Melbourne
9.4 Runoff to surface water

Supporting Information

The volume presented in the water accounting statements (3,222,163 ML) represents runoff from the landscape into rivers and storages in the Melbourne region.

Runoff was much higher in the 2010–11 year compared to the 2009–10 year (1,388,336 ML). This was attributed to very high rainfalls recorded in the region, which resulted in high soil moisture.

Quantification Approach

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): AWRA-L model parameters; monthly climatological average radiation grid data.

Provided by

Bureau of Meteorology.

Method

Runoff to surface water was estimated based on the AWRA-L version 1.0.0 (Van Dijk 2010)  model streamflow outputs

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

The average runoff depth from the landscape into the surface water was determined as the weighted mean of the relevant grid- points within the region boundary. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages).

Assumptions, Limitations, Caveats and Approximations

  • Estimated runoff was compared against historical flows at unimpaired catchments within the Melbourne region for years prior to the 2010–11 year and provided a suitable representation of the runoff. A comparison against flows for the 2010–11 water year was unable to be completed in time for the 2011 Account.
  • Runoff estimates were subject to the assumptions of the AWRA-L model detailed in Van Dijk (2010).
  • The method represents runoff from the landscape into rivers and storages in the region. The volume of inflows to storages is only a portion, and could not be separated from the overall runoff estimate.

 

Uncertainty Information

Uncertainty estimates were not available for the modelling method.

Comparative year

This item corresponds to line item 13.4.1 Rainfall runoff to connected surface water reported in the 2010 Account. In the 2011 Account, changes in methodology caused the 2009–10 year value to be restated. In the 2010 Account, the WaterDyn v26 and AWRA-L v0.5 models were used to estimate rainfall runoff. In the 2011 Account the AWRA-L v1.0.0 model was used to calculate rainfall runoff. This resulted in a material change (approximately 38%) to the volume reported in the 2010 Account. 

This change and the respective volumes are detailed in the following table. The restated comparative year volume is 1,388,336 ML.


Restated comparative year information for line item 9.4 Runoff to surface water

2010 Account line item name

Volume at 30 June 2010 reported in 2010 Account (ML)

Value difference to volume reported due to method change (ML)

Volume at 30 June 2010 reported in 2011 Account (ML)

13.4.1 Rainfall runoff to connected surface water

1,005,902

382,434

1,388,336