South East Queensland
9.4 Runoff to surface water
The volumetric value for the line item at the end of the 2011–12 year was 5,673,771 ML.
This line item consists of the runoff to all surface water (including river channels downstream of surface water storages listed in line item 1.1 Storages).
The volume of runoff to surface water was significantly less during the 2011–12 year (5,673,771 ML) than the 2010–11 year (12,286,627 ML). Further information on the rainfall within the region can be found in the Climate overview.
Once the receiving landscape has become saturated during a rainfall event all additional rainfall results in runoff. A number of small rainfall events during the year, with the landscape drying in between, resulted in considerably less runoff than one or two large rainfall events with the same overall rainfall volume.
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.
Bureau of Meteorology.
Runoff to surface water was estimated based on the Australian Water Resources Assessment system Landscape model (AWRA-L) version 2.0.0 Van Dijk (2010) streamflow model outputs.
Using climate grid data for the South East Queensland (SEQ) 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 connected surface water store 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 reservoirs).
Assumptions, limitations, caveats and approximations
The estimated runoff was compared against historical flows at unimpaired catchments within the SEQ region for the 2011–12 water year and provided a suitable representation of the runoff for this year.
The runoff estimates were subject to the assumptions of the AWRA–L model detailed in Van Dijk (2010).
This line item corresponds to line item '9.4 Runoff to surface water' reported in the 2011 Account. There was a method change in the 2012 Account.
In the 2012 Account, the following change was made that caused the 2010–11 year value to be restated:
- The methodology used to quantify the line item was improved and resulted in a material change in volume.
The volume of runoff estimated for the comparison year for the 2012 Account (12,286,627 ML) is less than the volume reported for the 2011 Account (12,694,060 ML). This was due to a change in modelling methods. The difference between the previously reported estimate of runoff and the estimate produced for the comparison year can be attributed to the choice of the AWRA-L v2.0.0 model instead of the previously used AWRA-L v1.0.0. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 407,433 ML represents a change of approximately –3.2% of the previously reported value.