The Murray–Darling Basin has a highly variable climate, with conditions differing each year and across regions. Planning for wet and dry conditions is a major challenge for river operators, water resource managers and the community.
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Planning for water use in the Basin is critical to ensure enough water is available in the River Murray system to meet critical human, system and environmental needs and enough is held for future reserves. After this water is set aside the amount of remaining water in the River Murray system is calculated by the MDBA and made available to the Basin states to allocate to their entitlement holders.
Special accounting is a form of state water sharing that comes into effect during dry conditions in the southern Basin when water availability and forecast reserves are low. Special accounting adjusts water sharing arrangements between New South Wales, Victoria and South Australia, which is scaled back when there is less water available. This ensures each state gets an agreed share of water even when there is less to share.
The rules for special accounting are set out in the Murray–Darling Basin Agreement (the Agreement). The Murray–Darling Basin Authority is responsible for sharing water according to the Agreement. It acts as an independent body to make sure New South Wales and Victoria share the agreed amount of water with South Australia.
The Murray–Darling Basin has a variable climate that requires planning ahead. This includes determining future needs and continually assessing risks to prepare for drier periods and ensure adequate water is available.
When does special accounting occur?
A period of special accounting is declared by the MDBA when either New South Wales or Victoria are forecast to have less than 1,250 GL in reserve at the end of the MDBA water year (31 May).
The MDBA uses worst-case scenarios to forecast water needed in reserve and determines if special accounting will come into effect. This conservative approach means there is a low risk that less water than expected will be available.
Special accounting has been activated in most recent years. Victoria tends to enter and leave Special Accounting within a year whereas NSW is more likely to remain in special accounts for multiple years. This is due to states different reserve policies and tributary inflows.
Did you know…
- The entitlements of South Australia are provided equally by New South Wales and Victoria.
- Special accounting is part of the water accounting rules in the Agreement but is only activated in certain conditions.
- Special accounting occurs during a dry period when either New South Wales or Victoria is forecast to hold less than 1,250 gigalitres in storage.
- The conveyance water reserve is the additional amount of water set aside for the next year to minimise the risk of not enough conveyance water being available.
- NSW and Victoria can each independently enter and exit a period of special accounting with South Australia.
- Critical human water needs are a practical measure to safeguard water for communities in drought.
Prioritising water needs
Water in the River Murray is prioritised to ensure communities have enough water to drink and water reaches its destination. Water is then shared between New South Wales, Victoria and South Australia before it is allocated to water users by Basin state governments.
Water that is critical for human needs
The highest priority water in the River Murray System is conveyance water, which is the amount of water needed to ensure water for drinking and sanitation gets to where it is needed without being reduced by evaporating or seeping into the riverbed.
Water for these critical needs is set aside every year as a must have, alongside the water needed to deliver this water to communities. Given the dry climate in Australia, the MDBA also places water in reserve for the following year to ensure towns don’t run out of water along the River Murray.
Water for states to share
After critical needs have been considered, the share of water for New South Wales and Victoria is then calculated and assigned to each state which equally supply the agreed share to South Australia.
South Australia receives a specific volume (currently 1,850 gigalitres) which it can rely on each year, except when forecast reserves are low. Victoria and New South Wales each provide half of South Australia’s share from the water they have in the system.
Each state is responsible for setting aside the water they need to meet critical human water needs. Once this water has been set aside states can allocate the remaining water to their entitlement holders.
Special accounting
When special accounting is declared, South Australia does not receive its agreed 1,850 gigalitres but receives one third of the shared water upstream and no inflows from the tributaries – this water remains with New South Wales and Victoria.
Periods of special accounting
Special accounting can be in effect between South Australia and one upstream state or with both New South Wales and Victoria. Special accounting can be in place for a period of months to multiple years.
During a period of special accounting, the MDBA is required to keep additional water accounts. These ‘special accounts’ record:
- state diversions (or use)
- useful tributary flow which is the tributary flow used by NSW and Victoria. It does not include any tributary flows that can't be captured or used and flows to South Australia.
- the volume of water NSW and Victoria have supplied to South Australia. Special accounting allows states flexibility in how they manage available water during dry periods without impacting on water available to other states.
During periods of special accounting, the 'imbalance in use' is used in the calculation of water available to each state. It records the relative usage by states and their useful tributary inflows.
Water sharing tiers
To help prioritise critical human water needs, the Basin Plan and the Murray–Darling Basin Agreement adopts a tiered approach to water sharing.
There are 3 water sharing tiers. The Basin Plan sets triggers for moving between water sharing tiers. These triggers are based on the risks to meeting and/or delivering critical human water needs, conveyance water targets and reserves, and water quality. Special accounting can be activated during any of the water sharing tiers.
Find out more about water sharing tiers
Leaving special accounting
The MDBA must terminate a period of special accounting declared for New South Wales or Victoria whenever it is satisfied that the reserve allocated to that state at the end of the following May will be greater than 1,250 gigalitres.
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