Melbourne
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.
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
Provided by
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 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.