Review of May 2026 Operations
Above average monthly rainfall totals were recorded across Grampians storages during May. The highest rainfall total received was at Lake Wartook with 106 mm (118% of the historic monthly average), followed by 102.6 mm recorded at Lake Bellfield (108% of the historic monthly average). The lowest monthly rainfall total was received at Taylors Lake with 59.4 mm (141% of historic monthly average).
Dry catchment conditions to start the month, followed by frequent rainfall throughout May started to generate inflow to headworks storages. A total of 1,937 ML of inflow was recorded to storages during May, representing 32.8% of the historic monthly average. Inflow recorded in the July to May period (59,397 ML) represent 32.5% of the historic average for the same period.
Between 6th May 2026 and 3rd June 2026, the total volume in storage increased from 185,540 ML (33.1%) to 189,110 ML (33.7%) representing a total increase of 3,570 ML (0.6% of total operating capacity). Lake Wartook continues to be of particular interest, currently holding 9,990 ML, a 560 ML (2%) increase since the May resource assessment.
Net evaporation from storages totalled -2.39 GL during May, with monthly rainfall on storages exceeding evaporation for the first month since August 2025. If average rainfall persists over winter, net evaporation is expected to remain negative providing additional resource.
Regulated environmental deliveries reduced in May with the following river systems receiving water:
- 175 ML delivered to the Wimmera River from Taylors Lake,
- 80 ML delivered to the upper Mt William Creek from Lake Fyans, and
- 59 ML delivered to the MacKenzie River from Lake Wartook and Moora Moora Channel.
Consumptive entitlement holders collectively used approximately 1,038 ML during the month to supply respective urban and rural demands.
June 2026 Operations and Climate Outlook
Above average rainfall in May has allowed for harvesting from unregulated catchments in the upper headworks, primarily from Scrubby Creek in the upper Glenelg Catchment and MacKenzie River in the Wimmera River Catchment. These flows are currently being directed to Taylors Lake. Catchment performance will continue to be monitored throughout June for opportunistic harvesting opportunities. Water harvesting into Taylors Lake has the potential to reduce the volume and duration of any Rocklands Transfer required during the August – September period.
The Bureau of Meteorology (BoM) report climatic conditions reflect a neutral El Niño–Southern Oscillation (ENSO) state. International and BoM models forecast a continued warming of the tropical Pacific, resulting in El Niño thresholds being met in early winter.
The Indian Ocean Dipole (IOD) is currently neutral, with models indicating it is likely to remain neutral until early winter. Although models are indicating a large range of outcomes for the IOD throughout the winter-spring period, most are suggesting a shift towards a positive threshold being reached.
Positive IOD and ENSO states have historically been observed during drier climatic periods across Australia; however their impact is not certain.
Short-term forecasts for the Wimmera-Glenelg headworks area suggest a below average chance (<40%) of receiving median rainfall conditions across the Grampians during June.
Medium-term outlooks indicate a less than 30% chance of rainfall exceeding the median for the June to August period. Daytime temperatures are expected to remain elevated, with the BOM projecting a >80% chance of exceeding median max temperatures for the June to August period.
With short- and medium-term outlooks suggesting dry climatic conditions are likely to persist across autumn and into winter. There is an increased likelihood of a delayed start to system inflows over the traditional inflow period (June-October).
Reservoirs are projected to hold suitable levels during June 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 Lake Lonsdale. Information on all current algae warnings, as well as general information on Blue-Green Algae, is available from the GWMWater Algae Warnings webpage.