9.4 Runoff to surface water
Runoff to surface water was estimated based on the AWRA-L (Australian Water Resources Assessment system Landscape) model version 2.0.0 (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 2011–12 year and provided a suitable representation of the runoff. A comparison against flows for the 2011–12 water year was unable to be completed in time for the 2012 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.
In the 2011 Account, the AWRA-L v1.0.0 model was used to calculate rainfall runoff. In the 2012 Account, the AWRA-L v2.0.0 model was used to calculate rainfall runoff. This new method was also applied to retrospectively calculate runoff to surface water for the 2010–11 year.
This resulted in a material increase (approximately 22%) to the volume reported in the 2011 Account. Consequently, the volume has been restated from 3,222,163 ML (as published in the 2011 Account) to 3,944,083 ML.