Melbourne
19.17 Adjustment and forfeiture of urban claim on surface water

Supporting information
Quantification approach

Supporting information

The volume presented (53,052 ML) represents the volume of allocated surface water under the bulk entitlements that was effectively forfeited at the end of the 2012–13 year. The volume includes:

decreases to the bulk entitlement share of storage through evaporation, spills, and other losses
an adjustment for the difference between the bulk entitlement harvest volume (reported as the volume of water taken by the retailers in the Melbourne Water annual report 2012–13) and the actual volume of surface water diverted for urban water supply under the bulk entitlements.

The calculation of these decreases and adjustment are shown in the following table.

Calculation of adjustment and foreiture for the urban claim on surface water
Source	Decrease/adjustment	Volume (ML)
Yarra River	evaporation from reservoirs	n/a*
Yarra River	unaccountable differences from operational management	12,444
Subtotal Yarra River		12,444
Tarago River	evaporation from Tarago Reservoir	1,756
Tarago River	releases for operation management	14,264
Subtotal Tarago River		16,020
Adjustment for unaccountable losses in the retailer's share of storage in the Yarra and Tarago reservoirs¹		17,626
Gisborne–Baringo Creek	estimated evaporation	4
	other losses (e.g., seepage plant losses)	4
	unaccountable losses from Western Water's share of storage	(3)
Lancefield	estimated evaporation	7
Lancefield	volume of environmental and passing flows	2,058
Macedon and Mount Macedon	estimated evaporation	22
	volume taken for supply to Macedon storage	443
	unaccountable losses from Western Water's share of storage	36
Maribyrnong—Rosslynne Reservoir	volume of environmental and passing flows	1,744
	other losses (e.g., seepage plant losses)	1,054
	unaccountable losses from Western Water's share of storage	159
Riddells Creek	estimated evaporation	10
Riddells Creek	other losses (e.g., seepage plant losses)	23
Romsey	other losses (e.g., seepage plant losses)	155
	estimated evaporation	39
	adjustment for potable water received from Melbourne Water	64
Subtotal Maribyrnong River		5,819
Werribee system—Merrimu and Djerriwarrh reservoirs	Djerriwarrh Reservoir—estimated evaporation	130
	Djerriwarrh Reservoir—other losses (e.g., seepage plant losses)	824
	volume of environmental and passing flows	241
	unaccountable losses from Western Water's share of storage	-52
Subtotal Werribee River		1,143
Total		53,052

* 2012–13 evaporation volumes for Cardinia and Silvan reservoirs are unavailable for the full period.
¹ Refer to table in quantification approach for further detail.

Unaccountable losses calculated by the Bureau of Meteorology (the Bureau) as the difference between the opening balance, inflows, outflows, and closing balance.

All data regarding adjustments/decreases to Western Water's share of storage was sourced from Appendix 2 of the Western Water annual report 2012–13, with the exception of the unaccountable losses which were calculated by the Bureau.

All data regarding adjustments/decreases to the Yarra and Tarago/Bunyip entitlements was provided by Melbourne Water from internal management records. This includes evaporation data which is different from evaporation data calculated by the Bureau and reported at line item 17.1 Evaporation from surface water.

Quantification approach

Data source

Melbourne Water – internal management records; Western Water – annual report

Provided by

The Bureau of Meteorology.

Method

The adjustment for the difference between the bulk entitlement harvest volume and the volume of water diverted from surface water for urban water supply reported at line item 11.12 was calculated according to the following table.

Calculation of adjustment for unaccountable losses
Relevant line item	Description	Volume (ML)
3.7	opening balance at 1 July 2012	588,618
11.18	share of inflow to reservoirs	368,435
11.12	diversion from the Yarra, Tarago, Melbourne Headworks system¹, Maribyrnong, and Werribee rivers	(408,812)
part of line item 19.17	decreases to share of storage due to evaporation and operational losses (sum of the four subtotal in table above)	(35,426)
3.7	closing balance at 30 June 2013	(495,189)
part of line item 19.17	Adjustment for unaccountable losses in the retailer's share of storage²	17,626

¹ Data provided by Melbourne Water which corresponds to rounded volumes reported in the Melbourne Water annual report 2012–13.
² No unaccountable losses were reported in retailer's share for Maribyrnong and Werribee rivers.

It is not possible to balance the bulk entitlement harvest volume (reported as the volume of water taken by the retail water authorities in the Melbourne Water annual report) with the actual volume of surface water diverted for urban water supply (refer to line item 11.12) in any given financial year. The reasons for this are:

The Yarra basin bulk entitlement harvest volume is calculated using the metered inflow to Sugarloaf Reservoir (from the Yarra River, Maroondah aqueduct and north-south pipeline). However the actual volume diverted for urban water supply (consumed) is the outflow from Sugarloaf Reservoir.
Similarly, the Yarra basin bulk entitlement harvest volume is calculated using the metered inflow to Silvan Reservoir; however, the actual volume diverted for urban water supply (consumed) is the outflow from various reservoirs that can be supplied from Silvan Reservoir (e.g., Cardinia and Greenvale Reservoir).
In years when storages are refilling (e.g. post-drought), the inflow to a reservoir may exceed its outflow.
Similarly, in dry years when storages are being drawn down, the outflow may exceed the inflow.

Assumptions, limitations, caveats and approximations

Nil.

Uncertainty information

The uncertainty estimate was not quantified.

Melbourne19.17 Adjustment and forfeiture of urban claim on surface water