Review of July 2025 Operations
Average to above average rainfall totals were recorded across Grampians storages during July. The highest rainfall total received was at Lake Bellfield with 161.6 mm (117% of the historic monthly average), followed by Lake Fyans with 112 mm (183% of the historic monthly average). The lowest monthly rainfall total was received at Taylors Lake, with 42.8 mm (101% of historic monthly average).
Average rainfall in July increased inflow to storages, with 14,018 ML being recorded across all storages. Lake Bellfield received the highest monthly inflow with 4,765 ML, followed by Lake Wartook with 3,283 ML and Rocklands Reservoir with 3,251 ML. Inflow received during July was equal to 48% of the total inflow received to all storages during the 2024/25 water year.
Average rainfall during July and lower daytime temperatures allowed for Grampians catchments to start producing runoff. While inflow volumes received in July 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 1st July 2025 and 6th August, the total volume in storage increased from 210,245 ML (37.5%) to 228,040 ML (40.7%), representing an increase of 14,910 ML (3.2% of total operating capacity). Of particular interest is the volume held in Lake Wartook, which increased from 7,780 ML (27% of storage capacity) to 11,200 ML (38% of storage capacity). 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 July, with approximately -2,680 ML of net evaporation calculated from headworks storages during the month.
There were no environmental deliveries in July, with reduced allocations and cooler weather decreasing demand.
Consumptive entitlement holders collectively used approximately 1,332 ML during the month to supply respective urban and rural demands.
August 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.
Short-term forecasts for the Wimmera-Glenelg headworks area suggest average to slightly below average rainfall conditions for the Grampians over the month. Below average rainfall is likely to 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 turn negative during spring. Although a negative IOD increases the likelihood of higher rainfall across southern Australia, there are very few observed signed that one is developing. 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 August and a 55-60% chance of rainfall exceeding the median for the August/ October period. Daytime temperatures are expected to remain elevated, with the BOM projecting a 70-80% chance of exceeding median max temperatures for the August to October period. Unless average to above average rainfall conditions develop throughout August, 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 40% capacity.
A 12,360 ML headworks transfer from Rocklands Reservoir to Taylors Lake has been scheduled to commence mid-August 2025. The timing of this transfer is to coincide with wetter / cooler conditions, as to limit any losses associated with a transfer. 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 August 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.