River. Its upper reaches are the higher relief landscapes in the Bathurst-Orange district, flowing down basin through Wellington and Dubbo, and finally across the flat alluvial plains before reaching the Darling.
The national groundwater flow systems classification defined the catchments as having a complex distribution of local and intermediate flow systems in the headwaters (Figure 30), moving to a predominantly intermediate system in the middle reaches. The flow system then changes to a regional flow system based on the Great Artesian Basin recharge beds, and finally a regional alluvial sediments flow system underlying the Riverine Plain to the north-west.
Figure 30. Australian groundwater flow systems: Macquarie catchment, New South Wales.
Two separate catchment classification exercises have been completed in the catchment. The first, as part of a modelling exercise carried out by Australian Bureau of Agriculture and Resource Economics/CSIRO to determine whole-of-catchment responses to land use change, used the mapping rules devised for the national groundwater flow systems map, but applied them to more detailed data coverage available for the catchment. This had the effect of not only increasing the level of detail that described the catchment, but also finetuning the classification in some areas in response to the better data (Figure 31).
A second classification was undertaken in a study of the mid-Macquarie catchment aimed at providing a framework for catchment management. This classification used a new dataset based on more detailed geological data, together with more detailed elevation data. The key to the new classification was the reformulation of conceptual models based on knowledge from local salinity experts. This enabled a new set of rules to be developed that generated a more detailed set of flow systems and mapped as a set of Land Management Units (Figure 32). The new set of units is at a finer scale than the groundwater flow system classification and will form the basis for definition of a range of management options for discussion with the local community.
Figure 31. Groundwater flow systems of the Macquarie-Bogan catchment (enhanced data inputs) and Figure 32. Seventeen Land Management Units were defined by the classification procedure (enhanced data and mapping rules).
South-west of Western Australia
The groundwater flow system classification project had defined a large area of south-west Western Australia as a series of local flow systems. Again, at the national scale this was sufficient to demonstrate the differences between this region and others across Australia. It was of limited use in defining the response and applicability of management options.
More detailed work shows that local flow systems behave differently. Work by Agriculture WA and CSIRO Land and Water (George et al. in press) was aimed at simulating groundwater responses in several catchments to a variety of land use change and engineering interventions. Results show that watertables, and hence the long-term salinity risk, in relatively flat catchments in the low to medium rainfall areas (representing much of the agricultural area) respond only to significant (greater than 50%) reductions in recharge. Lower levels (less than 50%) of intervention may buy significant (greater than 40 years in the modelled cases) time before salinity impacts are fully realised in catchments with a deep (10-20 m) watertable.
In the very long term, they do not significantly alter outcomes. By contrast, modelling suggests that catchments in undulating landscapes respond to the recharge reductions relatively quickly. In these areas, groundwater and salinity levels may decline (in upper and mid-slopes) within 10-20 years. However, the likelihood of the majority of the treatment options modelled being adopted is low; these treatment options are not well developed, and are not as economically attractive as current farming systems. This Western Australian work has focused on simulating the response for an area of high watertables, and does not make predictions about changes in salt loads in rivers and streams.
The work also concluded that some catchments within the flatter regions would develop significant areas with a shallow watertable as a result of their geomorphology. Evidence suggested convex or complex shaped land systems have a lower risk of salinity. Local topographic variations will also have a large impact on the area that actually becomes saline. As potential evaporation rates are very high (greater than 2000 mm per year), and discharge rates low (less than 100 mm per year), relatively small areas of the low gradient, high watertable areas need to discharge groundwater.
Conclusions
These conclusions highlight the fundamental characteristics of landscapes that operate at the finest, or paddock, scale. The national groundwater flow systems classification provides context and direction for these finer scale investigations.
The national groundwater flow systems map is a strategic planning tool. It is based on regional-scale information. It forms the technical base to consider regional salinity management options in the context of groundwater processes and the responsiveness of aquifers to changes in land use. Finer-scale analysis, building on these concepts and process understandings (such as the two-dimensional models from Western Australia), provides a tactical planning base at the local scale.
Figure 33. Australian groundwater flow systems: south-west Western Australia.
Table of Contents for the Australian Dryland Salinity Assessment 2000
Before you download
Most publications are downloadable as PDF files. Adobe Acrobat Reader is required to view PDF files.
If you are unable to access a publication, please contact us to organise a suitable alternative format.
Key
Links to an another web site
Opens a pop-up window