9.4 Runoff to surface water
The volume reported (598,040 ML) represents runoff into surface water features including rivers and major storages in the Adelaide region during the 2010–11 year.
Rainfall runoff to surface water was estimated based on the AWRA-L version 1.0.0 (Van Dijk 2010) model output – streamflow.
Using climate grid data for the Adelaide 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 was considered; therefore, the surface areas of the major storages and the off-channel water storages (local catchment reservoir e.g. farm dams) were excluded from the analysis.
Runoff from the landscape is divided into two components: runoff into the surface water (surface water storages and weirs, rivers and drains) and runoff into off-channel water storages. Only runoff into the surface water store was considered here because for the National Water Account, off-channel water storages are not included in the definition of surface water.
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 storages).
Finally, runoff into off-channel water storages (estimated using the STEDI model; see Sinclair Knight Merz 2010) was subtracted from the total modelled runoff into the surface water to establish the rainfall runoff into surface water storages, weirs and rivers only.
Assumptions, Limitations, Caveats and Approximations
- Runoff estimates were compared to flows at unimpaired catchments within the Adelaide region for the 2010–11 year and found to be a reasonable representation of the runoff in this region for this year
- Runoff estimates were subject to the assumptions of the AWRA-L model detailed by Van Dijk (2010).
- Where the volume of water intercepted by the off-channel water store has been calculated, the runoff estimates inherit the approximations, assumptions and caveats of the STEDI local catchment reservoir model and the associated parameters used.
This line item corresponds to 13.4.1 Rainfall-runoff to connected surface water, reported in the 2010 Account. Following publication of the 2010 Account, an error was identified in the volumes reported and improved data sets were made available. Restatement of the volume published in the 2010 Account was made as the difference was material and increased the accuracy of the information provided to the users of the National Water Account. The following summarises the changes that were made that caused the value to be restated for the 2009–10 year:
- In 2010 a bias correction applied to the satellite-derived solar radiation dataset was incorrect resulting in an overestimation of incoming solar radiation. When the bias correction was updated and rainfall runoff was re-estimated, the comparative year volume reported for rainfall runoff increased.
- In 2010, the WaterDyn v26 and AWRA-L v0.5 models were used to estimate rainfall runoff. In the 2011 Account the AWRA-L v1.0.0 model was used to calculate rainfall runoff, which resulted in an increase to the comparative year volume reported for rainfall runoff.
These changes and their respective volumes are detailed in the following table. The restated comparative year volume is 483,276 ML.
|2010 Account line item name||Volume at 30 June 2010 reported in 2010 Account (ML)||Value difference to volume reported due to bias error correction (ML)||Value difference to volume reported due to use of AWRA-L v1.0.0 model (ML)||Volume at 30 June 2010 reported in 2011 Account (ML)|
|13.4.1 Rainfall- runoff to connected surface water