South East Queensland
9.4 Runoff to surface water
This line item consistes 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 more than three times greater during the 2010–11 year (12,694,060 ML) than the 2009–10 year (3,714,190 ML). This was due to the 2010–11 year being significantly wetter, with major flood events occurring in December 2010 and January 2011. 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, will result in considerably less runoff than one or two large rainfall events with the same overall rainfall volume. The wet conditions during the 2010–11 year and major flood events contributed to the large runoff volumes.
Rainfall runoff to surface water was estimated based on the AWRA–L version 1.0.0 stream flow 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 2010–11 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 13.4.1 Runoff to connected surface water reported in the 2010 Account. There was a method change in the 2011 Account.
The following table compares the value at 30 June 2010 reported in the 2011 Account with the corresponding value reported in the 2010 Account.
|2011 Account line item||2010 Account line item||Volume reported in the 2010 Account
|Prior period error correction
|Volume difference due to scope change
|Restated volume reported in the 2011 Account
|9.4 Runoff to surface water||13.4.1 Runoff to connected surface water||2,722,267||0||991,923||3,714,190||Method for calculating runoff changed|
It was recognised that the volume of runoff estimated for the comparison year (3,714,190 ML) was greater than the volume reported for the 2010 Account (2,722,267 ML). This was due to a change in modelling methods. The difference between the prior estimate of runoff and the estimate produced for the comparison year can be attributed to the choice of the AWRA–L v1.0.0 model instead of the previously used average of WaterDyn v26 and AWRA–L v0.5. The difference of 991,923 ML represents a change of approximately 36% of the previously published volume. This was considered material and therefore a comparison year volume of 3,714,190 ML was presented in the 2011 Account.