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:

Calculation of unaccounted-for difference for the 2010–11 year

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 volumes of the water stores of the Ord region for the 2010–11 year

Balancing item

Volume (ML)

Balancing item – surface water store


Balancing item – groundwater store


Unaccounted-for difference


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.

Restatement of comparative year information for the unaccounted–for difference

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



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.