Picture: Lake Fyans

Review of May 2024 Operations

Below average rainfall totals were recorded across Grampians storages during May, with monthly rainfall totals between 12.6 mm and 26.6 mm recorded. These monthly rainfall totals were well below the historic average for each storage. The highest monthly rainfall total received was recorded at Moora Moora Reservoir with 26.6 mm (27% of the historic monthly average), followed by Lake Bellfield which recorded 20.6 mm (21% of the historic monthly average). The lowest monthly rainfall total was recorded at Lake Toolondo, with 12.6 mm (26% of historic monthly average). 

Persistent dry catchment conditions limited inflow during May to 58 ML. Increasing the year-to-date inflow to 52,245 ML or 28.6% of the historic average to the same time of year (182,500 ML).

Between 1 May and 5 June, the total volume in storage decreased from 317,300 (56.6%) to 308,950 (55.1%), representing a total decrease of 8,350ML (1.5% of total operating capacity).

The total monthly calculated net evaporation from all storages reduced in May, as mean daily temperatures continued to drop. Approximately 2.4GL of net evaporated was calculated for headworks storages during the month, with evaporation from storages continuing to exceeding rainfall. (Net evaporation considers rainfall on the reservoir as an evaporation offset).

Environmental demand from the headworks system continued to provide a baseflow to key river systems in May, in preparation for expected natural catchment flows throughout the winter months. A total of 1,625 ML of regulated environmental releases were made from Rocklands Reservoir to the Glenelg River. In addition to 1,700 ML of regulated releases to the MacKenzie River from Lake Wartook and Mt William Creek from Lake Lonsdale.

A total of 25.2 ML was supplied to wetlands from the Wimmera-Mallee Pipeline during the month, and 97 ML was delivered to Wimmera-Mallee Pipeline connected recreation lakes.

Consumptive entitlement holders collectively used approximately 1,035 ML during May to supply their respective urban and rural demands.

June 2024 Operations and Climate Outlook

The Bureau of Meteorology (BoM) indicate that the El Niño-Southern Oscillation (ENSO) is currently neutral. However, due to some early signs that a La Niña might form, the ENSO outlook is currently listed as La Niña watch.

The majority of international models are predicting a neutral outlook until at least August, at which point three out of the seven models indicate La Niña conditions will be met.

The Indian Ocean Dipole (IOD) is currently above the positive threshold but is considered to be neutral. Sustained values of the IOD index above the threshold are required for eight consecutive weeks for an IOD event to be declared. The most recent four weeks have seen the IOD index within the neutral threshold. Predictability of the IOD is low at this time of year.

The Southern Annular Mode (SAM) is currently negative, as of 25 May. Forecasts indicate SAM is likely to remain negative for the first two weeks of June.

Rainfall outlooks indicate a 30-40% chance of rainfall exceeding the median during June. Daytime temperatures are expected to remain elevated, with the BOM projecting a >80% chance of exceeding median max temperatures for the remainder of June. Given the expected dry conditions, the operational focus for June will continue to be supplying entitlement holders.

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 Green Lake (Horsham) and Lake Toolondo. Information on all current algae warnings, as well as general information on Blue-Green Algae, is available from the GWMWater Algae Warnings webpage.