About the area
- The Thomson basin is in south-east Victoria. The Thomson and Macalister rivers join the Latrobe River before flowing into the Gippsland Lakes.
- The large northern section of the basin, containing the headwaters of the Thomson and Macalister Rivers, is forested and mountainous.
- Annual rainfall is over 1,200 mm in the upper reaches, reducing to between 400-600 in the south of the basin, which is amongst the driest area in Gippsland on average.
- Water from the Thomson basin is used to supply agriculture, including a large irrigation area in the Thomson and Macalister river valleys, as well as urban supply for the Melbourne system.
More information on water management and accounting is detailed in Water .
2021-22 overview
Available water
Catchment inflow was higher than the previous year.
Licensed diversion restrictions
There were no restrictions on licensed diversions.
Seasonal determinations
Thomson-Macalister system reached 100% allocation to high-reliability water shares – the same as the previous year.
Water use
Less water was diverted for consumptive purposes than the previous year.
When compared to the previous year, in 2021-22:
- more rainfall was received, catchment inflows were higher and storage levels peaked and declined at higher levels
- seasonal allocations were the same as the year before, with both high and low-reliability entitlements reaching 100% allocation; spill entitlement was also available
- there were fewer restrictions (no restrictions) on licensed diversions from unregulated streams
- less water was diverted from the basin for consumptive uses.
Climate
Rainfall
In 2021-22, rainfall in the Thomson basin was very much above average in most areas, higher than the previous year. Across the basin, rainfall was:
- above average in the west around the Thomson Dam and Baw Baw National Park, as well as along the northern border towards Mt Buller
- very much above average across the rest of the basin from the Alpine National Park down to Heyfield, Sale, and Lake Wellington
- the highest on record in the east, close to Mitchell River National Park.
River basin water balance
Along with the very much above average rainfall, there were also higher inflows than the previous year and more water in storages. Above average inflows of 1,724,657 ML were received (184% of the long-term average of 936,400 ML), more than the previous year (146% of the long-term average). Storage levels also peaked and declined at higher levels in 2021-22 than they did the year before.
In 2021-22, 163,210 ML of water was diverted for consumptive uses: town, domestic and stock, irrigation and commercial supply. This was less than the 250,594 ML diverted in the previous year.
Water balance table
The table below shows the total volumes of water available and supplied from water resources in the Thomson basin in 2021-22.
Table 1: Water balance, Thomson basin
Storages
Major — greater than 1,000 ML — on-stream storages in the Thomson basin are included in the water balance.
Storage levels
Storage levels were 87% in June 2022.
Higher than the previous year
Storage levels were 75% in June 2021.
Table 2: Storage levels, Thomson basin
Catchment inflow
Catchment inflow represents the volume of water flowing into the waterways of a basin. It is calculated to be the difference between the total outflows, the known inflows and the net change in storage volume.
Above average inflow
Catchment inflow was 184% of the long-term average of 936,400 ML.
More water received than last year
This is higher than the previous year (146% of the long-term average).
Wastewater treatment plants
Water treated at wastewater treatment plants can be used to supplement water available in the basin. Water discharged to waterways from treatment plants is included as an inflow to the water balance.
Information on treatment plants is now reported in the Water Supply local reports for the water corporation responsible for managing the plant.
In this basin, wastewater treatment plants are managed by Gippsland .
Small catchment dams
Water harvested, used and lost by small catchment dams (farm dams) is included in the water balance.
Table 3: Small catchment dams, Thomson basin
Entitlements and compliance
Entitlements provide the basis for how water is shared in the basin.
Melbourne Water holds a bulk entitlement to surface water from the Thomson River. This entitlement is one of four that contribute to the Greater Yarra system – Thomson River Pool, which primarily supplies Melbourne and supports regional urban water corporations Barwon Water, Greater Western Water (formerly Western Water), South Gippsland Water and Westernport Water.
Surface water is also diverted by licensed diverters and is harvested in small catchment dams.
The VEWH holds environmental entitlements for the Thomson and Macalister rivers. Water available under these entitlements is used to support streamflows, and a small volume is diverted out of waterways to wetlands in the basin.
Entitlement volumes
Rights to water in the Thomson basin are shown in table 4 below.
