Melbourne: Climate and water overview
Annual rainfall in the region was below average. The combination of below average rainfall and soil moisture conditions led to below-average flows in the Bunyip and Yarra river systems. The Maribyrnong and Werribee river systems had very much below-average flows due to extensive diversions.
Introduction
The Melbourne region has a temperate climate and is well known for its changeable weather conditions due to its geographical location and proximity to the Southern Ocean. The region's rainfall occurs mostly during the winter and spring seasons (June–November). As a consequence, flows in the four major rivers Yarra, Bunyip, Maribyrnong and Werribee, is higher during the winter and spring seasons when evapotranspiration is relatively low. The region's rivers have relatively low flows during the summer and autumn seasons December–May (see Water resources in 'Region description' for more information).
Climate conditions
Rainfall
Rainfall across Australia was close to average, although there was significant regional and intra-annual variation. The high rainfall variability was largely due to a strong El Niño event and its breakdown in early autumn, warm ocean temperatures across the Indian Ocean, and strong fluctuations in the Indian Ocean Dipole (IOD). The period saw record high national area-averaged temperatures, as well as reduced rainfall; with particularly dry spells during September and October resulting in Australia's third-driest spring on record. Following the breakdown of the El Niño, a strong negative phase of the IOD developed in May–June. This event contributed to two very wet months, including Australia's second-wettest June on record.
The total area-averaged rainfall over the Melbourne region during the 2015–16 year was 695 mm, which is below the long-term area-averaged rainfall of 862 mm (based on the 1900–2016 period). Annual rainfall ranged from more than 1,200 mm in the northeast to less than 400 mm in the southwest of the region, which includes the western part of metropolitan Melbourne (Figure C1).
Figure C1 Total annual rainfall for the Melbourne region during the 2015–16 year
Rainfall across the region was below average or very much below average for the 2015–16 year (see Figure C2). These conditions are similar to that of the 2014–15 year when total area-averaged rainfall for the region was 696 mm (see the 2015 Account).
Figure C2 Annual and monthly rainfall deciles for the Melbourne region during the 2015–16 year
During the 2015–16 year, the Melbourne region experienced a nine-month stretch of below average rainfall comencing in August 2015 with the exception of January 2016 (Figure C3). Above long-term average rainfall occurred during the months of July 2015 and May–June 2016; however, the annual rainfall still remained below the long-term average.
Figure C3 Total monthly rainfall for the Melbourne region during the 2015–16 year compared with the long-term average and percentiles for the region
Evapotranspiration
The total area-averaged potential evapotranspiration over the Melbourne region during the 2015–16 year was 1,451 mm, which is slightly over the long-term area-averaged potential evapotranspiration of 1,389 mm (based on the 1911–2016 period). Potential evapotranspiration was lower in the northern eastern parts of the region due to forested and mountainous terrain (Figure C4) .
Figure C4 Total annual potential evapotranspiration for the Melbourne region during the 2015–16 year
Potential evapotranspiration across the Melbourne region was very much above average in most of the region and above average to average in the southern boundary areas for the 2015–16 year due to less rainfall (Figure C5).
Figure C5 Annual evapotranspiration deciles for the Melbourne region during the 2015–16 year
More information on potential evapotranspiration across the Melbourne region is available in Australian Landscape Water Balance.
Soil moisture
Figure C6 shows that soil moisture in the root zone (0–1m depth) for the 2015–16 year was very much below average in the Melbourne region, compared with the 1911–2016 period.
Figure C6 Annual and monthly soil moisture deciles in the root zone (0–1m depth) for the Melbourne region during the 2015–16 year
Figure C6 also shows monthly soil moisture deciles in the root zone (0–1m depth) during the 2015–16 year. Comparison of 2015–16 monthly rainfall and soil moisture decile maps shows that in most months there was an aligned response of soil moisture to changes in rainfall.
More information on soil moisture distribution across the Melbourne region is available in the Australian Landscape Water Balance.
Streamflow responses
Streamflow
There are four primary river catchments within the Melbourne region:
- Bunyip
- Yarra
- Maribyrnong
- Werribee.
