Melbourne: Climate and water
Annual rainfall in the region was slightly above average and the highest in five years. Heavy rainfall over the western part of the region in September 2016 contributed to very high streamflows in the Werribee, Maribyrnong and Yarra river catchments during September–October 2016. Rainfall and streamflow throughout most of the remainder of the year were below average.
For further information on the region's climate and water conditions during the 2016–17 year scroll down this page or click on the links below:
About the region
The Melbourne region has a temperate climate and is well known for its changeable weather conditions. The region's rainfall occurs mostly during the winter and spring months (June–November). The seasonal flow characteristics of the region's rivers reflect the distribution of rainfall over the year. Most of the streamflow and storage inflows occur during the winter and spring months; lower streamflow occurs during the summer and autumn months (December–May).
Climate conditions
Rainfall
Figure C1 Annual and monthly rainfall deciles for the Melbourne region during the 2016–17 year
- Total area-averaged rainfall for the year was 864 mm, slightly above the mean annual rainfall of 857 mm.
- Higher than usual rainfall occurred over the western part of the region during the year.
Figure C2 Total monthly rainfall for the Melbourne region during the 2016–17 year compared with the long-term average and percentiles for the region
- In the first four months of the year (July–October 2016), the climate was influenced by a negative Indian Ocean Dipole phase that is associated with higher winter-spring rainfall. Rainfall across the Melbourne region was above average during this 4-month period.
- Except for February and April 2017, rainfall was below average for the remainder of the year.
- Rainfall in June 2017 was well below the 10th percentile due to dominant high pressure systems that reduced the number of cold fronts and lows that typically occur across the region.
Actual evapotranspiration
Figure C3 Annual and monthly actual evapotranspiration deciles for the Melbourne region during the 2016–17 year
- Total area-averaged actual evapotranspiration was 717 mm, slightly above the mean value of 706 mm.
- Similar to rainfall, higher than usual evapotranspiration generally occurred over the western part of the region during the year; lower than usual evapotranspiration occurred over the eastern part.
Figure C4 Total monthly actual evapotranspiration for the Melbourne region during the 2016–17 year compared with the long-term average and percentiles for the region
- Actual evapotranspiration was generally average for most of the year.
Soil moisture
Figure C5 Annual and monthly soil moisture deciles for the Melbourne region during the 2016–17 year
- Soil moisture in the root zone (0–1 m depth) was average to above average across the region.
- Rainfall and soil moisture are typically closely aligned. For example, above-average soil moisture during July–November 2016 reflects the above-average rainfall that occurred during the first four months of the year (July–October 2016); however, there was less soil moisture response than expected following the well below-average rainfall in June 2017, which can be attributed to the relatively high soil moisture conditions across the region following a very wet April 2017.
- More information on soil moisture distribution across the Melbourne region is available in the Australian Landscape Water Balance.
Streamflow responses
Figure C6 Key flow gauging stations along the Bunyip, Yarra, Maribyrnong, and Werribee rivers within the Melbourne region
- There are four primary river catchments within the Melbourne region: Bunyip, Yarra, Maribyrnong and Werribee.
- Streamflows in the Maribyrnong, Werribee, and Yarra rivers are influenced by diversions for consumptive use, dam operations, and dam spilling. The Bunyip River is less influenced by these factors.
Figure C7 Total monthly flow for key rivers during the 2016–17 year compared with the long-term average and percentiles
- Total annual streamflow in the Bunyip River was below average, which reflects the generally below-average rainfall conditions experienced over the eastern part of the region.
- Most of the annual streamflow in the Maribyrnong, Werribee, and Yarra rivers occurred during September–October 2016. The well above-average monthly flow during this 2-month period was due to heavy rainfall during September 2016 across the western and central parts of the region, particularly in the upstream catchment areas of the Maribyrnong and Werribee rivers.
- There was less flow response than expected following the well above-average rainfall in April 2017, which can be attributed to the relatively low soil moisture conditions across the region following a very dry summer.
Major water reforms
- On 1 June 2017, due to the allocation of a 3.9% share of inflows and additional 8,000 ML storage capacity in the Thomson Reservoir to the Victorian Environmental Water Holder, bulk entitlement volume for City West Water, South East Water and Yarra Valley Water from Greater Yarra System-Thomson River Pool was reduced from 588,060 ML to 578,854 ML (Victorian Government 2017).