Review of December 2024 Operations
Below average rainfall totals were recorded across Grampians storages during December, with all storages except Mt Cole recording rainfall below the monthly average. The highest rainfall total received was at Mt Cole with 60.6 mm (133% of the historic monthly average), followed by Moora Moora Reservoir with 49.6 mm (92% of the historic monthly average). The lowest monthly rainfall total was received at Lake Toolondo, with 13.4 mm (44% of historic monthly average). Below average rainfall received during December represented the eleventh consecutive month of below average rainfall across headworks storages.
Below average rainfall in December struggled to generate meaningful inflow to storages with 510 ML (excluding Taylors Lake) being recorded. December inflow was equivalent to 10.9% of the historic average. Year to date inflow increased to 24,455 ML, or 14.7% of the historic average for the same time period.
Between 4th and 31st December, the total volume in storage decreased from 295,030 ML (52.7%) to 276,680 ML (49.4%) representing a total decrease of 18,350 ML (3.3% of total operating capacity).
With daytime temperature increasing and catchments continuing to dry out, any future harvesting from unregulated catchments will be reliant on catchments receiving sufficient rainfall to recommence flow, which during summer is unlikely.
The total monthly evaporation from all storages was greater than rainfall received to storages for December, with approximately 13.2 GL of net evaporation calculated from headworks storages during the month. High net evaporation throughout the summer period is expected to be the main influence on storage levels over the next few months.
A fire in the Grampians National Park limited operational access to a number of storages through December. Although deliveries were able to continue from effected storages, operational staff weren’t able to adjust releases during this period.
Environmental deliveries from the headworks system continued through December, with 305 ML of environmental water released to the Mackenzie River / Burnt Creek systems. Releases to the Mt William / Wimmera River system continued, with 360 ML released from Lake Lonsdale. Releases from Rocklands Reservoir to the Glenelg River continued, with 1,190 ML of regulated environmental flow, 310 ML of passing flow and 11 ML of Glenelg River Compensation Flow released. A total of 195 ML was also released from Moora Moora reservoir to the upper Glenelg River.
A total of 275 ML was delivered to Wimmera-Mallee Pipeline connected recreation lakes during December.
Consumptive entitlement holders collectively used approximately 1,545 ML during December to supply respective urban and rural demands.
January 2025 Operations and Climate Outlook
The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is likely to remain neutral.
Long-range forecasts for the Wimmera-Glenelg headworks area continue to suggest average to slightly above average rainfall conditions over the next few months. Previous predictions of favourable rainfall conditions have so far failed to develop, with consistent dry conditions being experienced across the headworks system since late December 2023 / early January 2024. While predictions for the January to March period are neutral to slightly positive, dry catchment conditions are likely to drive operational decision making over the next month, with additional monitoring likely required for less resilient storages such as Lake Wartook.
The Indian Ocean Dipole (IOD) has returned to a neutral state and is likely to remain until the end of the forecast period (April 2025).
The Southern Annular Mode (SAM) is also neutral, and expected to remain neutral over the next few weeks.
Rainfall outlooks indicate a 50% chance of rainfall exceeding the median during January and a 50% chance of rainfall exceeding the median between January and March. Daytime temperatures are expected to remain elevated, with the BOM projecting a >80% chance of exceeding median max temperatures for the January to March period. Unless average to above average rainfall conditions develop throughout January, the operational focus will continue to shift towards contingency planning for less secure storages, based on entitlement holder demand estimates for the 2024-25 water year.
Reservoirs are projected to hold suitable levels during January 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 is currently a Blue-Green Algae warning issued for Green Lake (Horsham). Information on all current algae warnings, as well as general information on Blue-Green Algae, is available from the GWMWater Algae Warnings webpage.