The location of the key gauging stations on the Bunyip River (Iona, 28213A), Yarra River (Chandler Highway, 229143A), Maribyrnong River (Keilor, 230200D) and Werribee River (Werribee Diversion Weir, 231204A) on these river systems is provided in Figure R8 in 'Region description'.
Total annual streamflows in these river systems during the 2015–16 year were below the long-term average (figures C7–C10), reflecting the below average rainfall and soil moisture conditions across the region.
Figure C7 Total monthly flow along the Bunyip River during the 2015–16 year compared with the long-term average and percentiles for the river
Total annual flow for the Bunyip River recorded at Iona gauging station (228213A) for the 2015–16 year was 37% less than the long-term average for the period 1962–2016. In August 2015, above average streamflow was recorded as a result of above average rainfall recorded in the same month and also in the previous month (July). The below average rainfall until April 2016 resulted in soil water stores being relatively low, resulting in very low flows. Streamflow was much lower than expected in May–June given the well above average rainfall that occurred during that period, probably because of the very poor antecedent soil moisture conditions.
Figure C8 Total monthly flow along the Yarra River during the 2015–16 year compared with the long-term average and percentiles for the river
Total annual flow for the Yarra River recorded at Chandler Highway gauging station (229143A) for the 2015–16 year was 41% less than the long-term average for the period 1976–2016. Below average streamflow was observed in all months for the 2015-16 reporting year. This observation is consistent with the rainfall pattern observed for the region (figures C1–C3). A strong relationship between monthly streamflow and monthly rainfall is unlikely for the Yarra River at this gauging station because of water diversion to and releases from the upstream storages. Streamflow was much lower than expected in May–June given the well above average rainfall that occurred during that period, probably because of the very poor antecedent soil moisture conditions.
Figure C9 Total monthly flow along the Maribyrnong River during the 2015–16 year compared with the long-term average and percentiles for the river
Total annual flow for the Maribyrnong River recorded at Keilor gauging station (230200D) for the 2015–16 year was 90% less than the long-term average for the period 1985–2016, and 35% less than the flow recorded for the 2014–15 year. Diverting a major portion of catchment runoff to Rosslynne Reservoir combined with very much below average rainfall recorded in the catchment (see Figure C9) contributed to low streamflow conditions observed at the gauging station.
Figure C10 Total monthly flow along the Werribee River during the 2015–16 year compared with the long-term average and percentiles for the river
Total annual flow for the Werribee River recorded at Werribee Diversion Weir gauging station (231204A) for the 2015–16 year was 4% of the long-term average for the period 1957–2016. Extensive diversions upstream of the gauge for Pykes Creek Reservoir, Bacchus Marsh Weir, Melton Reservoir, and Werribee Weir were likely the main reasons for observed low streamflows. As a result, there is hardly any relationship between monthly streamflow and monthly rainfall at this gauging station for the 2015–16 year.
Significant water events
The volume of water transferred from the Thomson Reservoir (247,098 ML) to the Melbourne region through Upper Yarra Reservoir in the 2015–16 year increased by 66% compared with the previous reporting year (see Surface water inflows).
Melbourne Water delivered environmental flows in the Yarra (8,817 ML), Tarago (2,117 ML), Maribyrnong (302 ML) and Werribee (440 ML) river systems, delivering 11,676 ML of water, compared with about 32,000 ML in the previous reporting year, to improve river health and meet ecological objectives (Melbourne Water 2016).
Major water reforms
- During 2015–16 year Melbourne Water embraced innovation through planning and design of the Western Treatment Plant upgrade to service the growing population of Melbourne (Melbourne Water 2016)
- Melbourne Water enabled connection of the Melbourne Water supply network to the Barwon Water system (Melbourne Water 2016)
- In March 2016, the Victorian Government placed an order for 50,000 ML of water from the Victorian Desalination Plant. Melbourne Water and Melbourne’s three water retailers (City West Water, South East Water and Yarra Valley Water) helped inform the Government’s desalinated water order decision by providing technical and operational advice. Consideration was given to current and forecast storage conditions, expected water demands, the Bureau of Meteorology's (the Bureau's) climate outlooks and costs to customers. The advice was supported by detailed analysis using a range of streamflow scenarios to examine risks to future water storage volumes. Furhter details on Melbourne Water website.