17.2 River outflow from the region

Supporting information

The volume of 1,918,282 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 2011–12 year.

The following table shows discharge from each catchment in the Melbourne region.

Discharge per catchment in the Melbourne region
Catchment Flow gauge number 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 109,510 53,321 162,831 33%
228213 Bunyip River at Iona 189,138 41,883 231,021 18%
228228 Cardinia Creek at Cardinia 26,142 31,018 64,828 48%
228217 Toomuc Creek at Pakenham 7,668
n/a Downstream ungauged catchments
555,006 555,006 100%
Bunyip discharge total 332,458 681,228 1,013,686 67%
Yarra 229143 Yarra River at Chandler Highway 782,629 50,445 833,074 6%
Maribyrnong 230200 Maribyrnong River at Keilor 44,141 711 44,852 2%
Werribee 231204 Werribee River at Werribee (Diversion Weir) 17,201 520 17,721 3%
n/a Downstream ungauged catchments
8,949 8,949 100%
Werribee discharge total 17,201 9,469 26,670 36%
Total discharge 1,176,429 741,853 1,918,282 39%

Quantification approach

Data source

Melbourne Water Waterworks database and Hydstra database for gauges in the Bunyip and Yarra catchments; Victorian Department of Environment and Primary Industries Hydstra database for gauges in the Maribyrnong and Werribee catchments.

Provided by

Melbourne Water, Victorian Department of Environment and Primary Industries.


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 2011–12 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).

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 on 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.