13.2 Adjustment and forfeiture of surface water allocation – urban water system
The volume presented (45,156 ML) represents the volume of allocated surface water under the bulk entitlements that was effectively forfeited at the end of the 2011–12 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) and the actual volume of surface water diverted for urban water supply under the bulk entitlements.
|Yarra River||Evaporation from reservoirs||26,948|
|Unaccountable differences from operational management||4,686|
|Subtotal Yarra River||22,262|
|Tarago River||Evaporation from Tarago Reservoir||1,372|
|Releases for operation management||26,951|
|Scour releases from treatment plant for maintenance work||638|
|Extra diversions from weir and pump for operation management||808|
|Subtotal Tarago River||29,769|
|Adjustment for difference between volume reported at line item 17.12—the volume of surface water taken for urban water supply and the volume of water taken by the retailers in 2011–12 under the bulk entitlements to the Yarra and Tarago and Bunyip rivers (reported in the Melbourne Water Annual Report)1||(10,523)|
|Gisborne—Baringo Creek||Estimated evaporation||8|
|Other losses (e.g. seepage plant losses)||3|
|Unaccountable losses from Western Water's share of storage2||4|
|Macedon and Mount Macedon||Estimated evaporation3||(61)|
|Volume taken for supply to Macedon storage||631|
|Unaccountable losses from Western Water's share of storage4||7|
|Maribyrnong—Rosslynne Reservoir||Volume of environmental and passing flows||1,269|
|Riddells Creek||Estimated evaporation5||(5)|
|Other losses (e.g. seepage plant losses)||9|
|Unaccountable losses from Western Water's share of storage6||12|
|Romsey||Other losses (e.g. seepage plant losses)||5|
|Subtotal Maribyrnong River||2,904|
|Werribee system—Merrimu and Djerriwarrh reservoirs||Djerriwarrh Reservoir—estimated evaporation||76|
|Djerriwarrh Reservoir—other losses (e.g. seepage plant losses)||141|
|Lake Merrimu—unaccountable losses from Western Water's share of storage7||527|
|Subtotal Werribee River||744|
1 Refer to quantification approach for further detail.
2,4,6,7 Unaccountable losses calculated by the Bureau of Meteorology as the difference between the opening balance, inflows, outflows and closing balance.
3,5 The negative value means there was more rain than evaporation.
All data regarding adjustments/decreases to Western Water's share of storage was sourced from 'Appendix 2', Western Water Annual Report 2011–12, 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.
|Bulk entitlement harvest volume—volume of water taken by the retailers in 2011–12 as reported in the Melbourne Water Annual Report1||Yarra River||343,398|
|Melbourne Headworks system||5,108|
|Subtotal volume of water taken by retailers
|less—volume of water diverted from surface water for urban water supply reported at line item 17.12||Melbourne Water—bulk water supplied to retail water authorities: Yarra River, Tarago River, Bunyip River, Melbourne Headworks system2||367,773|
|equals||Adjustment for difference between volume reported at line item 17.12 and volume of water reported as taken by retailers||(10,523)|
2 Does not include the volume of surface water diverted from the Maribyrnong and Werribee catchments by Western Water.
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 17.12) 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 reservoirs).
- In years when storages are refilling (eg 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
In the 2011 Account, it was reported that the bulk entitlements held by Melbourne's retail water authorities did not have formal allocation announcements. Consequently, the volume of water effectively allocated was equivalent to the volume of water diverted from surface water storages in the Melbourne region and the volume reported as adjustment and forfeiture was zero. This does not accurately reflect the nature of the bulk entitlements operating in the Melbourne region.
In the 2012 Account, decreases to these bulk entitlements were reported as the decreases to this share of storage through evaporation, spills and other losses and other adjustments.
This represents a significant change compared to the 2011 Account, but provides improved and more accurate information for users of the 2012 Account and ensures consistency in the approach used to report on other claims to water (refer to line item 17.17 Decrease of inter–region surface water claim on water). However data on outflows from and adjustments for these storages were not available at the time of publication. Therefore, the comparative year volume was unquantified.