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Printing help Glossary problems? |
What can be transferred? There are three distinct themes that the project seeks to transfer between catchments. 1. An understanding of the processes occurring within a catchment, specifically those that lead to dryland and stream salinisation. 2. The physical state parameters of the aquifer systems, which can be used in more detailed process modelling of the aquifer for specific catchments. 3. The management options and strategies that can be employed in catchments to control or remedy dryland and stream salinisation. Each of these can most reliably be transferred to other catchments of the same type that are geographically nearby. There is less confidence as the distance between case study and unstudied area increases. 1. Process understanding Catchment process understanding is the basis for all aquifer modelling and management. Without strong conceptual models of the flow paths and processes within catchments, we cannot start to describe the critical data sets needed. This aspect of Catchment Classification is already well advanced, and the documents by Coram (1998) and Coram et al. (2000) are the essential starting point for this. 
Groundwater flow systems can be broadly classified by the underlying geology that provide robust descriptors of the aquifer type, e.g. fractured rock, deeply weathered, alluvial deposits and colluvial fans.
The next essential level of Classification is by size, where local, intermediate and regional groundwater flow systems have been defined. Aquifers with the same geology and aquifer material can exhibit different flow characteristics and salinity expression depending on their size, so this information is just as important.
A national map of groundwater flow systems, differentiated by both size and geology was developed within Coram et al. 2000. The current project has refined this large-scale mapping (approximately 1:5M data sources) down to individual regions at a scale of 1:250,000 where data exists. This has been achieved largely through workshops conducted in the regions covered by the respective state departments with authority over land and water resources. We assert that process understanding is highly transferable between catchments. 2. Aquifer parameters Process understanding may be the basis for developing a numerical model of an aquifer, but without estimates or distributions of the necessary physical parameters, modelling is not possible. So the second aspect of transferability is how easily can parameters, measured or fitted for modelling in one catchment, be used in another of the same type? In general, there are two important aquifer parameters:
Transmissivity - the rate at which water can pass through a unit of aquifer, usually one metre wide, in one unit of time, usually one day. Very high values are found in sand and gravel aquifers, and within individual cracks of a fractured rock system, with values decreasing in highly weathered and clayey materials.
Specific yield - the proportion of the aquifer that can be filled with water, and is most commonly associated with unconfined aquifers. When an aquifer becomes confined, the compressibility of the water and aquifer itself must be taken into account with a storage coefficient. In some situations this is a significant proportion of total stored water.
The transferability of aquifer parameters is somewhat dependent on the aquifer itself.
Well-sorted sand and gravel alluvial aquifers are likely to exhibit relatively consistent hydraulic conductivity and specific yield values. These values are likely to be relatively large also, so measurement errors are a smaller proportion of the value itself.
Conversely, fractured rock aquifers may have great variation in distributions of fracture width, density and orientation.
Thus a representative bulk transmissivity, conductivity or pore volume estimate is likely to exhibit large variation between aquifer systems across the continent. We assert, however, that aquifer parameters are moderately transferable between aquifers with the context of local data and experience. 3. Management options The ultimate aim of data collection and aquifer modelling and simulation, is to derive some set of management options to control groundwater levels into the future. Managing groundwater might be:
to stop it rising by reducing recharge,
to control the maximum height of the water level by enhancing surface discharge or pumping from the aquifer directly, or
to manage the groundwater as a resource by ensuring the level never drops below a certain height.
In the dryland salinity context we are interested in biological and engineering options to control the maximum height of a water level, usually to prevent saline discharge to streams, shallow local groundwater flow systems, or to the land surface directly. Management options can often be related to the aquifer geology and processes. In large and highly transmissive alluvial systems, broad-scale recharge control by manipulating surface vegetation is not a viable option. Recharge directly from stream and river reaches along with episodic events may be all that is required to drive rising water levels and surface discharge. In this case, engineering options become favoured, since finding water in such aquifers is easy, salinity of the groundwater is often low, and pumping is likely to have wide spread rather than localised effects. Biological options, such as upgrading annual to perennial pastures, inclusion of lucerne into cereal cropping rotations, or the reintroduction of trees or other native vegetation, are subject to a greater range of variables than engineering. For example, while it may be theoretically possible to reduce recharge with lucerne, in some areas soil type or climate may hinder water uptake or preclude establishment altogether. Similarly with reforestation, there may be very important considerations of tree species, establishment, planting density, and thinning management. Likely management options for salinity in generic groundwater flow systems Coram et al. (2000) have provided first estimates of the likely management options for salinity in generic groundwater flow systems. Depending on the aquifer type, we assert that:
general control types, biological or engineering or living with salt, are highly transferable,
some specific management options, trees or lucerne or cropping practices, are moderately transferable with local data and experience.
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| design & production by Talkin' Technical Communications | last updated: April 2002 |