Quantification approach
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 using the following table.
| |
Account |
Volume (ML) |
| |
Opening balance connected surface water store (as at 01 July 2009) |
126,334 |
| add |
Total 13 Connected surface water inflows |
449,280 |
| minus |
Total 14 Connected surface water outflows |
301,461 |
| minus |
Closing balance connected surface water store (as at 30 June 2010) |
151,256 |
| |
Unaccounted-for difference |
122,897 |
The unaccounted-for difference is equal to approximately 28% of the inflows and outflows of the connected surface water store.
It is likely that the unaccounted-for difference is primarily attributed to errors associated with the following line items:
- 13.2 Connected surface water flow from outside region: the averaging of two stream gauges to determine the inflow from upstream may have resulted in inaccuracies in the estimate.
-
- 13.4.1 Runoff to connected surface water: this value is based on modelled data only and was not validated against observed streamflow records. A reasonable error for such hydrology models is 10–20% (compared to observed data), which would account for the majority of the unaccounted-for difference value.
-
- 14.2 Surface water flow out of region: the surface water outflow from the region is assumed to be equal to the volume of discharge measured at the most downstream station (Halls Crossing, Site ID 410777). There is no adjustment made for the contributing area below the gauging station which also falls within the region boundary. This Therefore, the surface water flow out of region is likely to be underestimated. There is also potential for gauging error at the gauging station, which would introduce error in the calculations.