The unaccounted-for difference is the difference between total groundwater outflows (including groundwater extractions) and total groundwater inflows measured during 2009-10, as shown in the table below.
| 
 | Account | Volume (ML) | 
|---|---|---|
| 
 | Opening balance of groundwater store (as at 1 July 2009) | - | 
| add | Total of 15 Groundwater inflows | 1,800,124 | 
| minus | Total of 16 Groundwater outflows | 1,812,521 | 
| minus | Closing balance of groundwater store (as at June 30 2010) | - | 
| 
 | Unaccounted-for difference | -12,397 | 
Because the volume of water in the groundwater stores within the Perth region could not be quantified accurately, the volume disclosed in the unaccounted-for difference (-12,397 ML) represents the net change in groundwater store throughout the reporting period.