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Perth

                                                                                                   

30.2 Unaccounted-for difference—connected surface water store

                             

Supporting information   


The volume disclosed in the water account statements represents the difference between the measured opening and closing balances of surface water assets, after physical inflows and outflows have been applied.

The unaccounted-for difference is calculated according to the table below:

 

 

Account

Volume (ML)

 

Opening balance of connected surface water store (as at 1 July 2009)

223,775

add

Total of 13 Connected surface water inflows

1,239,241

minus

Total of 14 Connected surface water outflows

1,103,099

minus

Closing balance of connected surface water store (as at 30 June 2010)

253,878

 

Unaccounted-for difference

106,039

 

It is likely that the unaccounted-for difference is primarily attributed to errors associated with the following line items:

  • 13.4.1 Runoff to connected surface water – the estimated volume of rainfall-runoff to rivers, a large component of the total runoff value, was derived from rainfall-runoff models. A reasonable error for such hydrology models is 10-20%, compared to actual observed data, which would account for the entire unaccounted-for difference value.
  • 14.2 Surface water flow out of region – the surface water outflow to the sea is assumed to be equal to the volume of discharge measured at the most downstream station along a river. Therefore, there is no adjustment made for the contributing area below the gauging station to calculate the outflow. Outflow to sea is likely to be underestimated by about 10-20%, which would also account for the unaccounted-for difference.