Review of August 2024 Operations
Below average rainfall totals were recorded across Grampians storages during August, with all storages recording rainfall below the monthly average. The highest rainfall total received was at Moora Moora Reservoir which recorded 71.2 mm (54% of the historic monthly average), followed by Lake Bellfield with 67 mm (51% of the historic monthly average). The lowest monthly rainfall total was received at Taylors Lake, with 18.6 mm (43% of historic monthly average). Below average rainfall received during August represented the seventh consecutive month of below average rainfall across headworks storages.
Inconsistent rainfall and warmer daytime temperatures restricted the generation of inflow to storages during August with 3,900 ML (excluding Taylors Lake) being recorded. August inflow was equivalent to 8.4% of the historic average. Year to date inflow increased to 8,690 ML, or 10.8% of the historic average for the same time period.
Between 7 August and 4 September 2024, the total volume in storage increased from 312,460 ML (55.8%) to 316,290 ML (56.5%), representing a total increase of 3,830 ML (0.7% of total operating capacity).
Opportunities to harvest unregulated catchment flow are being actively pursued with harvesting occurring from Scrubby Creek, Mackenzie River and Mt William Creek providing some small inflow to Taylors Lake.
A transfer from Moora Moora Reservoir to Taylors Lake commenced during August, supported by natural catchment flows in the upper Glenelg. Initial planning indicates a total of 1,400 ML may be transferred to Taylors Lake, subject to favourable catchment conditions continuing. This volume may be extended or truncated in response to changing catchment conditions.
The total monthly evaporation from all storages was greater than rainfall received to storages for August, which represents the first month since May to record positive net evaporation (evaporation on storage exceeding rainfall). Approximately 1.17 GL of net evaporated was calculated for headworks storages during the month.
Environmental deliveries from the headworks system continued through August with 835 ML of environmental releases to the Wimmera River and Mackenzie River / Burnt Creek systems. Passing flow releases commenced from Rocklands Reservoir to the Glenelg River with 155 ML delivered, in addition to 60 ML released as Glenelg River Compensation Flow.
A total of 185 ML was delivered to Wimmera-Mallee Pipeline connected recreation lakes during August.
Consumptive entitlement holders collectively used approximately 840 ML during August to supply their respective urban and rural demands.
September 2024 Operations and Climate Outlook
The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is currently neutral. However, due to a number of models suggesting a La Niña might form by October 2024, the ENSO outlook is currently listed as La Niña watch.
While climate indicators such as La Niña can provide broad indications of the expected climate, the long-range forecast provides better guidance for local rainfall and temperature patterns. Long-range forecasts for the Wimmera-Glenelg headworks area have been relatively optimistic, with early indications of average to above average rainfall conditions developing as the year progresses. These predictions 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 September to December period remain positive (average to slightly above average rainfall), current catchment conditions are likely to drive operational decision making over the next month, with the likelihood of transfers and contingency planning expected to increase.
The Indian Ocean Dipole (IOD) is currently neutral and is predicted to remain neutral until at least early spring.
The Southern Annular Mode (SAM) is currently positive but is expected to become neutral in early September. A neutral SAM has no strong influence on rainfall or temperature patterns.
Rainfall outlooks indicate a 50% chance of rainfall exceeding the median during September and a 60-65% chance of rainfall exceeding the median between September and November. Daytime temperatures are expected to remain elevated, with the BOM projecting a >70% chance of exceeding median max temperatures for the remainder of September. Unless average to above average rainfall conditions develop throughout September, 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 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 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.