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National Water Account 2015

Melbourne: Climate and water overview

City of Melbourne,Yarra River Victoria (iStock © Blues and Views)

Climate conditions

Rainfall

The total area-averaged rainfall over the Melbourne region during the 2014–15 year was 696 mm, which is below the long-term area-averaged rainfall of 864 mm (based on the 1900–2015 period). Annual rainfall ranged from more than 1,200 mm in the northeast of the region to less than 600 mm in the southwest of the region including the western part of metropolitan Melbourne (Figure C12).

 

Figure C12 Total annual rainfall for the Melbourne region during the 2014–15 year

Figure C12 Total annual rainfall for the Melbourne region during the 2014–15 year

 

Rainfall across the region was below average or very much below average for the 2014–15 year (Figure C13). These conditions represent a decrease in rainfall compared to the 2013–14 year total of 845 mm (see the 2014 Account). Consequently, flows in the major rivers within the region were 37% lower than the previous year (see Streamflow responses).

Decreased inflows resulted in a 10% decrease in total storage volume at end of the 2014–15 year compared with the previous year volume (see Surface water assets and liabilities in 'Supporting information'). If there had not been an increase in the amount of water transferred into the region from the Thomson Reservoir, storage volumes within the region would have been further depleted by the end of the 2014–15 year (see Water received under inter-region agreements in 'Supporting information').


Figure C13 Annual and monthly rainfall deciles for the Melbourne region during the 2014–15 year

 

Rainfall was below average, tending very much below average for large parts of southeastern Australia extending from western South Australia into western and central Victoria. Climate across Australia was largely influenced by persistent near El Niño conditions in the Pacific Ocean from late-2014 and the Bureau of Meteorology declared an El Niño event in May 2015. There were no strong influences on the region from other large-scale climate drivers during the year (such as the Indian Ocean Dipole).

The area-averaged monthly rainfall was below the long-term average for all months except July 2014 and January 2015 (Figure C14). June 2015 was the driest month, recording below the 10th percentile of long-term average rainfall. July 2014 was the wettest month during the year, recording just above the long-term average rainfall.

 

Figure C14 Total monthly rainfall for the Melbourne region during the 2014–15 year compared with the long-term average and percentiles for the region

Figure C14 Total monthly rainfall for the Melbourne region during the 2014–15 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 2014–15 year was 1,404 mm, which is slightly over the long-term area-averaged potential evapotranspiration of 1,395 mm (based on the 1911–2015 period). Potential evapotranspiration was highest in the central and northern parts of the region (Figure C15).

 

Figure C15 Total annual potential evapotranspiration for the Melbourne region during the 2014–15 year

Figure C15 Total annual potential evapotranspiration for the Melbourne region during the 2014–15 year

 

Potential evapotranspiration across the Melbourne region was above average in northwestern areas, below average in southern boundary areas, and average in the remaining areas for the 2014–15 year (Figure C16).

 

Figure C16 Annual evapotranspiration deciles for the Melbourne region during the 2014–15 year

Figure C16 Annual evapotranspiration deciles for the Melbourne region during the 2014–15 year

 

More information on potential evapotranspiration across the Melbourne region is available in Australian Landscape Water Balance.

 

Soil moisture

Figure C17 shows that soil moisture in the root zone (0 to 1m depth) for the 2014–15 year was generally average to below average in the Melbourne region with small areas showing very much below average (compared with the 1911–2015 period).


Figure C17 Annual and monthly soil moisture deciles in the root zone (0 to 1m depth) for the Melbourne region during the 2014–15 year

 

Figure C17 also shows monthly soil moisture deciles in the root zone (0 to 1m depth) during the 2014–15 year. Comparison of monthly rainfall and soil moisture deciles shows that the response of soil moisture to changes in rainfall closely aligns for the majority of months in the 2014–15 year.

Below average rainfall and generally dry conditions in the soil translated into below average streamflows in the rivers.

More information on soil moisture distribution across the Melbourne region is available in the Australian Landscape Water Balance.

 

Streamflow responses

Significant water events

The volume of water transferred from the Thomson Reservoir to the Melbourne region (148,744 ML) in the 2014–15 year increased by 13% compared with the previous year (see Surface water inflows).

