3 March 2011
Melbourne’s “Stubborn” Dam Levels Explained
Key Points
- Summer rain tops off huge 20-month turnaround in storage levels
- Thomson Dam largest contributor to recovery so far
- Significant storage rises in 2011 depend on more rain over Thomson
Any significant increase to Melbourne’s storage levels during 2011 will depend chiefly on conditions over the massive Thomson Dam.
General Manager of Asset Planning, Paul Pretto, said above-average rain over the major catchments since spring 2009 had practically filled most of the city’s smaller dams, but that the Thomson still had room for more than 650 billion litres.
“We understand that it seems strange for total dam levels to rise only slightly in the face of heavy rain and flooding in many parts of the state,” said Dr Pretto.
“The fact is that with most of our smaller dams now full any significant increase depends on how much rain falls over the 49,000 hectares of forested catchment that feeds the Thomson Dam.”
Melbourne’s water supply system is made up of 10 dams ranging in capacity from 3 billion to more than 1,000 billion litres. The city uses an average of 1 billion litres a day.
Since 1 January the catchments have received a total of 223mm of rain, which is almost double the average for this time of year. Despite normally dropping by around 0.4% each week in summer, dam levels have actually increased overall by 0.4% (7 billion litres) to reach 54.2%. This compares with 34.8% at the same time last year.
Dr Pretto said Thomson Dam represented almost two-thirds of Melbourne’s total water storage capacity and had contributed 225 billion litres – more than any other harvesting reservoir – to the recovery of the city’s overall dam levels from just 25% in June 2009.
“The Thomson is very large for the size of the catchment that feeds it. It captures water over many years and protects the city against extended drought periods,” he added.
“In percentage terms Thomson may seem to distort how much water we have but in real terms it is designed as the city’s term deposit – it grows slowly and you only draw down from it when you need to, but you’d be in strife without it.”
Thomson has been full three times in its 26 year history – 1992, 1993 and 1996. During the previous drought, it fell from around 99% in 1997 to just 16% in mid-2009, protecting Melbourne from running out of water in the process.
Thomson will continue to ‘rest’ during 2011 to keep recovering, with most of the city’s water coming from the Upper Yarra, Cardinia and Sugarloaf reservoirs. This strategy of not taking water from Thomson is expected to continue during 2012.
Dr Pretto said innovation around more water recycling and stormwater reuse for non-drinking purposes, together with avoiding a return to pre-drought levels of water use, were critical to a sustainable water future for Melbourne.
Melbourne Water recently released figures showing how the city’s dams performed during one of their wettest summers on record. More information on how our dams work and how Melbourne Water is running the system is available from our website.
Thomson Facts
- Thomson is the third-largest dam connected to a major Australian city behind Warragamba in Sydney (2,027 billion) and Wivenhoe in Brisbane (2,600 billion).
- Thomson’s capacity of 1,068 billion litres when full is almost equivalent to Melbourne’s total water use for the past 3 years.
- For Thomson to increase by 1% of its storage capacity requires 10 billion litres to flow into it – enough water to supply Melbourne for over a week.
Media Contact: Andrew McGinnes (03) 9235 7277; 0403 930 846