Melbourne
17.2 River outflow from the region

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

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.

Melbourne Water Corporation and Department of Sustainability and Environment.

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.

• 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.

 

The uncertainty estimate was not quantified.

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