Melbourne

Thomson Reservoir is located approximately 125 kilometres east of Melbourne. It provides Melbourne with multiyear carryover storage capacity and hence long-term drought security. It has a total storage capacity of 1,068,000 ML. Further information is at the Melbourne Water website.

Diversions are made from Thomson Reservoir to:

• provide water for the Melbourne metropolitan area
• satisfy environmental flow requirements of the Thomson River
• accord with the Southern Rural Water diversion orders for irrigators
• the Thomson Hydroelectric Power Station, depending on the volume stored in Thomson Reservoir.

For supplying water to Melbourne, water is generally transferred as required to Upper Yarra Reservoir under gravity via the Thomson–Yarra Tunnel from about November to April. The Thomson–Yarra Tunnel is approximately 35 kilometres long and 3 metres in diameter. From Upper Yarra, water is transferred daily to Silvan Reservoir via the Yarra–Silvan and Yarra Valley conduits.

Bulk entitlements

Five bulk entitlements exist for water harvested by the Thomson Reservoir. These are:

• Transfer of Bulk Entitlement (Thomson River – Melbourne Water Corporation) Conversion Order 2001 to Yarra Valley Water 2006.
• Transfer of Bulk Entitlement (Thomson River – Melbourne Water Corporation) Conversion Order 2001 to City West Water 2006.
• Transfer of Bulk Entitlement (Thomson River – Melbourne Water Corporation) Conversion Order 2001 to South East Water 2006.
• Bulk Entitlement (Thomson/Macalister – Southern Rural Water) Conversion Order 2001.
• Bulk Entitlement (Thomson River – environment) Conversion Order 2005.

Bulk entitlements for the three Melbourne retail water authorities are essentially pooled. This means that the volume of water specified in the entitlements is available to be accessed by all three authorities and that Melbourne is effectively served by a single bulk entitlement.

Share of storage capacity

Details of Share of Storage Capacity can be found in each of the respective bulk entitlements (http://waterregister.vic.gov.au/Public/Reports/BulkEntitlements.aspx). In summary:

• The Minister for Environment is entitled to a storage capacity of 10,000 ML in the Thomson Reservoir. The environment is also entitled to store water in the ‘air-space’ within the reservoir, however when the Thomson Reservoir spills, the environmental share of storage in excess of 10,000 ML is reduced by the amount of the spill.
• Southern Rural Water is entitled to a storage capacity of 45,000 ML in the Thomson Reservoir.
• The Melbourne retail water authorities are entitled to the remaining storage volume in the Thomson Reservoir (total storage capacity of 1,068,00 ML).

Share of flow

Details of Share of flow can be found in each of the respective bulk entitlements. In summary:

• The first 6% of annual inflow into the Thomson reservoir is attributed to Southern Rural Water.
• After assigning 6% of inflows to SRW and after making diversions to meet minimum flow requirements, the next
• 10,000 ML of inflow must be attributed to the environmental share of storage.
• Remaining inflows are attributed to the Melbourne retail water authorities.

Temporary qualification of rights

Since the publication of the Thomson bulk entitlement conversion orders, there have been a number of qualifications made to the environment’s bulk entitlement.

Two qualifications active during 2009–10 were:

• Transfers 2,000 ML from Southern Rural Water’s drought reserve to irrigators in Werribee and Bacchus Marsh.
• Transfers 2,300 ML from Southern Rural Water’s drought reserve to irrigators in Werribee and Bacchus Marsh.

Resource manager

West Gippsland Catchment Management Authority is the resource manager for the Thomson catchment. This role includes, but is not limited to:

• preparing Thomson catchment water account
• monitoring whether entitlement holders are complying with the conditions of their entitlements
• directing the diversion of any water set aside for maintaining water quality in the waterway
• investigating and mediating disputes between entitlement holders
• investigating and dealing with significant unauthorised use of water in the Thomson catchment
• supervising the qualification of any rights to water made by the Minister during periods of declared water shortage.

Storage operator

• operating the Thomson headworks system (including making diversions from Thomson Reservoir)
• managing and monitoring flows into and out of the Thomson headworks system.
• monitoring water accounts.

Environmental flow manager

West Gippsland Catchment Management Authority is the environmental flow manager as appointed by the Minister for Environment. The environmental flow manager is responsible for managing the environment’s bulk entitlement on behalf of the Minister for Environment. This includes the development of annual watering plans that document intended diversions of water from the environmental water reserve to maximise potential benefit to the environment. The storage operator is then obliged to divert water as prescribed in these watering plans.

Due to persistent drought conditions, in recent years Southern Rural Water has also transferred water from its Thomson Reservoir drought reserve to the Bacchus Marsh and Werribee irrigation districts. Melbourne Water infrastructure has been used to facilitate this transfer.

Seepage data for the Thomson Reservoir is currently not available.

The following table shows water balance monthly breakdowns.

Water balance methods

Melbourne Water uses different methods and different results to manage the Thomson Reservoir.

Opening and closing balances

Precipitation

Run-off

Evaporation