Calculating water storage data

Find out how we calculate our water storage levels and why we report data in a particular way.

Aerial shots of Silvan Reservoir
Aerial shots of Silvan Reservoir

Check out the data available:

  • water storage levels at all ten reservoirs
  • rainfall at our four water catchment areas
  • flow into our four major reservoirs
  • water use for all of Melbourne
  • historical reservoir levels and inflows from when we began data collection.

Apart from rainfall, which is measured by depth, we report all water data by volume:

  • gigalitres (GL) — billion litres (1GL = 1000ML)
  • megalitres (ML) — million litres (1ML = 1000KL)
  • kilolitres (KL) — thousand litres (1KL = 1000L)

Data is calculated at 8 am each day, with our water storage levels page updated between 2-3 pm on weekdays, and 4-5 pm on weekends and public holidays.

Customise the data

You can customise what’s displayed on the total water storages graph, including:

  • choosing between daily or weekly data
  • focusing on a specific date
  • hovering over a date to view the storage volume figure.

Selecting a date will display summary data in the tabs below the graph. If you’re viewing weekly data, you’ll also see the weekly water report summarising key highlights.

Selecting a tab will display more detailed information that can be displayed in tables and charts, or downloaded in .CSV format.

O'Shannassy Reservoir
O'Shannassy Reservoir

Understanding the data

Water storage levels

Melbourne’s total water storage level is based on the combined capacity of our ten reservoirs. Changes to this figure are calculated by subtracting all reservoirs’ incoming flows from total water use — including water consumption, releases to the environment and water held for Southern Rural Water.

Reservoirs' daily volume data is periodically checked against readings taken in person. As a result we sometimes adjust the figures, but do not usually update past data  — so there can be discrepancies.

Storage levels occasionally go over 100%, meaning the reservoir is holding more than its set operating capacity (the figure we report against). This is similar to bottles exceeding their labelled capacity when they are filled to the brim.

Historical storage levels show the changes in Melbourne’s total water storage capacity as reservoirs were built, upgraded, brought under Melbourne Water’s responsibility or disconnected from the water supply system. This dramatically impacts storage levels, which are reported as a percentage of total capacity.

Rainfall at water storages

Verified rainfall data is only available for the four major catchments that supply most of Melbourne’s drinking water, as rainfall elsewhere does not significantly impact our storages. However, you can still view unverified hourly rainfall and river level data from our electronic rain gauge network.

Flow into water storages

Incoming flows are reported in detail for only the four major reservoirs. Flows into other reservoirs are used to calculate changes in the total water storages level, but these usually have little impact.

Water use

Daily and weekly data is based on household and business water usage. However, the changes in the total water storage figure also factor in environmental water releases, water supplied to Western Water and other water losses.

Yearly water use figures are in our annual report.

Last updated:
2 October 2020