Review of July 2020 Operations
Rainfall totals during July were well below average at all Grampians reservoirs, with recorded rainfall ranging from 28% to 53% of the monthly average. The highest rainfall total for the month was at Lake Wartook with 44.8 mm (32% of average), followed by 40.6 mm recorded at Lake Bellfield (29% of average). The lowest rainfall total recorded for the month was Taylors Lake with 11.8 mm (28% of average).
July rainfall resulted in around 5,940 ML of inflow to reservoirs, equivalent to approximately 16% of the historic monthly average. As July represents the first month of inflow for the 2020/21 season, the total year to date inflow is 5,940 ML (5,530 ML, excluding Taylors Lake).
During July, the total volume in storage increased from 172,380 ML (30.8%) to 177,290 ML (31.7%), representing a total increase of 4,910 ML.
Calculated net evaporation from reservoirs was approximately -300 ML for the month; with rainfall on reservoirs only slightly exceeding evaporation (Net evaporation considers rainfall on the reservoir as an evaporation offset). Net evaporation for July 2019 was -1,290 ML, further illustrating the effect of below average rainfall in July 2020.
A total of 620 ML of passing flow obligations were released to the Glenelg River from Rocklands Reservoir during the month. The Wimmera and Glenelg Rivers Environmental Entitlement requires the Storage Manager to make passing flow releases in accordance with the entitlement rules.
Wimmera-Mallee Pipeline supplied recreation lakes received 41 ML throughout the month of July, while no supply was made to Wimmera-Mallee Pipeline connected wetlands during the month.
Consumptive entitlement holders collectively used 910 ML during July to supply their respective urban and rural demands.
Planned August 2020 Operations
The Bureau of Meteorology (BOM) i
in their most recent El Niño-Southern Oscillation (ENSO) outlook have advised the El Niño–Southern Oscillation (ENSO) remains at La Niña WATCH. Cooling in the tropical Pacific Ocean has continued, and the majority of climate models indicate this cooling trend may be close to La Niña thresholds by early spring. La Niña events during spring are typically accompanied by above average rainfall for northern, eastern and southern parts of Australia.
Despite recent cooling in the eastern Indian Ocean, some climate models still indicate a possibility that a negative Indian Ocean Dipole (IOD) may develop during late winter or early spring. Negative IOD events typically coincide with increased winter-spring rainfall for southern parts of Australia.
The Storage Manager’s operational focus for August will be water harvesting activities in response to rainfall events, and a planned water transfer.
Catchment conditions at the beginning of August remain in a position to produce further runoff, however good rainfall events will be necessary for an increase in runoff to be observed. The extent of runoff during August will depend on the frequency and magnitude of rainfall events during the month. Where possible, natural inflows to waterways downstream of reservoirs will be used to service entitlement holder demands, in order to preserve water in storage.
A planned water transfer of up to 6,500 ML from Rocklands Reservoir to Taylors Lake is scheduled to commence on 13 August 2020. The Storage Manager’s planning has determined that a transfer is necessary to secure sufficient water volumes in Taylors Lake to supply the planned environmental watering requirements for the Wimmera River, and to provide a backup for the Wimmera-Mallee Pipeline. Transfer plans will be regularly reviewed, and may be modified in in response to observed rainfall and reservoir inflows
Although many reservoirs are anticipated to remain at suitable levels through August for a range of recreational activities, some restrictions may remain in place to comply with the Victorian Government’s Coronavirus response. Please refer to GWMWater'sReservoir information page for further information on current restrictions and recreation activities permitted at each reservoir.
While Blue-Green Algae blooms are generally less common during the cooler months, it is still possible for these to occur. Recent Blue-Green Algae warnings for reservoirs can be found on the Storage Manager News Feed and all current warnings are available from the GWMWater Algae Warnings webpage.
Note: 1 ML = 1 megalitre = 1 million litres