Perth

Riparian releases

Riparian releases are generally made seasonally from reservoirs and water mains within the Perth region. These releases are made from reservoirs within the public and irrigation water supply systems.

The timing, volume and point of release are determined through a liaison between the Department of Water, Water Corporation and downstream water users.

Public water supply system

The table below shows the volume of riparian releases made during the 2009-10 reporting period from the public water supply system. Almost the entire volume of water released occurred between November and May (Water Corporation 2010).

There is currently only a statutory requirement for releases to be made specifically for environmental benefit from the Canning River. Water for environmental benefit is provided through operating rules for the Canning River, as outlined in the Lower Canning River surface water allocation plan (Department of Water 2010).

The plan outlines a flow regime required to maintain current ecological conditions in the Lower Canning River. These ecological requirements for the Canning River were developed by Radin et al. (2010), and include the maintenance of:

• flow connectivity
• pool depth as refuge habitat
• a minimum flow over riffles for macroinvertebrates
• prevention of anoxic conditions in pools.

In order to maintain these ecological requirements, a daily volume of water to be released from the six environmental release points has been determined for the Lower Canning River. Water release is only required during the drier months between November and May. The standard summer-flow schedule at each release point is given in the table below. The summer-flow schedule commences when flow at a key downstream gauging station (616027) along the Canning River falls below 9.3 ML/day for three consecutive days in late spring.

There is also provision for up to three higher volume ‘pulse’ flows to aid upstream fish migration during summer. These pulse flows are scheduled to be released up to three times a year during the summer period. The pulse duration is five days after which flows return to the above standard summer-flow schedule. Pulse flows may be cancelled if winter rainfall is low.

A more detailed description of these riparian release schedules are provided in Appendix B of the Lower Canning River surface water allocation plan (Department of Water 2010).

Water Corporation operates the reservoirs and manages the release of water from the environmental release points. The Department of Water monitors flows at each station to ensure these minimum flows are maintained.

Irrigation supply system

There are environmental water requirements for river reaches downstream of the following reservoirs that supply water to irrigation schemes within the Perth region:

• Drakes Brook
• Logue Brook
• Samson Brook
• Stirling
• Wokalup Pipehead.

These environmental requirements are largely based on three flow components:

• continuous (ecological) flow provisions
• fish migration and reproduction
• maintenance of channel form and riparian vegetation.

Water Corporation is responsible for the release of water from irrigation supply reservoirs to ensure that these environmental flow provisions are met at specific compliance points downstream of the reservoirs.

During 2009-10, the minimum flow requirements downstream of these reservoirs were largely met by releases made for irrigation supply, except for Samson Brook, where specific environmental releases were much larger than those made for irrigation supply.

Managed aquifer recharge

Treated wastewater from the Kwinana Wastewater Treatment Plant is infiltrated to the groundwater aquifer, which assists in maintaining environmental values at The Spectacles wetlands on the Swan Coastal Plain. The groundwater in this region is also extracted and recycled by Alcoa.

During the 2009-10 reporting period, 1,475 ML of treated wastewater was infiltrated to the groundwater aquifer. This is equivalent to approximately 1% of the total treated wastewater.

Water is released from Harvey Reservoir during the irrigation season (generally November to May) to ensure that there is flow along Harvey River downstream of the Harvey Reservoir. It is considered important that Harvey River has flow for aesthetic reasons, primarily because this region of the Harvey River is a tourist precinct.

The minimum flow requirement along this section of the Harvey River during the irrigation season is 62.5 L/s. During 2009-10, a total of 1,269 ML was released from Harvey Reservoir for this social benefit. There are no environmental water requirements for this section of the Harvey River.