Water across the southern-connected Basin is managed. This means flows in the River Murray are controlled to some extent to ensure water is stored and delivered when and where it’s needed. The term used to describe the management of water in this way is 'regulated'.
When rainfall falls downstream of storages and cannot be captured, it’s referred to as unregulated flow. In the River Murray system, 'unregulated flow' occurs when water in the river, or water that is forecast to flow through the river system:
- exceeds the amount of water required to meet all system demands, and
- cannot be captured and stored in Lake Victoria.
Both of these elements need to occur at the same time in order to declare unregulated flows.
When this occurs, the flow to South Australia can no longer be held or 'regulated', meaning excess or 'surplus' water will flow into South Australia.
When it comes to river management, the priority for the Murray–Darling Basin Authority (MDBA) is capturing and storing as much water as possible, so it can be allocated to entitlement holders and made available for use.
Unregulated flow commonly occurs when there is persistent, high inflows downstream of Hume Dam, or as the result of a spill at Hume Dam or the Menindee Lakes – these flows all significantly boost flows into the Murray.
Unregulated flow occurs mostly during average to wetter years and can last for many months. During most drier years, unregulated flow does not typically occur.
Forecasting and declaring unregulated flows
The forecasting and management of unregulated flow is the responsibility of the MDBA as part of its role in directing the operation of the River Murray system. It is an important component of the overall system operations and is part of the Murray–Darling Basin Agreement.
To inform Basin state governments of the potential occurrence of unregulated flow, the MDBA regularly reviews current and forecast storage levels, inflows and demands in the coming weeks to predict if and when unregulated flow might begin. If these forecasts indicate that unregulated flow is likely to occur, the MDBA works with state water agencies to confirm the forecast. It will then determine if unregulated flow can be announced (or 'declared'), as well as the likely volume and location of unregulated flow along the river.
When there is potential for an unregulated flow to occur, the MDBA carries out an assessment of the situation to determine if and when an unregulated flow might begin. To do this, the MDBA considers a range of factors, which include:
- where the rainfall occurred
- current and forecast water storage levels
- how long it will take for the water to travel through the river system
- whether any water will be lost to evaporation or seep into the ground
- the upcoming weather forecasts
- demands for water for the environment
- River Murray system requirements such as conveyance water
- future River Murray system demands and inflows
- the time of year.
The MDBA always takes a cautious approach when forecasting unregulated flow. This ensures the system is managed efficiently and that as much water as possible can be made available for entitlement holders.
The MDBA and states must work together to ensure that any water being taken during the unregulated flow period will not impact:
- the priority of providing water to South Australia, by filling Lake Victoria to its full capacity, or
- state allocations for entitlement holders.
The MDBA must consider the likely demand of water for priority entitlement holders across New South Wales, Victoria and South Australia. If forecasting shows all the water in the system will be required to supply South Australia or to meet demands of entitlement holders and there is not any surplus water, an unregulated flow will not be declared.
If forecasting suggests these needs can be met, discussions with states are then held to determine:
- the expected demands of irrigators who would access this water
- potential use of the flows
- the expected volume that may be taken during an unregulated flow.
The MDBA will then decide and announce formal unregulated flow advice to New South Wales, Victoria and South Australia and the Chair of the Southern Connected Basin Environmental Watering Committee (SCBEWC). This announcement includes which areas of the River Murray and timeframes where unregulated flows will occur and will be accessible.
Basin state governments are then responsible for determining how much water is allocated to their licence holders who can access unregulated flows, as per the state allocation frameworks.
It is important that water diverted from an unregulated flow does not exceed the volume surplus to that required to meet demands of entitlement holders seeking to use water that has been allocated to them. If this were allowed to occur the extra water used would have to be met from water that could otherwise be held in store and thereby reduce the volume of water available to be allocated.
Accessing unregulated flows
States have different rules when allocating and accessing declared unregulated flows.
When the MDBA announces a period of unregulated flow in the River Murray system, states publish the details of the sections of river and timeframes, where water is accessible.
Water license holders can only pump water against these licenses during the announced periods. The state water authorities are responsible for debiting retail licences and are able to advise on the actual volume, percentage of use and other licensed take once the event is complete.
For more information on state allocations view our guide on allocations or visit:
Any unused water from an unregulated flow is considered and can be accessed by environmental water holders for use to achieve environmental outcomes. This is water that remains after water licence holders have taken their allocation and is used based on environmental watering priorities.
To see how water for the environment is being used in the River Murray system, look at the latest flows in the River Murray.
Ceasing unregulated flow
Once unregulated flow begins, the MDBA continually monitors, assesses and updates the declaration of unregulated flow, the reaches where flows are available and if the unregulated flow will last longer, particularly if additional rain and inflow takes place.
When inflows decrease, the amount of unregulated flow along the river normally decreases downstream towards Lake Victoria and upstream reaches will again become ‘regulated’.
The period of unregulated flow ends once the flow to South Australia can be maintained at the required rate and Lake Victoria has maximised its water storage on the date unregulated flow ceases.
