Probability scenarios provide a guide to the likelihood of future irrigation allocations. They are based on River Murray system modelling, undertaken by the Murray-Darling Basin Authority (MDBA), and South Australia’s River Murray Water Allocation Framework, included in the Water Allocation Plan for the River Murray Prescribed Watercourse.
The probability scenarios are not predictions, but rather a tool to help better understand potential future water availability. They represent allocations under different water availability scenarios (based on historical variability in rainfall and temperature, in combination with current policy and operational settings).
The modelling sets all storages and flows in the system to current conditions and uses historical inflow and climate conditions to create over 100 unique inflow sequences for the next 12 months. These simulations are used to calculate the probability scenarios.
What about carryover?
Allocations in the probability scenarios do not include carryover. Going forward, carryover availability would also be announced by no later than mid-April to align with the announcement of minimum opening allocations.
Changes to the private carryover policy included in the Water Allocation Plan for the River Murray Prescribed Watercourse are proposed.
Further information can be found in the Private Carryover fact sheet.
Probability scenarios example
Below is an example of the types of information that are proposed to be included as part of allocation announcements, when allocations are less than 100 per cent.
- The projected minimum opening allocation is XX per cent.
- The projected minimum amount of water that will be delivered to South Australia as part of its Entitlement for the following water year is XX GL, up to 1850 GL (excluding additional environmental water deliveries, e.g. water held by the Commonwealth Environmental Water Holder).
- Carryover will [or will not] be provided.
- XX GL of water is currently held in storage for private carryover - see Figure 1 below.
Table 1 and Figure 2 provide information on potential allocations for a range of probability scenarios that could be provided in advance of the water year. The projected minimum opening allocation is XX per cent. This is reflected in the 1 July allocation under the minimum allocation scenario (in this EXAMPLE it is 36 per cent).
Probability scenarios are based on River Murray system modelling and South Australia’s River Murray Water Allocation Framework. They represent allocations under different water availability scenarios based on historical variability in rainfall and temperature, in combination with current policy and operational settings (this includes Basin-wide storage volumes).