Review of June 2025 Operations
Average rainfall totals were recorded across Grampians storages during June, breaking a run of 16 consecutive monthly of below average rainfall. The highest rainfall total received was at Lake Bellfield with 108.2 mm (95% of the historic monthly average), followed by Moora Moora with 98.2mm (86% of the historic monthly average). The lowest monthly rainfall total was received at Taylors Lake, with 37.6 mm (85% of historic monthly average).
Average rainfall in June started to generate some inflow to storages, with 2,310 ML being recorded across all storages. Year to date inflow increased to 29,310 ML, or 14.7% of the historic average for the same time period. 2024/25 ranked as the second lowest inflow year of record (1900-2024), approximately 4,460 ML ahead of the worst year (1999/2000) and 450 ML behind the third lowest year (2006/07).
Between 4th June and 2nd July 2025, the total volume in storage increased from 206,330 (36.8%) to 210,245 (37.5%), representing an increase of 3,915ML (0.7% of total operating capacity). Of particular interest is the volume held in Lake Wartook, which currently sits at 7,780 ML (27% of storage capacity). This volume is just above the nominated 7,500 ML Critical Needs Reserve agreed to by entitlement holders. If the volume drops below the Critical Needs Reserve, all water within Lake Wartook is reserved for urban and rural consumptive use associated with the townships of Horsham and Natimuk. Environmental deliveries can only occur while the volume remains above the reserve or through agreement with impacted entitlement holders. Regular meetings between the Storage Manager and relevant stakeholders will occur while Lake Wartook remains under or close to the Critical Needs Reserve.
Average rainfall during June and lower daytime temperatures allowed for some Grampians catchments to start producing runoff in response to rainfall event. While this is a positive sign, consecutive average to above average rainfall months will be required to sufficiently saturate catchments and provide any meaningful inflow to storages.
The total monthly rainfall received on storages exceeded evaporation from storages during June, with approximately -2,430 ML of net evaporation calculated from headworks storages during the month.
There were no environmental deliveries in June, with reduced allocations and cooler weather decreasing demand.
Consumptive entitlement holders collectively used approximately 1,313 ML during the month to supply respective urban and rural demands. A total of 175 ML was delivered to Wimmera-Mallee Pipeline connected recreation lakes during June.
July 2025 Operations and Climate Outlook
The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is neutral and likely to remain neutral through to December 2025.
Long-range forecasts for the Wimmera-Glenelg headworks area suggest well below average rainfall conditions over the month. Below average rainfall is likely to further delay the development of any significant inflow to storages.
The Indian Ocean Dipole (IOD) is currently in a neutral state, with the majority of international models forecasting it may briefly reach negative conditions before returning to neutral sometime between July to September 2025. This forecast does not indicate a high likelihood of a strong negative IOD event occurring during winter.
The Southern Annular Mode (SAM) is currently positive, with a tendency towards positive values over the next week.
Rainfall outlooks indicate a less than <30% chance of rainfall exceeding median during July and a 50% chance of rainfall exceeding the median for the July / September period. Daytime temperatures are expected to remain elevated, with the BOM projecting a >80% chance of exceeding median max temperatures for the July to September period. Unless average to above average rainfall conditions develop throughout July, operational decision making will continue to focus on maximising harvesting opportunities and contingency planning for less secure storages.
With the system entering the historic inflow period (July – September) storage recovery will be closely monitored and communicated to stakeholders. Of particular importance will be the recovery of Lake Wartook which is currently below 30% capacity and unable to provide complete security of supply for 2025/26.
Reservoirs are projected to hold suitable levels during July for a range of recreational activities. Recreation users are reminded to take care and abide by signage at reservoirs. Please refer to GWMWater’s Reservoir information page for further information on water levels and recreation activities permitted at each reservoir.
There are currently no Blue-Green Algae warnings issued. Information on all current algae warnings, as well as general information on Blue-Green Algae, is available from the GWMWater Algae Warnings webpage.