9.11 Delivery of water under inter–region agreement to surface water
The volume presented in the water accounting statements (2,192 ML) represents the volume of water delivered to Melbourne's retail water authorities City West Water, South East Water and Yarra Valley Water under bulk entitlements to Thomson Reservoir and Silver and Wallaby creeks (refer to line item 1.5 Inter–region claim on water for further information). Although an agreement currently exists entitling Melbourne's retail water authorities to water sourced from the Goulburn River and stored in Lake Eildon, no water was diverted under this agreement during the 2011–12 year (see below for further detail).
The following table presents the breakdown of the delivery of water to the Melbourne Region during the 2011–12 year.
|Silver and Wallaby creeks||1,130|
The volume of water delivered to the Melbourne region during the 2011–12 year was substantially lower than in previous years. This is largely due to:
- above-average rainfall across the Melbourne region for the previous two years, reducing reliance on sources of water from outside the region
- Melbourne Water (in its capacity as the storage operator) optimised harvesting of water within the Melbourne region to allow storage volumes in the Thomson Reservoir to recover.
Water stored in Thomson Reservoir is delivered to Upper Yarra Reservoir for distribution to Melbourne's water supply system.
Water is diverted from Silver and Wallaby creeks, and delivered to Tooroorung Reservoir and Yan Yean Reservoir for distribution to Melbourne's water supply system.
The Water Savings Supply and Transfer Agreement allows the Melbourne retail water authorities to divert up to 75,000 ML per year of water they collectively hold in Lake Eildon via the North–South Pipeline.
In late 2010, the Victorian Government introduced its policy to phase out Melbourne's reliance on the North–South Pipeline. In future years, the pipeline will only be used for critical human needs and only when there is sufficient water in the Goulburn system.
The total volume of water delivered to the Melbourne region under an inter–region claims is the sum of the following components
The volume of water delivered from Thomson Reservoir was measured by the depth of water flowing over the spillway chute at the inflow point to Upper Yarra Reservoir (which behaves as an Ogee weir) using a Mindata 2100P level sensor. The water depth was converted to flow using a stage–discharge relationship.
The volume of water delivered from the Silver and Wallaby creeks to the Toorourrong and Yan Yean reservoirs occurs via Clearwater Channel. Clearwater Channel also diverts water from the Plenty River (in the Melbourne region) to Yan Yean Reservoir. The total volume of water transferred along the Clearwater Channel was metered after the point where water enters from Plenty River.
Based on historical records of catchment runoff, the flow along Clearwater Channel was estimated to comprise 64% of the water delivered from the Silver and Wallaby catchments, and 36% of water diverted from the Plenty River.
The volume of water transferred-in from the Silver and Wallaby catchments was derived by multiplying the Clearwater Channel meter reading by a factor of 0.64.
The volume of water delivered from Lake Eildon was measured at the North–South Pipeline Pump Station. There are six pumps at the pump station, which are divided evenly into two pump-well groups. The total volume of water diverted from the Goulburn River was calculated as the sum of the volume pumped from the two wells, measured using a Siemens Sitrans SONOKIT dual-beam ultrasonic flow meter.
Assumptions, limitations, caveats and approximations
The factor applied to derive the flow from water delivered from Silver and Wallaby creeks is an assumption based on historical data.
The uncertainty relates to the accuracy of the measurement device used to measure the volume of water transferred-in. The uncertainty for each delivery is shown below:
- Thomson Reservoir: estimated uncertainty is +/– 2%
- Silver and Wallaby creeks: estimated uncertainty is +/– 1%
- Lake Eildon: estimated uncertainty is +/– 1%.