Melbourne
17.2 River outflow from the region

Supporting Information

The volume of 2,502,127 ML recognised in the water accounting statements represents the volume of water discharged from rivers and streams in the Melbourne region to Port Phillip Bay and Western Port Bay during the 2010–11 year.

The following table shows discharge from each catchment in the Melbourne region.The discharge measured at each of the gauge locations used to calculate catchment outflows, plus the estimate of the ungauged discharge downstream of those gauges.

Discharge per catchment in the Melbourne region
Catchment

Flow
gauge
no.

Flow gauge
location

Gauged
discharge (ML)

Estimated
ungauged
discharge (ML)

Total
discharge (ML)

Ungauged
flows
(% of total)

Bunyip 228209 Lang Lang River at Lang Lang 121,514 59,165 180,679 33
228213 Bunyip River at Iona 186,074 41,204 227,278 18
228228 Cardinia Creek at Cardinia 36,513 42,669 89,178 48
228217 Toomuc Creek at Pakenham 9,996
n/a Downstream ungauged catchments - 661,061 661,061 100

Bunyip discharge total

354,097 804,099 1,158,196 69
Yarra 229143 Yarra River at Chandler Highway 886,580 57,145 943,725 6
Yarra discharge total 886,580 57,145 943,725 6
Maribyrnong 230200 Maribyrnong River at Keilor 218,723 3,522 222,245 2

Maribyrnong discharge total

218,723 3,522 222,245 2
Werribee 231204 Werribee River at Werribee (Diversion Weir) 100,494 3,038 103,532 3
n/a Downstream ungauged catchments -
74,429 74,429 100

Werribee discharge total

100,494 77,467 177,961 44

Total discharge

1,559,894 942,233 2,502,127 38


Quantification Approach

Data Source

Melbourne Water Corporation: Waterworks database and Hydstra database for gauges in the Bunyip and Yarra catchments, Department of Sustainability and Environment: Hydstra database for gauges in the Maribyrnong and Werribee catchments.

Provided by

Melbourne Water Corporation and Department of Sustainability and Environment.

Method

Discharge from the Melbourne region to Port Phillip Bay and Western Port Bay was calculated using gauged streamflow data, plus the estimated ungauged flows in downstream subcatchments.

Gauged streamflow information is the sum of discharges during 2010–11 from the lowest stream gauges in the catchment that are above the tidal limit.  Discharge was derived from a telemetered flow level gauge and the water level was converted to a volume using a stage–discharge relationship.

Ungauged streamflow was calculated using mean average flows (MAF) to provide a relationship between gauged streamflow data and flow magnitudes for adjacent or nearby catchment areas.  Recorded flows were extrapolated to ungauged areas using the ratio of the MAF computation for the gauged area compared with the MAF computed for ungauged areas.  The MAF values were based on the Sustainable Diversions Limit Project Department of Sustainability and Environment 2010a. These were updated in some catchments for the Central Region Sustainable Water Strategy (2006).

This is a change from the method used in the 2010 Account, which used measured discharges at the most downstream gauges in the region above the tidal limit to represent outflow.  An estimate of surface discharge downstream of the gauges was not made.

Assumptions, Limitations, Caveats and Approximations

  • Gauged streamflow measurements are based on the the stage–discharge relationship at each gauge location. Stage–discharge relationships change over time and the accuracy measurement is dependent the location.
  • The portion of the catchment that is ungauged is 38%.
  • Conceptually the accuracy of the MAF in the ungauged portion of a catchment is likely to be low whereas the accuracy of the gauged outflows is likely to be high.

 

Uncertainty Information

The uncertainty estimate was not quantified.

Comparative year

This item corresponds to line item 14.2 Surface water flow out of region reported in the 2010 Account. In the 2011 Account, improved datasets were made available, which caused the 2009–10 value to be restated. The improved datasets were identified following a review of the method used to estimate discharge. The changes to the datasets used to estimate this line item are summarised as:

  •  inclusion of estimates of ungauged discharge from catchment areas
  •  exclusion of streamflow gauges 228203, 228356, 228363, 228365 and 229660 from the calculation of the gauged Bunyip River discharge
  •  exclusion of streamflow gauge 229624 from the calculation of the gauged Yarra River discharge
  •   exclusion of streamflow gauge 229643 from the calculation of the gauged Maribyrnong River discharge
  •  exclusion of streamflow gauges 231107 and 231110 from the calculation of the gauged Werribee River discharge.

A restatement of the 2010 year volume was made. Increased accuracy in the measurement technique has resulted in a difference considered material. The restated comparative year volume is 795,254 ML and is summarised in the following table.

Restated comparative year information for line item River outflow from the region

2010 Account line item name

Volume at 30 June 2010 reported in 2010 Account (ML)

Value difference to volume reported due to improved datasets (ML)

Volume at 30 June 2010 reported in 2011 Account (ML)

14.2 Surface water flow out of region

418,938

376,316

795,254