Sydney
25.1 Unaccounted-for difference

In the Sydney region, the unaccounted-for difference is 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. It is calculated according to the following table.

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

	Account	Volume (ML)
	Opening water storage (30 June 2010)	1,709,649
add	Total water inflows	4,915,014
minus	Total water outflows	4,165,258
minus	Closing water storage (30 June 2011)	2,241,310
	Unaccounted-for difference	218,095

The unaccounted-for difference of 218,095 ML represents 9% of the total water storage for the Sydney region. The unaccounted-for difference in the Sydney account found in both the urban water system and the surface water can be attributed to:

• modelling errors, particularly in reference to line item 9.4 and line item 17.2
• uncertainty in estimating leakage (as in line item 19.2)
• errors in measurement equipment and/or meter accuracy on gauging stations and flow meters.

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 volumes for the Sydney region in the 2010–11 year

Balancing item	Volume (ML)
Balancing item - surface water store	167,268
Balancing item - urban water system store	50,827
Unaccounted-for difference	218,095

Detail of the balancing items calculation is given in the following Resources and systems notes:

• Surface water note (Table 3)
  
• Urban system: System's interactions with the region note (Table 7)

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
43,368	(364,542)	Methodology change: several of the physical water inflows and outflows for the Sydney region were restated due to new models and revised datasets used.