Review of September 2025 Operations
Well below average rainfall totals were recorded across Grampians storages during September, with headworks storages receiving a combined total equal to 34% of the historic average. The highest rainfall total received was at Moora Moora Reservoir with 36 mm (34% of the historic monthly average), followed by Lake Bellfield with 32.6 mm (31% of the historic monthly average). The lowest monthly rainfall total was received at Lake Fyans, with 12 mm (22% of historic monthly average).
A total of 10,118 ML of inflow was recorded to storages during September, representing 24.3% of the historic monthly average. Inflow recorded in the July to September period (37,125 ML) represented 30.4% of the historic average for the same period. Inflow received during September was largely the result of average rainfall received during July and August, which generate a positive response from most catchments. With well below average rainfall total received throughout September, catchments have started to dry, with future system inflow likely to depend heavily on additional rainfall.
Between 3rd September and 1st October 2025, the total volume in storage increased from 241,840 ML (43.2%) to 247,030 ML (44.1%), representing an increase of 5,190 ML (1.1% of total operating capacity). Lake Wartook continues to be of particular interest, currently holding 14,030 ML (48%), having shown a modest recovery throughout July and August, only to plateau during September.
September represented the first month of the water year where evaporation from storages exceeded rainfall, with approximately 2.71 GL of net evaporation calculated from headworks storages during the month.
Environmental deliveries continued through September to the Lower Burnt and Bungalally Creeks, with 555 ML delivered during the resource assessment period.
Consumptive entitlement holders collectively used approximately 860 ML during the month to supply respective urban and rural demands.
October 2025 Operations and Climate Outlook
The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is neutral, but could briefly turn negative during spring, before returning to neutral by summer.
Short-term forecasts for the Wimmera-Glenelg headworks area suggest below average rainfall conditions for the Grampians over the month.
The Indian Ocean Dipole (IOD) is currently in a negative phase, with recent values indicating it will remain negative throughout spring. Accuracy on IOD model predictions is generally limited to one month, making longer term predictions less accurate.
The Southern Annular Mode (SAM) is currently neutral and is expected to remain neutral over the next week.
Rainfall outlooks indicate a less than 45% chance of rainfall exceeding median during October and a 55-65%% chance of rainfall exceeding the median for the October/December period. Rainfall outlooks continue to be moderated after showing optimism for early spring, with recent dry conditions indicated forecasts for above average conditions may be difficult to achieve in the current climate. Daytime temperatures are expected to remain elevated, with the BOM projecting an 80% chance of exceeding median max temperatures for the October to December period. Unless average to above average rainfall conditions develop throughout October, operational decision making will continue to focus on maximising harvesting opportunities and contingency planning for less secure storages.
A 12,360 ML headworks transfer from Rocklands Reservoir to Taylors Lake will continue into October, with the harvesting of unregulated catchment flows assisting in fulfilling the required transfer volume. Harvesting opportunities to Taylors Lake will continue to be leveraged with the potential to truncate the transfer if sufficient volume in storage is achieved. This volume is required to ensure an adequate supply of water is held in Taylors Lake to meet expected environmental demand for the 2025/26 water year, while providing some recreational value and providing a backup supply for the Wimmera Mallee Pipeline.
Reservoirs are projected to hold suitable levels during October 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.