Page 96 - VEWH Seasonal Watering Plan 2020-21
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 3.2 Yarra system
Scenario planning
Table 3.2.2 outlines the potential environmental watering and expected water use under a range of planning scenarios.
Environmental flow planning in the Yarra River primarily focuses on providing sufficient low flow throughout the year to maintain habitat for aquatic life and providing
high flows at critical times to support the migration and breeding requirements of native fish. The extent to which the required flows are met by natural tributary inflows varies between dry, average and wet scenarios. Water for the environment is used to fill the main shortfalls under each scenario, where possible.
The highest priorities for watering in the Yarra River are summer/autumn low flows and freshes, an autumn high flow, a spring high flow and winter/spring low flows and freshes. Summer/autumn low flows are not identified as a planned watering action under a wet scenario, as it is anticipated they would be met by natural flows.
The summer/autumn low flows and freshes help maintain water quality and improve aquatic habitats. A higher-than- normal carryover volume from 2019–20 will potentially allow an autumn high flow and a spring high flow to be delivered in 2020–21 under all scenarios. The autumn high flow is a priority because it was not delivered in 2017–18 or 2018–19 and is needed in most years to support Australian grayling breeding. Spring high flows trigger Australian grayling migration back up the system and scour sediments in the mid-reaches, to improve spawning habitat for Macquarie perch.
The potential watering actions for the Plenty River are winter/spring low flows and freshes. This is the first time that environmental flows are planned to be delivered
in the Plenty River. Flows are planned as a trial to help waterway managers understand the operational and safety requirements for the system and to better understand the river’s response to water for the environment.
Watering at Yering Backswamp and Annulus Billabong is considered a high priority under all scenarios in 2020–21. There are numerous billabongs throughout the Yarra catchment that are drier than natural, due to river regulation and modifications to natural flow paths. Melbourne Water
is currently developing a landscape-scale approach
to watering floodplain billabongs that will consider the ecosystem services provided by different billabongs and how many billabongs need to be watered at any given time to support regionally-important plant and animal populations. This is the first year that Annulus Billabong is planned to receive water for the environment. Water levels will be monitored at Annulus Billabong to inform future management of the site and the broader billabong assessment.
The environmental entitlement for the Yarra system is
highly secure, and it is expected that the volume of water available for use in 2020–21 will be sufficient to deliver all the potential watering actions under the average and wet scenarios. Lower tributary inflows under a dry scenario means that larger volumes of environmental water will likely be needed to deliver potential watering actions in a dry year. The expected volume of available water should be sufficient to deliver all of the potential watering actions under a dry scenario except for winter/spring freshes in the Yarra River.
A critical carryover volume of 3,000 ML has been identified to provide sufficient water to deliver high-priority actions (summer/autumn low flows and freshes, an autumn high flow and targeted billabong watering) in 2021–22.
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