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 4.3 Wimmera system
Scenario planning
Table 4.3.2 outlines the potential environmental watering and expected water use under a range of planning scenarios.
If dry conditions continue in the Wimmera system, the type of environmental watering actions that can be delivered
to individual reaches will likely be influenced by water availability in the storages directly above each target reach. This is especially true for the MacKenzie River, Burnt Creek and Mount William Creek, which rely on water from Lake Wartook and Lake Lonsdale. Management in the Wimmera system necessitates for periods of cease-to-flow in the rivers and creeks, with additional or longer periods without flow in drier conditions, and planned continuous flows during wetter conditions when there is additional water available through rainfall runoff and allocations made to the environment.
The highest-priority (tier 1a) watering actions are to deliver summer/autumn low flow, summer/autumn freshes, winter/ spring low flow in MacKenzie River reaches 2 and 3, upper Burnt Creek, Wimmera River reach 4 and to provide top- ups to Ranch Billabong. These flows aim to improve water quality and provide habitat for native fish, platypus, crayfish and waterbugs.
Under extreme dry, very dry and dry scenarios, flows in tributaries will mostly target reach 2 of the MacKenzie River and upper Burnt Creek to protect habitat for native fish, platypus and instream vegetation. While these flows will not target reach 3 of the MacKenzie River, flows delivered in reach 2 may pass into the upper sections of reach 3 of the MacKenzie River and top up small refuge pools for native fish and waterbugs throughout 2020–21. During winter
and spring, flows may be delivered through to reach 3 to support native vegetation and allow some fish and platypus to move between the MacKenzie River and Wimmera River. Small flows are also planned to be delivered to maintain refuge habitats in upper Mount William Creek under dry conditions. Environmental flows will not directly target lower Burnt Creek under dry conditions but flows from upper Burnt Creek will likely pass into the reach. Under drought and very dry conditions, water quality and a lack of water availability in Lake Lonsdale is likely to prevent the targeted delivery of water for the environment to lower Mount William Creek, although it may receive some water en route to the Wimmera River or passing flows if available. Under average or wet scenarios environmental water may be used to deliver bankfull flows to Bungalally Creek and lower Burnt Creek and to partially fill Dock Lake.
If more water is available, the next highest priority — tier 1b watering actions — will be to increase the duration of low flow to reduce the number of cease-to-flows and increase the magnitude and/or frequency of freshes in reach 4 of the Wimmera River, reach 3 of MacKenzie River, upper Burnt Creek and lower Mount William Creek.
Reserving water for carryover into the 2021–22 water year will be a priority under all scenarios, to ensure sufficient water to deliver the highest-priority flows during summer and autumn 2022. The volume carried over against the Wimmera-Glenelg environmental entitlement will be decided in consultation with the Wimmera and Glenelg Hopkins CMAs during the year, and it will be based on use during 2020–21, environmental conditions and seasonal outlooks for 2021–22.
With drier conditions expected in the western region, waterway managers are exploring contingency measures (such as using the Wimmera-Mallee pipeline network) to pipe water for the environment directly into critical refuges sites in some river systems.
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