Picture: Lake Fyans

Review of December 2020 Operations

December saw below average rainfall recorded at all but one reservoir, with rainfall ranging from 51% to 102% of the monthly average. The highest rainfall total for the month was recorded at Mt Cole Reservoir with 45.8 mm (102% of average), followed by 37.8 mm recorded at Lake Bellfield (68% of average). The lowest rainfall total recorded for the month was Taylors Lake with 15.8 mm (57% of average).

With below average rainfall across much of the Grampians catchment area, inflow to reservoirs was approximately 7% of the December average. Total 2020-21 year to date inflow increased by 320 ML to 63,080 ML (58,790 ML excluding Taylors Lake), equivalent to 35% of the historic average inflow for the same period (166,300 ML excluding Taylors Lake).

During December, the total volume in storage decreased from 219,680 (39.2%) to 204,510 ML (36.6%), representing a total decrease of 15,170 ML.

Calculated net evaporation from reservoirs was approximately 9,570 ML for the month; with evaporation from reservoirs exceeding rainfall. Lower rainfall, warmer temperatures and windy weather increases evaporation losses. (Net evaporation considers rainfall on the reservoir as an evaporation offset)

A total of 701 ML of passing flow requirements were released from Rocklands Reservoir to the Glenelg River and from Lake Lonsdale to Mt William Creek during December. A further 1,780 ML was released from environmental water entitlements to the lower MacKenzie River and Glenelg River below Rocklands Reservoir. 

Wimmera-Mallee Pipeline supplied recreation lakes received 203 ML during the month of December, and 2.8 ML was delivered to Wimmera-Mallee Pipeline connected wetlands.

Consumptive entitlement holders collectively used 1,744 ML during December to supply their respective urban and rural demands.

Planned January 2021 Operations

The Bureau of Meteorology (BOM) in its most recent Climate Driver Update advised that the current La Niña event has likely to have peaked in terms of sea surface temperature. However, associated impacts such as increased rainfall are expected to persist across eastern states throughout the summer months. Rainfall may continue to be sporadic, and while some areas may receive high rainfall, others may not.

The Indian Ocean Dipole (IOD) index remains in a neutral phase, and is unlikely to significantly influence Australian climate in the coming months.

Grampians catchments have experienced further drying over the past month. As a result, conditions at the beginning of January suggest that large rainfall events are necessary to prime catchments before any significant runoff or inflows to reservoirs would be observed.

With catchments now relatively dry, the Storage Manager’s operational focus for January will be the delivery of entitlement holders’ water requirements. Should large rainfall events eventuate, the Storage Manager will transition to planning and coordination of water harvesting activities.

A number of reservoirs are projected to remain at suitable levels through 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 current restrictions and recreation activities permitted at each reservoir.

Blue-Green Algae above the levels safe for recreational contact have been recorded at Toolondo Reservoir and 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.


Note: 1 ML = 1 megalitre = 1 million litres