Melbourne

14.2 Surface water flow out of region

Supporting information

The volume recognised in the water accounting statements (418,938 ML) represents the volume of water discharged from waterways within the Melbourne region to Port Phillip Bay and Western Port Bay.

The table below shows the discharge measured at each of the gauge locations used to calculate the total outflow to sea from the Melbourne region.

Discharge measured at each streamflow gauge location
Catchment	Flow gauge no.	Flow gauge location	Discharge (ML)	Uncertainty (%)
Yarra	229143A	Yarra River at Chandler Highway*	188,054	+/–10
	229624A	Gardiners Creek at Great Valley Road, Gardiner**	21,400	+/–10
	Yarra discharge subtotal		209,454
Bunyip	229660A	Elsternwick Main Drain at Elsternwick*	6,282	+/–10
	228363A	Deep Creek at Pakenham***	735	+/–5
	228365A	Gumscrub Creek at Pakenham***	269	+/–5
	228228A	Cardinia Creek at Cardinia***	9,114	+/–5
	228209B	Lang Lang River at Lang Lang***	25,194	+/–5
	228217C	Toomuc Creek at Pakenham***	2,143	+/–5
	228203C	Eumemmering Creek at Lyndhurst*	26,359	+/–5
	228213A	Bunyip River at Iona***	49,041	+/–5
	228356B	Dandenong Creek at Pillars Crossing Bangholme*	60,970	+/–10
	Bunyip discharge subtotal		180,107
Maribyrnong	229643A	Moonee Ponds Creek at Flemington**	7,150	+/–10
	230105A	Maribyrnong River at Keilor**	8,801	+/–10
	Maribyrnong discharge subtotal		15,951
Werribee	231107A	Kororoit Creek at Brooklyn*	11,899	+/–20
	231110A	Skeleton Creek at Hoppers Crossing*	969	+/–40
	231204A	Werribee River at Werribee (Diversion Weir)*	558	+/–10
	Werribee discharge subtotal		13,426
Total discharge			418,938

* Stream discharges into Port Phillip Bay
** Stream discharges into the Yarra River
*** Stream discharges into Western Port Bay

Quantification approach

Data source

Melbourne Water: Waterworks database; Hydstra database.

Data provider

Melbourne Water.

Method

Outflow to sea was calculated as the sum of the discharge during 2009–10 from the most downstream stream gauges in the region 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.

Uncertainty

The uncertainty values presented in the table relate to the confidence in the stage-discharge relationship for each gauge location.

Approximations, assumptions, caveats/limitations

The method of using the most downstream stream gauges in the region that are above the tidal limit to represent outflow to sea assumes that no surface run-off occurs downstream of these stream gauges. In reality, some run-off did occur from that portion of the landscape whose catchment area lies downstream of those points. However, given that it cannot be quantified with representative faithfulness, no estimate of the magnitude of that run-off has been made. Instead, measured run-off has been reported.
The Victorian Department of Sustainability and Environment uses a different methodology that accounts for water running off the downstream, ungauged part of the catchment. Such differences in methodology significantly affect the run-off (surface water outflow) data.

The following are limitations and caveats about the data used:

Elsternwick Main Drain (229660A) did not record data between 01 July 2009 and 05 August 2009. The site was damaged.
Gumscrub Creek (228365A) data recorded between 01 July 2009 and 05 July 2009 were lost.
Cardinia Creek (228228A) did not record data between 24 July 2009 and 27 July 2009 due to logger memory failure; and did not record data between 25 August 2009 and 08 September 2009.
Bunyip River (228213A) did not record data on 02 July 2009; and data recorded between 12 August 2009 and 15 September 2009 were lost.
Dandenong Creek (228356B) did not record data between 01 July 2009 and 15 July 2009; and data recorded between 24 July 2009 and 27 July 2009 were lost.
Yarra River at Chandler Highway (229143A) data recorded between 22 December 2009 and 30 December 2009 were lost. No data were recorded between 31 December 2009 and 04 January 2010.