Melbourne

The volume recognised in the water accounting statements (1,005,902 ML) represents surface run-off into the connected surface water store. Connected surface water store includes all river channels and reservoirs. The figure represents run-off from the whole Melbourne region.

Data source

Bureau of Meteorology, National Climate Centre (NCC): daily climate grids (rainfall, temperature and solar radiation); Australian Hydrological Geospatial Fabric (AHGF), waterbody feature class.

Commonwealth Scientific and Industrial Research Organisation (CSIRO): WaterDyn and AWRA-L model parameters; monthly climatological average radiation grid data.
Geoscience Australia, southwest Western Australia: human-made waterbody feature class.

Data provider

Bureau of Meteorology.

Method

Run-off was calculated as the average ‘discharge’ from the CSIRO WaterDyn water balance model and ‘streamflow’ from the CSIRO AWRA-L water balance model.

Using climate grid data for the Melbourne region (including precipitation, temperature and solar radiation data), WaterDyn and AWRA-L were used to estimate the run-off depth at each grid-point within the region. Only run-off from the landscape is considered; therefore, the surface areas of the major reservoirs and local catchment reservoirs were excluded from the analysis.

Run-off from the landscape is divided into two components: (1) run-off into the connected surface water store (major reservoirs, rivers and drains); and (2) run-off into local catchment reservoirs. Only run-off into the connected surface water store is considered here.

The average run-off depth from the landscape into the connected surface water store was determined as the unweighted arithmetic mean of the relevant grids falling within the region boundary. Mean run-off depth was converted to a run-off volume by multiplying average run-off depth by the total area of the region (excluding reservoirs).

Uncertainty

Modelled data. Uncertainty is ungraded.

Approximations, assumptions, caveats/limitations

• The estimated run-off into rivers was not verified by real-time analysis of streamflow records. The reported value was an estimate of water that was likely to have entered the connected water store from the landscape.
• The run-off estimates were subject to the assumptions of the WaterDyn model detailed in Raupach et al. (2008) and the AWRA-L model detailed in van Dijk (2010).
• The estimated run-off corresponds to the run-off expected from an unimpaired catchment. The impairment on run-off from local catchment reservoirs is estimated using a local catchment reservoir water balance model (STEDI). Where this is applied, the run-off estimates inherit the approximations, assumptions and caveats of the water balance model (STEDI) and parameters used.
• Grid-cells that intersected the reporting region boundary (i.e. had some part of the cell outside the region boundary) were included in the calculation of total run-off. This had limited influence on the calculation of total run-off because the area by which average run-off was multiplied was determined from the region boundary, not from the grid-cells themselves.
• Melbourne Water uses a different methodology and different results in managing their reservoirs. Melbourne Water uses a mass balance approach to determine run-off into its harvesting reservoirs. This approach yields different results to that determined by the Bureau of Meteorology using the methodology described above.