Understanding the water storages data

Understand how our water storages report data is calculated and the history behind our reporting.

Water storage data

We report on:

  • today
  • yesterday
  • same day last year
  • this week

Each report provides data comparisons for:

  • today compared to yesterday
  • yesterday compared to the previous day
  • same day last year compared to the previous day to same day last year
  • this week compared to last week

Water storage dashboard

The dashboard provides a high level summary of Melbourne’s water data.


Dam storage levels The figure reports Melbourne's water storages as  a percentage of the combined capacity of our ten storage reservoirs.​
Average catchment rainfall the average weighted rainfall across the four major catchments.​
Flow into dams ​ the total flow measured at the four major catchments.​
Water use ​ the daily consumption for Melbourne​

Storage calculations 

The measured storage loss/gain is validated against the calculated value.

  •    storage loss/gain = total water use - flow
  •    total water use = consumption + releases

The Water report only shows the residential and business water consumption.  Total water use also inlcudes environmental releases, refreshening flows and extra allocations required by Southern Rural Water.

Flow only shows the intake from the four major water sources - Thomson, Upper Yarra, O'Shannassy and Maroondah.  The storage loss/gain calculation also takes into account the intakes from the other reservoirs.

In most situations, releases and flow from the other reservoirs have minimal impact to the overall result.

Water storage measurements

We measure the volume of our water storages, flow and water use. 

Volumes are measured over a 24 hour period to 8am each day and are validated at the end of each week.

We refer to dam storage levels, but we do not report on the level of the storages, only the volume of each reservoir.  Rainfall is measured by depth.

  • GL Gigalitre - billion litres (1GL = 1ML x 1000)
  • ML Megalitre - million litres (1ML = 1KL x 1000)
  • KL Kilolitre - thousand litres (1KL = 1L x 1000)

It is possible for a reservoir to measure more than 100% full.  This is because the reservoir has a set operating capacity that we report against, but the reservoir can hold more water.  This is similar to how a bottle can be filled to the brim to go over its labelled capacity.

Similarly, there is an optimal minimum operating capacity.  The minimum level is set at each reservoir and is dependent on factors such as water quality, operational factors, infrastructure (eg towers, pumps, tunnels), 'head' and pressure for gravity systems, water hammer and environment (eg erosion).

Dam storage levels

The storage level section reports how much water Melbourne has in our ten major storage reservoirs.

Breakdown by reservoir

Information available for each reservoir includes:

  • reservoir – links to a webpage with further information about the reservoir
  • total capacity – maximum capacity (millions of litres)
  • current volume – amount currently stored (millions of litres), including changes since the previous day and previous year
  • percentage full – current volume as a percentage of total capacity
  • rainfall recorded – millimetres of rain in the 24 hours to 8am (see ‘Average catchment rainfall’ for further information)

Daily volume data is based on the best available information, which we periodically validate by comparing it with manual readings taken at the reservoirs. As a result we may sometimes adjust the figures.

We do not normally update data retrospectively, so there may be discrepancies from day to day.  If data is updated we will include a message to indicate the changes and the date they were made.

Any water from the desalination plant will be included in the figures for Cardinia Reservoir.

Total storages over the years

Figures are expressed as a percentage of the combined capacity of the storage reservoirs at the time.  See the table below to find out when reservoirs have come online.

The graph shows a repeated trend of storage levels decreasing during the first half of the year, then increasing or stabilising during the second half of the year when inflows to the reservoirs are more frequent.

Changes to water storage reporting since the 1940s

Total capacity has changed over the years as reservoirs were built, upgraded, brought under Melbourne Water's responsibility or disconnected from the water supply system. eg Tarago has only been included in Melbourne Water's reporting since 1991, but it has been operational since 1969. 

​Date Change​ Total capacity (ML)​
​16 Sep 1957 Upper Yarra Reservoir comes online​ 296,230​
13 Aug 1971​ Greenvale Reservoir comes online​ 323,230​
7 Sep 1973​ Cardinia Reservoir comes online​ 610,230​
19 Jul 1980​ Sugarloaf Reservoir comes online​ 706,230​
31 Jul 1984​ Thomson Reservoir comes online ​ 1,774,230​
1 Jan 1986​ O’Shannassy Reservoir's total volume decreases from 4,230ML to 3,000ML 1,773,000​
10 Sep 1991​ Tarago and Devilbend reservoirs included due to water authorities merging 1,825,000​
1 Jan  1999​ Tarago Reservoir taken offline 1,787,500​
1 Jun 2002 Devilbend Reservoir decommissioned​ 1,773,000​
24 Jun 2009​ Tarago Reservoir reconnected to the system following major upgrade​ 1,810,500​
1 Jul 2010​ Reporting aligned with Bureau of Meteorology (previously we reported to the nearest 1,000 ML for most reservoirs)​ 1,812,175​
​1 Jan 2012 Sugarloaf Pipeline allocation removed (water held in Lake Eildon on Melbourne's behalf), decreasing total volume by 12.3 billion litres 1,812,175

Average catchment rainfall

Validated rainfall data is provided for the four major catchments only – Thomson, Upper Yarra, O’Shannassy and Maroondah – which supply most of Melbourne’s drinking water.  Rainfall in other catchments has little impact on the total volume of Melbourne’s water storages.

The daily report includes data for:

  • rainfall in the 24 hours to 8am today (millimetres) – reporting times are based on Melbourne Water standards
  • daily average rainfall for the current month (millimetres)
  • monthly rainfall recorded to date (millimetres)
  • average rainfall for the month (millimetres), based on data from the previous 30 years (1985 - 2014)

The weekly report includes data for:

  • this week's rainfall
  • the long term average for the week, based on data from the previous 30 years (1985-2014)
  • the same week last year

Rainfall is calculated as a weighted average. 

You can also view unverified hourly data automatically sent by our electronic rain gauges. This is available for all other catchments and monitoring stations:

Flow into dams

Flow into dams is the amount of water flowing into our four major reservoirs -Thomson, Upper Yarra, O'Shannassy and Maroondah. This includes rivers, streams and rainfall run-off.

The daily report includes data for:

  • flow for the last 24 hours to 8am today (ML)
  • daily average flow for the current month
  • month to date flow
  • average flow for the month, based on data from the previous 30 years (1985-2014)

The weekly report includes data for:

  • this week's total flow
  • the long term average, based on data from the previous 30 years (1985-2014)
  • the same week last year 

The streamflow for Maroondah is calculated by the flow from teh Watts River and Graceburn diversion.

While ups and downs are a constant feature, the average inflow dropped rapidly by almost 40% during the 1997-2009 drought. This decrease included a devastating drop in 2006, and was consistent with CSIRO predictions.

Water use for Melbourne

Daily data on Melbourne’s total water use includes consumption by businesses and households, as well as water supplied to Western Water. Data is available for the:

  • 24 hours to 8am today (millions of litres)
  • daily average for the month (millions of litres), based on the previous five years (2010-2014)

The weekly water update only includes households, and does not include water supplied to Western Water. This week's data is reported as an average consumption per person per day.

Water use does not include releases or other water losses.

Annual water use figures are in our Annual Report:

Changes to water use reporting

Water use reporting has varied over the years. It originally included all water usage for the Melbourne metropolitan area, which has expanded over time to include suburbs on the urban fringe of Melbourne.

Other reporting changes:

  • March 2000 – June 2012 – water supplied to Western Water was included in the weekly water update
  • 2009-2011 – the Victorian Government ran its Target 155 program, which encouraged Victorians to conserve water.  It compared the average per person consumption to the target usage of 155L.  The same reporting mechanisms are still in place
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