Ord
25.1 Unaccounted-for difference - surface water
Supporting Information
The unaccounted–for difference represents the volume necessary to reconcile the opening water storage and closing water storage with the total water inflows and total water outflows reported in the water accounting statements: physical water flows. It is calculated according to the following table:
| Account | Volume (ML) | |
|---|---|---|
| Opening water storage (30 June 2010) | 8,824,962 | |
| add | Total water inflows | 38,635,928 |
| less | Total water outflows | 20,369,314 |
| less | Closing water storage (30 June 2011) | 15,148,491 |
| Unaccounted-for difference | 11,943,085 |
The unaccounted–for difference can also be calculated by summing the volumes necessary to reconcile (balance) the opening and closing storage with the water inflows and outflows of each of the separate water stores of the region, as shown in the following table:
Balancing item |
Volume (ML) |
Balancing item – surface water store |
11,943,085 |
Balancing item – groundwater store |
0 |
Unaccounted-for difference |
11,943,085 |
The unaccounted–for difference volume is entirely attributed to the balancing item of the surface water store. Detail of the balancing item's calculation is given in Table 2 of the Surface water note.
The volume of water in the groundwater stores within the Ord region could not be quantified. The Department of Water, in managing groundwater in the region, estimate that the volume of recharge to groundwater was equivalent to the volume of extraction. Therefore the volume of the balancing item for the groundwater store was quantified as 0 ML.
Comparative year
Substantive changes in the unaccounted–for difference for the 2009–10 year reported in the 2011 Account with the corresponding value reported in the 2010 Account are presented in the following table.
2011 Account volume for the 2009–10 year (ML) |
2010 Account volume for the 2009–10 year (ML) |
Change from the 2010 Account to the 2011 Account |
2,575,746 |
4,352,385 |
Methodology change: several of the physical water inflows and outflows for the Ord region were restated due to new models and revised datasets used |
In the 2010 Account, the surface water balancing item was the biggest contributor to the unaccounted-for difference reported for the 2009–10 year. That volume was attributed to inaccuracies in the estimation of rainfall-runoff, which is the largest contributor to total surface water inflows.
Improvements in the rainfall-runoff model used in the 2011 Account (see line item 9.4 Runoff to surface water) is the main reason the restated value of unaccounted-for difference for the 2009–10 year has reduced from that reported in the 2010 Account.