Melbourne Water delivered 11 separate environmental flow events in the Werribee, Tarago, and Yarra rivers, delivering about 32,000 ML of water to improve river health and meet ecological objectives (Melbourne Water 2015).

 

Streamflow

There are four primary river catchments within the Melbourne region:

  • Bunyip
  • Yarra
  • Maribyrnong
  • Werribee.

More details on these river catchments are provided in the Water resources section of the 'Contextual information'.

Consistent with the below average rainfall conditions and dry conditions in the soil, total annual flows for all four rivers were below long-term averages at selected gauging stations. Figure C6 in the Water resources section shows the location of these gauging stations and the catchment boundary used to prepare hydrographs for the four main rivers in the Melbourne region. Figures C18–C21 compare the total monthly flow for the Bunyip, Yarra, Maribyrnong, and Werribee rivers respectively with long-term average and percentile flows.

 

Figure C18 Total monthly flow along the Bunyip River during the 2014–15 year compared with the long-term average and percentiles for the river

Figure C18 Total monthly flow along the Bunyip River during the 2014–15 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 2014–15 year was 16% less than the long-term average for the period 1962–63 to 2014–15. This observation is consistent with the below average rainfall and soil moisture conditions. In July 2014, above average streamflow was recorded as a result of above average rainfall recorded in the same month and the previous month. January 2015 recorded below average streamflow, although average rainfall was recorded in the same month, presumably because soil water stores were relatively low after five months with below average rainfall.

 

Figure C19 Total monthly flow along the Yarra River during the 2014–15 year compared with the long-term average and percentiles for the river

Figure C19 Total monthly flow along the Yarra River during the 2014–15 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 2014–15 year was 14% less than the long-term average for the period 1976–77 to 2014–15. Below average streamflow was observed in all months except July 2014 and March and May 2015. This observation is consistent with the rainfall pattern observed for the region (figures C12–C14) except for March and May 2015. 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 storages.

 

Figure C20 Total monthly flow along the Maribyrnong River during the 2014–15 year compared with the long-term average and percentiles for the river

Figure C20 Total monthly flow along the Maribyrnong River during the 2014–15 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 2014–15 year was 86% less than the long-term average for the period 1985–86 to 2014–15, and 67% less than the flow recorded for the 2013–14 year. Diverting a major portion of catchment runoff to Rosslynne Reservoir combined with very much below average rainfall recorded in the catchment (see Figure C13) contributed to low streamflow conditions observed at the gauging station.

 

Figure C21 Total monthly flow along the Werribee River during the 2014–15 year compared with the long-term average and percentiles for the river

Figure C21 Total monthly flow along the Werribee River during the 2014–15 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 (231204) for the 2014–15 year was 3% of the long-term average for the period 1957–58 to 2014–15. 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 2014–15 year.

 

Major water reforms

Organisational changes

The establishment of the Victorian Department of Environment, Land, Water, and Planning (DELWP) came into effect on 1 January 2015 and was announced by the Premier of Victoria on 4 December 2014. The department has taken the responsibilities for the functions earlier delivered by the Victorian Department of Environment and Primary Industries.

 

Changes to bulk entitlements

Prior to 1 July 2014, water authorities held bulk entitlements separately for water resources from the Thomson River, Yarra River, Silver and Wallaby creeks (Goulburn River Basin), and Tarago and Bunyip rivers. With effect from 1 July 2014, the bulk entitlements held by the water authorities to these water resources were revoked, and replaced with a single delivery entitlement for the Greater Yarra–Thomson system (Yarra Valley Water 2015). Since 1 July 2014, the following water authorities (primary entitlement holders) have held bulk entitlements to the water resources of the Greater Yarra River–Thomson system:

  • Barwon Water
  • City West Water
  • South East Water
  • South Gippsland Water
  • Western Water
  • Westernport Water
  • Yarra Valley Water.

Melbourne Water manages the source bulk entitlements for water from the Greater Yarra River–Thomson system for the purpose of supplying water, while the primary entitlement holders manage delivery entitlements. This reform empowers primary entitlement holders to individually manage their available water against customers’ demand.

City West Water, South East Water and Yarra Valley Water hold source bulk entitlements to the water resources of the Victorian Desalination Plant, and River Murray and Goulburn systems.