Review of December 2025 Operations
Well below average rainfall totals were recorded across Grampians storages during December. The highest rainfall total received was at Moora Moora Reservoir with 30.4 mm (57% of the historic monthly average), followed by 29.8 mm recorded at Lake Bellfield (55% of the historic monthly average). The lowest monthly rainfall total was received at Toolondo Reservoir with 10.6 mm (35% of historic monthly average).
Below average rainfall during December and increased temperatures resulted in very limited inflow being recorded to system storages over the assessment period. A total of 999 ML of inflow was recorded to storages during December, representing 21% of the historic monthly average. Inflow recorded in the July to December period (56,111 ML) represent 33.7% of the historic average for the same period.
Between 3rd December and 7th January, the total volume in storage decreased from 255,010 ML (45.5%) to 235,830 ML (42.1%), representing a total decrease of 19,180 ML (3.4% of total operating capacity). Lake Wartook continues to be of particular interest, currently holding 13,600 ML (46%), a 1,745 ML (6%) reduction since the December resource assessment.
Evaporative losses from storages increase significantly during December, with 11.78 GL of net evaporation recorded during the month. This is up from the 5.22 GL recorded in November. Losses are expected to remain elevated throughout summer, with rainfall providing the only offset.
Environmental deliveries were predominately supplied from Rocklands Reservoir to the Glenelg River during December, with 2,625 ML of regulated releases. This was in addition to 715 ML of passing flow and 365 ML of Glenelg River compensation flow. In the Wimmera system, the majority of releases occurred from Taylors Lake to the Wimmera River, with 438 ML delivered. A total of 94 ML was also delivered to the Mackenzie River.
Consumptive entitlement holders collectively used approximately 2,185 ML during the month to supply respective urban and rural demands.
January 2026 Operations and Climate Outlook
The Bureau of Meteorology (BoM) report that La Niña is currently underway, with a coupling of the Pacific Ocean and atmosphere. La Niña conditions are expected to persist into early 2026, before returning to neutral conditions towards the end of summer.
Short-term forecasts for the Wimmera-Glenelg headworks area suggest average rainfall conditions for the Grampians over the month, although average rainfall totals would be low compared to winter/spring months.
The Indian Ocean Dipole (IOD) remains neutral and is likely to remain neutral until the end of autumn.
The Southern Annular Mode (SAM) is neutral and is not expected to impact climate conditions over the coming weeks.
Rainfall outlooks indicate a 50% chance of rainfall exceeding median during January and a 50% chance of rainfall exceeding the median for the January to March period. Daytime temperatures are expected to remain elevated, with the BOM projecting a 75% chance of exceeding median max temperatures for the January to March period.
Reservoirs are projected to hold suitable levels during January for a range of recreational activities, however high level of evaporation through summer are likely to cause a steady decline in water levels. 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.