Melbourne
17.2 River outflow from the region

Supporting information

The volume of 1,377,155 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 2012–13 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

57,965

28,223

86,188

33%

228213

Bunyip River at Iona

128,851

28,533

157,384

18%

228228

Cardinia Creek at Cardinia

14,511

13,313

27,824

48%

228217

Toomuc Creek at Pakenham

4,685

4,298

8,983

48%

n/a

downstream ungauged catchments

332,244

332,244

100%

Bunyip discharge total

206,012

406,611

612,623

66%

Yarra

229143

Yarra River at Chandler Highway

611,388

39,407

650,795

6%

Maribyrnong

230200

Maribyrnong River at Keilor

62,654

1,009

63,663

2%

Werribee

231204

Werribee River at Werribee (Diversion Weir)

38,816

1,174

39,990

3%

n/a

downstream ungauged catchments

10,084

10,084

100%

Werribee discharge total

38,816

11,258

50,074

22%

Total discharge

918,870

458,285

1,377,155

33%

Quantification approach

Data source

Melbourne Water: Waterworks database, 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.

Method

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

Gauged streamflow information is the sum of discharges during 2012–13 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%.
  • 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.