The Storage Manager for the Wimmera-Mallee system headworks has conducted an assessment of the risk of spill for the system headworks, as per the methodology defined in the Storage Management Rules. The result of this assessment has enabled the Storage Manager to declare a low risk of spill as of 1 July 2026.
Discussion
Analysis of the system headworks risk of spill has shown that under 50% probability of exceedance (PoE) inflow (equivalent to 50th percentile), average net evaporation and entitlement holder demand as per the Storage Manager’s 2026-27 Annual Operating Plan, the total volume in storage on 30 June 2027 is forecast to be 55% of maximum operating capacity.
The Storage Manager must declare a low risk of spill if the estimated end of year volume in storage is projected to be less than 70% of maximum operating capacity.
As of 1st July 2026, the headworks reservoirs were collectively holding approximately 38.6% of the system wide maximum operating capacity. The risk of spill is linked to the forecast storage volume across the Wimmera-Mallee system headworks at the end of June 2027. The Storage Manager’s modelling assessment indicates that under 50% PoE inflow conditions, no reservoirs spills are likely to occur (or require target curve releases).
With a low risk of spill having been declared, no deduction from entitlement holder accounts will be made in the event that any spills from reservoirs do occur during 2026-27.
Additional note for 2026-27 Risk of Spill Assessment
A proposed change that was been investigated as part of the 2019 Headworks Operations Review was to modify the criteria for the Storage Manager to declare a low risk of spill. It was proposed that the Storage Manager should not declare a low risk of spill when the end of season storage volume is predicted to be less than 70%, if spills from individual reservoirs are forecast to occur during the year using the 50% PoE inflows.
Headworks Operations Review modelling demonstrated that this approach led to better accounting of spills, and less unaccounted spills. While this change to the risk of spill criteria was supported by entitlement holders and stakeholders, it has not yet been approved for implementation. Approval is expected within the 2026/27 water year, at which point it will be used for next years assessment.
To ensure the risk of spill assessment was consistent with the expectations of entitlement holders, the proposed rules were also tested which indicated no spills or target curve releases would be required during 2026-27, with 50% PoE statistical inflow volumes and nominated entitlement holder demand.
Download the Rules here:
Storage Management Rules for the Wimmera-Mallee System Headworks (updated August 2016).pdf