Melbourne
9.15 Increase of inter–region surface water claim on water

Supporting information

The volume presented in the water accounting statements (259,701 ML) represents the increase in the water available from the Melbourne retail water authorities City West Water, South East Water and Yarra Valley Water entitlement to the Thomson Reservoir, Silver and Wallaby creeks and water held in Lake Eildon.

  • For the bulk entitlement to Thomson Reservoir, this represents the increase in the water available from the Melbourne retail water authorities' share of storage capacity obtained through their share of gross inflow to and rainfall on the Thomson Reservoir.
  • For the bulk entitlement to Silver and Wallaby creeks, there is no entitlement to share of storage capacity, therefore the increase in the entitlement is recognised as equal to the volume delivered during 2011–12 (see 9.11 Delivery of water under inter–region agreement to surface water). There was no allocation announcement on the volume that can be taken from the resource, although an upper limit is specified in the bulk entitlements held by the Melbourne retail authorities.
  • For the agreement to water held in Lake Eildon, allocation announcements for the Goulburn and Murray rivers are made once a year around November, and are based on audited water savings achieved from modernisation projects in northern Victoria in the previous year (e.g. the 2010–11 year). The volume of water allocated is stored in Lake Eildon.

The following table presents the breakdown of increases to inter–region claims for the Melbourne region during the 2011–12 year.

Increases to inter–region claim on water
Source Component Volume (ML)
Thomson Reservoir Share of gross inflow (runoff) and rainfall
248,520
Silver and Wallaby creeks delivery from Silver and Wallaby creeks
1,130
Lake Eildon Allocation announcements: Goulburn River 8,638
Allocation announcements: Murray River 1,413
Total 259,701

Quantification approach

Data source

Waterworks database.

Provided by

Melbourne Water.

Method

Increases to inter–region claims held by the Melbourne Region were calculated using the following methods.

Thomson Reservoir

Inflow into Thomson Reservoir is not directly measured/gauged. Net inflow is determined using a mass–balance approach that considers the reservoir's measured daily outflows and changes in storage level. Inflow is the net of evaporation and referred to as net inflow.

Silver and Wallaby creeks

The volume of inflows from 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.

Lake Eildon

Allocation announcements, made once a year around November, are based on audited water savings achieved from modernisation projects in Northern Victoria in the previous year (e.g. the 2010–11 year).

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. 

Uncertainty information

The uncertainty estimate was not quantified.