Picture: Lake Fyans

Review of October 2022 Operations

Rainfall recorded during October was well above average at all Wimmera-Glenelg System reservoirs. The highest rainfall total for the month of 316.6 mm was recorded at Lake Bellfield (388% of the October average). Also noteworthy were the rainfall totals at Lake Fyans (190 mm) and Lake Lonsdale (200.9 mm) which were 394% and 417% of their October averages respectively. The lowest rainfall total was recorded at Lake Toolondo (118.4 mm), 255% of the October average.

Consistent rainfall events during the month resulted in a significant increase in runoff and reservoir inflows relative to the previous month, September. A total inflow of 130,690 ML was recorded to Grampians reservoirs, excluding inflows to Taylors Lake (135,110 ML, including Taylors Lake). This inflow equates to approximately 465% of the historic average inflow for the month. The total year to date inflow for 2022-23 season is tracking at about 142% of the historic average inflow for the July to October period (213,670 ML excluding Taylors Lake, to allow statistical comparison).

Between 5 October 2022 and 2 November 2022, the total volume in storage increased from 273,050 ML (48.8%) to 393,660 ML (70.3%), representing a total increase of 127,460 ML (21.5%).

Operational releases were made from Taylors Lake throughout October to manage levels in advance of inflows from forecast rain events and keep the reservoir close to its target curve. Where possible, water has been routed through the reservoir to assist with managing water quality. Releases from Taylors Lake were impacted by rising levels on the Wimmera River and were paused at times during the month. The Wimmera Inlet Channel has been operated for flood conditions since early October. This means all flows from the Lower Mt William Creek, spills and releases from Lake Lonsdale, and any flood flows from the Wimmera River that over-topped the gates at Huddleston’s Weir, have been directed back to the Wimmera River via the Big Pipe, Middle Creek and Mt William escapes.

Moora Moora Reservoir inlet has remained closed throughout October. Despite this, inflows from its own catchment have caused it to rise above its full supply level and spill back to the Glenelg River for the entire month. The reservoir and spillway are being managed to bring down the storage level, but it is still responding rapidly and rising in response to rainfall events.

Harvesting to Lake Fyans from Fyans Creek ceased in September when the reservoir reached its temporary target level, which is in place while dam embankment upgrades are undertaken. The reservoir has seen a moderate rise in reservoir levels due to natural inflows into the Fyans Inlet Channel downstream of the offtake from Fyans Creek. The reservoir level does not pose any risk to the embankment work but controlled releases have been made to bring the reservoir back down to the target level.

Lake Bellfield received increased inflows in response to significant rainfall events over the month. The Wannon Diversion was closed early in mid-October to reduce the volume flowing to Lake Bellfield. The majority of inflows were coming from the Fyans Creek catchment upstream of the Bellfield wall. The reservoir reached its full supply level in the final week of October. As of 8 November, the reservoir was still spilling and is estimated to have spilled 6,600 ML to this date.

Lake Lonsdale received substantial inflow over the month and rose from 14,600 ML at the beginning of the month to its full supply volume (65,480 ML) before spilling at the beginning of November. Releases from the reservoir up to 330 ML/day began in mid-October and have been maintained since then. The reservoir was still spilling as of 8 November and the total volume released and spilled to that date is estimated to be 10,250 ML.

Lake Wartook saw a sharp increase in its storage level in early October in response to a significant rainfall event. The response was so rapid that a reservoir spill was unavoidable. The total volume spilled, not including controlled releases, in that single event was estimated to be approximately 1,000 ML. The reservoir received approximately 48 mm of rain from another rainfall event that was not forecast which lead to a second unavoidable spill event for the reservoir later in October. The estimated spill volume from the second event was also about 1,000 ML. In addition to these spills approximately 8,600 ML of water has been released from the storage over the month of October. Releases were made to manage the reservoir between the full supply level (equivalent to 29,300 ML) and the spillway (equivalent to approximately 31,000 ML), and to mitigate potential spill impacts on roads, properties and other infrastructure downstream of Lake Wartook.

Throughout October, flows from Burnt Creek have been directed to Green Lake. The volume directed into Green Lake has varied in response to catchment conditions and the flows along Burnt Creek. Green Lake filled and began to spill into Dock lake in early October. An estimated 3,300 ML has flowed into Dock lake which means the reservoir is approximately 75% full. Excess flows from the Lake Wartook spills and pick-up downstream of Lake Wartook have also were directed to Burnt Creek North, Bungalally Creek and Mackenzie River throughout the month.

The calculated net evaporation from reservoirs was approximately -8,580 ML during the month of October, with rain falling on reservoirs exceeding evaporation. (Net evaporation considers rainfall on the reservoir as an evaporation offset).

