Melbourne
17.2 River outflow from the region

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%

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.

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).

• 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.

 

The uncertainty estimate was not quantified.