Review of October 2024 Operations
Below average rainfall totals were recorded across Grampians storages during October, with all storages recording rainfall below the monthly average. The highest rainfall total received was at Lake Bellfield with 52 mm (64% of the historic monthly average), followed by Mt Cole with 42.8 mm (88% of the historic monthly average). The lowest monthly rainfall total was received at Lake Lonsdale, with 23.0 mm (48% of historic monthly average). Below average rainfall received during October represented the ninth consecutive month of below average rainfall across headworks storages.
Modest rainfall received in the middle of October generated some minor inflow to storages with 5,467 ML (excluding Taylors Lake) being recorded. October inflow was equivalent to 19.5% of the historic average. Year to date inflow increased to 22,390 ML, or 14.9% of the historic average for the same time period.
Between 2 October and 6 November, the total volume in storage decreased from 316,540 ML (56.5%) to 308,220 ML (55.0%) representing a total decrease of 8,320 ML (1.5% of total operating capacity).
There were limited opportunities to harvest unregulated catchment flow during October with only minor flows recorded on the upper Glenelg River, MacKenzie River and Fyans Creek. Even with some natural catchment flow observed in the system during October, increasing daytime temperature leading to higher daily evaporation ensured any flows were short lived. Any future harvesting from unregulated catchments will be reliant on catchments receiving sufficient rainfall to recommence flow.
A transfer from Moora Moora Reservoir to Taylors Lake was finalised in mid-October, after being extended due to the additional harvesting of natural catchment flow in the upper Glenelg River into Moora Moora Reservoir. This transfer provided Horsham Rural City Council the opportunity to purchase 1,000 ML of recreation water from GWMWater, which was supplied to Green Lake (Horsham) as part of this transfer. A transfer of approximately 1,660 ML from Lake Bellfield to Lake Fyans was completed during October, as previously flagged in the Annual Operating Plan. An additional transfer is likely to take place in early Autumn to secure adequate supply for consumptive users in addition to supporting recreational uses at the lake.
The total monthly evaporation from all storages was greater than rainfall received to storages for October, with approximately 7.31 GL of net evaporation calculated from headworks storages during the month. This was a significant increase on the 3.56 GL of net evaporation calculated for September and is expected to increase further heading into the summer period.
Environmental deliveries from the headworks system continued through October, with 400 ML of environmental water released to the Mackenzie River / Burnt Creek systems. Releases from Lake Lonsdale commenced in early October, with a total of 1,680 ML released during the assessment period. Smaller volumes were also released from Taylors Lake (345 ML) and Lake Fyans (140 ML).
A total of 230 ML was delivered to Wimmera-Mallee Pipeline connected recreation lakes during October.
Consumptive entitlement holders collectively used approximately 2,170 ML during October, (which includes 690 ML delivered to Green Lake, Horsham) to supply respective urban and rural demands.
November 2024 Operations and Climate Outlook
The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is likely to remain neutral through to February 2025.
Long-range forecasts for the Wimmera-Glenelg headworks area continue to be relatively optimistic, with early indications of average to 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 November to February period remain positive (average to slightly above average rainfall), 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) is currently neutral and is predicted to remain neutral for the remainder of the calendar year.
The Southern Annular Mode (SAM) is forecast to remain relatively neutral over the coming fortnight, with the potential become positive over the November to December period.
Rainfall outlooks indicate a 60-65% chance of rainfall exceeding the median during November and a 60-65% chance of rainfall exceeding the median between December and February. Daytime temperatures are expected to remain elevated, with the BOM projecting a >70% chance of exceeding median max temperatures for the remainder of November. Unless average to above average rainfall conditions develop throughout November, 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 November 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.