No passing flow obligations or regulated environmental deliveries were released to waterways in the Wimmera Catchment or the Glenelg River during October. Passing flow obligations have accumulated at Rocklands Reservoir but all passing flow obligations for Lake Lonsdale have spilled.

There was no delivery to Wimmera-Mallee Pipeline supplied recreation lakes during October. A total of 16.9 ML was delivered to Wimmera-Mallee Pipeline connected wetlands during the month.

Consumptive entitlement holders collectively used about 650 ML during October to supply their respective urban and rural demands.

November 2022 Operations

The Bureau of Meteorology (BoM) Climate Driver Update released on 25 October 2022 advised that a negative Indian Ocean Dipole (IOD) event is continuing, and that negative IOD conditions are likely to be maintained until November and return to neutral in January 2023. A negative IOD tends to increase the chance of above average winter–spring rainfall for much of Australia.

The El Niño Southern Oscillation (ENSO) outlook states that a La Niña event is now underway. BoM have stated that, “[m]odels indicate a return to ENSO-neutral conditions (neither La Niña nor El Niño) early in 2023”. La Niña events typically increase the chance of above average winter–spring rainfall across much of northern and eastern Australia.

Catchments remain saturated and responsive to small rainfall events. The Storage Manager expects that inflows will continue during November, though harvesting opportunities are now limited with most reservoirs at or above the maximum operating or full supply levels. Operational releases and spills are likely to continue throughout the month but ultimately depend upon inflows received.

Operations at Taylors Lake will aim to keep the reservoir below its full supply level and maintain some airspace when there are rainfall events forecast. Where appropriate, water may be routed through Taylors Lake to manage inflows and accumulation of salts and nutrients in the reservoir. Water levels in Wimmera River will be closely monitored and releases from Taylors Lake will cease if river flow is too high. Flows entering the Wimmera Inlet Channel from the Wimmera River or the Mt. William Creek will likely be directed back to the Wimmera River, rather than harvested to Taylors Lake throughout November.

Flows in excess of waterway needs will continue to be directed into Green Lake and Dock Lake, as allowed for by the Storage Management Rules. The Storage Manager will liaise with the Waterway Manager (Wimmera CMA) in relation to the sharing of flows and the target volume for Dock Lake.

Water levels in Lake Fyans will continue to be closely monitored. Rainfall on the storage in addition to natural inflow and drainage into the inlet channel means there is the potential for the storage to rise further. Releases will continue to be made to the Mt. William Creek to bring the Storage down to its target level.

Lake Wartook releases will be managed with the intent to avoid the reservoir spilling but keep the reservoir at its full supply level. While there is some space between the full supply level and the spillway level, operations over October have demonstrated that the catchment response to large rainfall events may mean reservoir spills could become unavoidable. If a spill is likely, the Storage Manager will advise affected stakeholders and GWMWater issue information via media.

The spillway at Moora Moora Reservoir will continue to be operated to attempt to bring the reservoir back down to its full supply level.

Releases from Lake Bellfield may be made in advance of forecast rainfall events, But, as the reservoir’s full supply level is the same as the spillway level, releases may be avoided. Releases and spills from Lake Bellfield are currently directed to Lake Lonsdale. Where possible, there is a preference to create airspace in Lake Lonsdale, which can have downstream impacts when it spills significant volumes. Lake Lonsdale will continue to spill as long as inflow from rainfall events exceeds evaporation and release volumes. The maximum release volume from Lake Lonsdale is 600 ML/day which is significantly smaller than daily inflow volumes received during October. Therefore, releases are unable to rapidly create airspace in Lake Lonsdale to absorb any further inflows. Hence controlled releases will be maintained over several weeks to bring the reservoir back down to its maximum operating level (equivalent of 53,300 ML).

The Toolondo transfer trigger was exceeded in September and further significant inflow to Rocklands in October meant the transfer rules were fully met. Favourable wet conditions and forecasts of above average rainfall through to January 2023 meant the Storage Manager determined it was preferable to commence a transfer now, rather than wait until after summer. The Storage Manager will begin a transfer from Rocklands Reservoir to Toolondo Reservoir in early November. The transfer is expected to take a number of months. The progress of transfers will be closely monitored and the flow plan adjusted in response to catchment conditions and environmental flows.

All 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.

Most reservoirs are projected to hold suitable levels during 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.

Blue-Green Algae above the levels safe for recreational contact has been recorded at Toolondo Reservoir. Information on all current algae warnings as well as general information on Blue-Green Algae is available from the GWMWater Algae Warnings webpage.