9.4 Runoff to surface water
Runoff to surface water was estimated based on the Australian Water Resources Assessment system Landscape (AWRA-L) Version 3.0 model streamflow outputs (Van Dijk 2010).
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 2012–13 year and provided a suitable representation of the runoff.
- 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.
The comparative year volume for line item 9.4 Runoff to surface water was restated in the water accounting statements from the 2012 Account due to an error identified in the method used to calculate this volume. Please refer to Restatement of comparative year information in the 'Water accounting policies' for more information.