Review of September 2021 Operations
Rainfall recorded during September was below average at all reservoirs, with the exception of Taylors Lake. The highest monthly rainfall total was recorded at Lake Bellfield (54.4 mm or 50% of the historic average), with the lowest monthly total being 30.4mm, recorded at both Mt Cole (44% of the historic average) and Lake Toolondo (57% of the historic average).
With generally below average rainfall and increasing daytime temperatures during September, catchment runoff and inflow to storages slowed over the month. A total of 5,840 ML of inflow was recorded to Grampians reservoirs (excluding Taylors Lake) during September. This equates to approximately 14% of the historic average inflow for the month.
In the five weeks between 1 September and 6 October, the total volume in storage increased from 236,070 ML (42.1%) to 243,410ML (43.4 %) representing a total increase of 7,340 ML. This increase was well below the 25,900 ML observed in August.
Calculated net evaporation from reservoirs was approximately 2,170 ML for the month; with evaporation from storages exceeding rainfall for the first month this water year. (Net evaporation considers rainfall on the reservoir as an evaporation offset).
There was 65 ML of environmental water released to waterways in the Wimmera catchment during the month. In addition, September saw 1,080 ML of environmental entitlement passing flow requirements released from Rocklands Reservoir to the Glenelg River.
Wimmera-Mallee Pipeline supplied recreation lakes received 284 ML during the month of September, and 24 ML was delivered to Wimmera-Mallee Pipeline connected wetlands during the month.
Consumptive entitlement holders collectively used 823 ML during September to supply their respective urban and rural demands.
Planned October 2021 Operations
The Bureau of Meteorology (BoM) in its most recent Climate Driver Update advised that the El Niño Southern Oscillation (ENSO) remains neutral, although the BoM have currently issued a La Niña WATCH. The BoM advise that historically when a La Niña WATCH has been issued, La Niña conditions are reached 50% of the time.
The Indian Ocean Dipole (IOD) has weakened over the month, and it is expected that the negative IOD conditions will soon end. Although IOD conditions are expected to ease out of a negative state, the influence from sea surface temperatures in the Indian Ocean are still expected to influence rainfall across Australia in the coming months.
Below average rainfall across the Grampians catchments during September, along with an increase in daytime temperatures resulted in natural catchment flow and inflow to storages declining. With daytime temperatures and evaporation anticipated to increase during October, significant rainfall events over the next month will be needed to produce good inflow to reservoirs, and runoff into waterways. October has historically been considered as the end of the primary inflow period for storages across the Wimmera-Glenelg headworks system.
The Storage Manager’s operational focus for October will continue to be the monitoring of catchment conditions and flows, and coordinating water harvesting in response to rainfall events where possible. Water harvesting is undertaken in accordance with Storage Management Rules, the Storage Manager’s Annual Operating Plan, and takes into consideration factors such as water quality and environmental objectives.
A 2,500 ML recreation delivery to Green Lake (Horsham) commenced in early October. The Storage Manager intends to use a combination of natural catchment flows, supplemented by a small transfer from Rocklands Reservoir, to complete this delivery by early November.
A number of reservoirs are projected to remain at suitable levels through October 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, including any restrictions in place to comply with the Victorian Government’s COVIDSafe Settings.
Blue-Green Algae above the levels safe for recreational contact have been recorded at 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.
Note: 1 ML = 1 megalitre = 1 million litres