Entitlement volumes represent a maximum volume that can be taken in a one-year period. The volume available in a particular year is dependent on the rules for allocating water set out in the entitlement and the seasonal conditions in that year, which varies. The rules for allocating water under an entitlement can differ between entitlements and systems. This affects the ability and likelihood of water being taken in a particular year.
Table 4: Annual entitlement volumes at 30 June, Thomson basin
Available water and take under entitlements
Total water available under entitlements represents the volume of water that was available to be taken by entitlement holders in 2021-22. The volume includes carryover from the previous year, seasonal allocations and net trade into the basin.
Water taken
There was 178,179 ML taken under entitlements in 2020-21, less than the year before (274,272 ML).
Seasonal allocations and restrictions on licensed diversions from unregulated rivers
- High-reliability entitlement allocations opened the water year at 100% (the same as the previous year). Low-reliability entitlements reached 100% in March 2022 (a month earlier than the previous year), spill entitlement was also available for use during the year.
- There were no restrictions on licensed diversions from unregulated streams in 2021-22. This is compared to the previous year when there was a peak of 2 streams on restrictions in March 2021.
Available water and take table
This table shows the volume of available water and the volume taken under entitlements in 2021-22.
More information on available water and take has been detailed in the How do we account for surface section on the How do we account for water page.
In 2021-22, less water was taken under entitlements than the previous year.
Table 5: Available water and take, Thomson basin
Compliance
Compliance against water entitlements is reported for this basin in three areas:
- entitlement issued: the volume of entitlements issued in a basin does not exceed formal caps, and has not increased without appropriate approvals
- water taken: the volume of water taken during the year does not exceed the volume considered to be available for consumptive and/or in-stream use during that year
- bulk entitlement provisions: holders of entitlements do not breach any provisions that are documented in their bulk entitlement orders.
Total entitlement volume
There was no net increase in the total entitlement volume from the previous year.
Total volume diverted
The total volume diverted (178,179 ML) was within the volume available for the year (522,133 ML).
Individual bulk entitlements
No individual bulk entitlement holder took more than the annual volume made available to them.
Exceptions to compliance
Individual bulk entitlement holders complied with all provisions in their entitlements.
Water for the environment
Environmental watering sites
Environmental watering sites and environmental values in the Thomson basin that depend on water for the environment include:
- the Gippsland Lakes, which are listed as internationally significant wetlands under the Ramsar Convention and partially rely on freshwater inputs from the Thomson basin to function ecologically
- the upper Thomson River (a heritage river reach) and its Australian grayling populations
- the Macalister River, supporting seven migratory native fish species, platypus and the native water rat (Rakali).
Environmental water reserve
In 2021-22, water for the environment in the Thomson basin comprised:
- the Bulk Entitlement (Thomson River – Environment) Order 2005 comprising a 3.9% share of inflows — on average, 8,000 ML a year — and 10,000 ML of high-reliability entitlement held by the VEWH and water set aside for the environment through the operation of passing flow conditions on the entitlement
- the Macalister River Environmental Entitlement 2010 comprising 12,461 ML of high-reliability and 6,230 ML of low-reliability entitlements held by the VEWH
- water set aside for the environment through the operation of passing flow conditions:
- on consumptive bulk entitlements held by Southern Rural Water
- on licensed diversions
- all other water in the basin not allocated under entitlements: this water also provides social, recreational and cultural benefits.
Environmental water use
In 2021-22, a total of 19,350 ML of environmental water was delivered in-stream in the Thomson basin.
Management responsibilities
Management of water in the Thomson basin is undertaken by various parties
| Authority | Management responsibilities |
|---|---|
| Melbourne Water |
|
| Southern Rural Water |
|
| Gippsland Water |
|
| West Gippsland Catchment Management Authority |
|
| Water supply system | 2021-22 (ML) | 2020-21 (ML) |
|---|---|---|
| Coongulla System | 43 | 43 |
| Heyfield | 199 | 216 |
| Maffra System | 1,095 | 1,127 |
| Water supply system | 2021-22 (ML) | 2020-21 (ML) |
|---|---|---|
| Macalister Irrigation District | 80,875 | 158,360 |
