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Catchment Classification is a rigorous attempt to bring about informed decision making for salinity management. The project aims to classify types of catchments, their salinity risk status, the management options available to the catchment community and the opportunities for risk reduction. Effective salinity management options will differ for the different parts of the landscape. The type of groundwater flow system is one of the main determinants of the choice of salinity management whether it be biological recharge reduction, engineering options for managing shallow water tables, or saline agriculture. The groundwater flow systems approach enables existing groundwater information to be used to effectively target funds and resources for further investigations and salinity management. The effectiveness of biological recharge reduction is dependent on both the level of recharge reduction required for sustainability and the time-lags before sufficient salinity improvements result from the recharge reduction. The level of recharge reduction required to avoid salinity is largely determined by the so-called discharge capacity of the aquifer at the outlet of the catchment (i.e. the amount of water the aquifer can carry out of the catchment). The time-lag for recharge reduction to impact on an amount of saline land is also sensitive to the size of the groundwater flow system and its characteristics, with times to equilibrium varying from about 20 years to several hundred years. To reach end-of-valley or regional salinity targets with a 20 to 50 year timeframe, such time-lags are very important. In the meantime, to protect assets such as water resources, urban infrastructure and ecologically important areas, groundwater engineering schemes will be required. The effectiveness of these will also depend on the groundwater flow system properties. A groundwater flow systems classification approach While the above concepts are not new, applying them across large areas has been difficult. Obtaining the prerequisite hydrogeological parameters and the conceptual understanding requires expensive investigations. To address this problem, a classification approach based on groundwater flow systems is being gradually implemented. Such classification approaches build upon land management units (LMUs) defined in Victoria, unique mapping areas (UMAs) developed in the Liverpool Plains (NSW) and hydrogeomorphic units (HGUs). A significant area of Australia is at risk from dryland salinity. But, across the landscape, there is a broad range in the expression of salinity, timescales of response, and possible action to mitigate rising water levels and salinisation. The case studies prepared for the National Land and Water Resources Audit (NLWRA) and the Murray-Darling Basin Commission (MDBC) Catchment Classification projects have covered some of this range. 1. In general, smaller flow systems have the greatest potential for remediation on a 20-year time horizon. The changes required are mainly through adoption of biological measures, although the scale of change is no doubt very large. Recharge reductions in these systems would be most effective through improved farming systems in elevated parts of the landscape. Case study catchments in this category are Kamarooka, and parts of Wanilla and Kyeamba Creek. 2. Larger scale flow systems generally have higher conductivity and more throughput of water, thus making the balance between filling and draining more complex and site specific. Such systems are more amenable to engineering solutions than exclusively biological recharge reduction methods. Local responses in these catchments may be rapid although may take 20 to 50 years for the full benefits to be manifested. Catchments in this category are the South Loddon Plains and Billabong Creek. 3. Some catchments are naturally either relatively unresponsive or so complicated that they defy simple solutions in any reasonable time frame. The low lying areas in the Wanilla catchment (SA, Eyre Peninsula) have been subject to primary salinisation processes over geologic time-scales, so even conversion back to a native state would not eliminate waterlogging and salinisation. In the Axe Creek catchment (Victoria), there is so much complexity in the nature and configuration of the aquifer that an overall catchment solution seems unlikely within a realistic time frame. Much more detailed investigations would be needed in these cases to provide robust process understanding for management advice. Next Page: Groundwater Flow Systems Classification - developing, applying, and the likely impact of... |
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