objective of the BSMS is to: maintain water quality; control the rise in salt loads; control land degradation and maximise net benefits of salinity control across the Basin. 10. The BSMS provides the framework for Basin States to account for and monitor land management activities that dispose of salt into the river (salinity debit) or those that have a salinity benefit by preventing salt from entering the river (salinity credit). All land use activities that have the potential to change the salt influx must be investigated by each State using an approved modelling approach and expressed in common terms of EC impact at Morgan. The Murray Darling Basin Authority (MDBA) quantifies the size of each accountable action using a computer model of the river system known as MSM BigMod. The MDBA maintains the official record in the Salinity Register equivalent to a financial ledger requiring each Basin State to maintain a positive balance of salinity credits against debits. 11. The BSMS framework recognises the difficulty in measuring many of the parameters important to quantifying physical processes active in the aquifer/river system being modelled. Accordingly, an adaptive management approach is implemented whereby the original estimates of salinity impact are reviewed every five years, known as ‘rolling reviews’. These reviews incorporate new knowledge, monitoring data and re-test the assumptions that underpin the model to reduce the uncertainty and refine the salinity impact estimates. It is recognised in the BSMS that modelling linked to monitoring is the only practical way to measure progress of actions directed towards salinity management. 12. The Mallee Catchment Management Authority (CMA), as agents of the state of Victoria, maintains a Mallee regional salinity register of credits and debits. An annual program of groundwater and surface water monitoring is undertaken to collect data that inform the annual reporting and scheduled five year reviews for each of the seven accountable actions pertaining to the region.
Modelling tools used to estimate salinity impact 13. Modelling is used within the BSMS to better understand the complex interactions of salinity processes and provide a predictive tool for determining appropriate solutions to landscape and water management. A ground-water model provides the scientific means to draw together the available data into a numerical characterisation of a groundwater system. Salt loads in the river, salinity in the water and salt in the regolith is not easily quantified. Water flows in the river have a major influence on salt load, salinity concentration and salt movement into and out of the river. 14. Without doubt the greatest advancement in our hydrogeological knowledge over the past 20 years and improved understanding of the patterns of variation and replication within and between landforms has been realised through improved data and modelling capacity, specifically: -
Improved technologies in field measure of geophysical parameters (e.g. electromagnetic surveys);
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Data collection through an annual program of monitoring of bore networks, stream gauging, irrigation footprint and climate data;
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Improved computing capacity and imagery from global information systems (GIS);
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Improved conceptualisation and representation of complex natural systems captured within surface water and groundwater models.
15. While many of the parameters important to hydrogeological process are not able to be directly 3 measured with an acceptable level of accuracy , these advances in technology have provided an insight into their degree of complexity and enabled estimations to be calculated with a known degree
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For example it is not possible to measure recharge for the simple reason that the errors associated with the overall water balance are greater than the magnitude of the recharge. Rootzone drainage can only be reasonably inferred with knowledge of soil water content changes. Recharge can only be reasonably estimated from information gained from observation wells.