Melbourne
19.17 Adjustment and forfeiture of urban claim on surface water

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*

unaccountable differences from operational management

12,444

Subtotal Yarra River

12,444

Tarago River

evaporation from Tarago Reservoir

1,756

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 reservoirs

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

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

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.