Review of September 2020 Operations
September was the first month this water year to record average to above average monthly rainfall across Grampians catchments, with rainfall ranging from 92% to 122% of the monthly average. The highest rainfall total for the month was recorded at Lake Wartook with 113 mm (104% of average), followed by 110.8 mm recorded at Moora Moora Reservoir (102% of average). The lowest rainfall total recorded for the month was Taylors Lake with 37.2 mm (92% of average).
Despite average to above average rainfall being recorded across Grampians catchments, inflows to reservoirs remained well below the September average. This can be attributed to drier than normal catchments resulting from below average rainfall over the preceding months. A total of 14,780 ML of inflow was received to reservoirs during the month (33% of September average). This increased the 2020-21 year to date inflow total to 30,920 ML (28,690 ML, excluding Taylors Lake), equivalent to 23% of the historic average inflow for the same period (122,300 ML).
During September, the total volume in storage increased from 186,980 ML (33.4%) 199,440 ML (35.6%), representing a total increase of 9,460 ML.
Calculated net evaporation from reservoirs was approximately -150 ML for the month; with rainfall on reservoirs exceeding evaporation (Net evaporation considers rainfall on the reservoir as an evaporation offset).
215 ML of passing flow requirements were released from Rocklands Reservoir to the Glenelg River during September. No other environmental water releases were made during the month.
Wimmera-Mallee Pipeline supplied recreation lakes received 312 ML during the month of September, and 39 ML was delivered to Wimmera-Mallee Pipeline connected wetlands.
Consumptive entitlement holders collectively used 1,090 ML during September to supply their respective urban and rural demands.
The transfer of water from Rocklands Reservoir to Taylors Lake continued through September, with a total volume of 3,860 ML released from Rocklands Reservoir over the month.
Planned October 2020 Operations
The Bureau of Meteorology (BOM) in its most recent El Niño-Southern Oscillation (ENSO) outlook advised that a La Niña event has now established. La Niña events during spring are typically accompanied by above average rainfall for northern, eastern and southern parts of Australia. Climate models surveyed by BoM suggest that La Niña conditions will likely continue well into summer 2020-21, but are unlikely to be as strong as the 2010-2011 event.
The latest values of the Indian Ocean Dipole (IOD) index have again entered a negative dipole state. Five of the six climate models surveyed by BoM indicate the IOD will remain negative throughout October, and three models indicate negative IOD values will persist into November.
Both La Niña and negative IOD events typically result in above average rainfall across eastern Australia. When both occur together they can reinforce each other and result in much wetter than average conditions. Even under such conditions, rainfall can be sporadic and while some areas may receive high rainfall, others may not.
Catchment conditions at the beginning of October remained in good a position to produce further inflow to storages, however continued rainfall events are necessary for meaningful inflows volumes to be observed. The extent of inflow received to storages during October will depend on the frequency and magnitude of rainfall events during the month. Where possible, natural inflows to waterways downstream of storages will be used to service entitlement holder demands, in order to preserve water in storage.
The transfer of water from Rocklands Reservoir to Taylors Lake will continue until mid-October. At the conclusion of the transfer, sufficient volume will be positioned in Taylors Lake to meet the anticipated environmental water requirements for the Wimmera River during 2020-21, and to provide adequate back-up supply for the Wimmera-Mallee Pipeline. There will also be sufficient ‘airspace’ remaining within Taylors Lake to capture further catchment flows during October.
Although many reservoirs are anticipated to remain at suitable levels through October 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's Reservoir 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