Review of August 2025 Operations
Average rainfall totals were recorded across Grampians storages during August, with a spread of storages recording above and below average rainfall. The highest rainfall total received was at Lake Bellfield with 112.4 mm (86.3% of the historic monthly average), followed by Moora Moora Reservoir with 98 mm (75.2% of the historic monthly average). The lowest monthly rainfall total was recorded at Toolondo Reservoir, with 46.2 mm (96% of historic monthly average).
Average rainfall in August sustained modest inflow to storages started in July, with 12,988 ML being recorded across all storages. Rocklands Reservoir received the highest monthly inflow with 4,088 ML, followed by Lake Bellfield with 3,673 ML and Lake Wartook with 2,108 ML. Inflow received during July and August was equal to 92% of the total inflow received to all storages during the 2024/25 water year.
Average rainfall and lower daytime temperatures allowed for Grampians catchments to continue producing runoff. While inflow volumes received in August should be considered a very positive sign, consecutive average to above average rainfall months will be required to sustain catchment flow and provide meaningful inflow to storages.
Between 6th August and 3rd September, the total volume in storage increased from 228,040 ML (40.7%) to 241,840 ML (43.2%), representing an increase of 13,800 ML (2.5% of total operating capacity). Of particular interest is the volume held in Lake Wartook, which increased from 7 11,200 ML (38% of storage capacity) to 13,630 (47%). This increase provided an additional buffer above the nominated 7,500 ML Critical Needs Reserve, at which point supply is reserved for urban and rural consumptive use associated with the townships of Horsham and Natimuk. Additional inflow is required before the volume held in Lake Wartook could be considered to provide a multi-year supply.
The total monthly rainfall received on storages exceeded evaporation from storages during August, with approximately -1,170 ML of net evaporation calculated from headworks storages during the month.
Environmental deliveries commenced in early September to the Lower Burnt and Bungalally Creeks, with 20 ML delivered during the resource assessment period.
Consumptive entitlement holders collectively used approximately 900 ML during the month to supply respective urban and rural demands.
September 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 January 2025.
Short-term forecasts for the Wimmera-Glenelg headworks area suggest average to slightly above average rainfall conditions for the Grampians over the month.
The Indian Ocean Dipole (IOD) is currently in a neutral state, with recent values indicating a shift to a negative phase, with further development in 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 50% chance of rainfall exceeding median during September and a 65-70%% chance of rainfall exceeding the median for the September/November period. Daytime temperatures are expected to remain elevated, with the BOM projecting an 80% chance of exceeding median max temperatures for the September to November period. Unless average to above average rainfall conditions develop throughout September, operational decision making will continue to focus on maximising harvesting opportunities and contingency planning for less secure storages.
With the system currently in 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 47% capacity.
A 12,360 ML headworks transfer from Rocklands Reservoir to Taylors Lake commence mid-August 2025, 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 September 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.