South East Queensland
9.4 Runoff to surface water

The volumetric value for the line item at the end of the 2012–13 year was 7,182,219 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 higher during the 2012–13 year than the 2011–12 year due to flood events occurred at the end of January 2013. 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 (the Bureau).

Runoff to surface water was estimated based on the Australian Water Resources Assessment system Landscape model (AWRA-L) version 3.0 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).

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).

There was a prior period error in the line item volume reported in the 2012 Account.  The prior period error was a result of using solar radiation data with bias correction errors. Solar radiation is an input for line item volume calculation.  The line item volume for the 2011-12 year has been recalculated accommodating the prior period error correction. As a result, the volume for the 2011-12 year has been changed from 5,727,597  ML reported in the 2012 Account to 5,874,622  ML in the 2013 Account.