Perth: Statement details
- A wet July-August period contributed to high runoff and storage inflows across the region.
- Surface water storage volumes in October 2018 were the highest in more than 10 years.
For further information on the region's water accounting statements scroll down this page or click on the links below:
Water assets
Surface water assets
- Total surface water asset volumes in the Perth region at 30 June 2019 were 415,416 ML, 99% of which is water held in storages. The remaining amount is water held in rivers.
Figure S1 Total storage volume in the region at 30 June 2019 compared with the previous 9 years
- Storage volumes increased from 45% full at 30 June 2018 to 47% full at 30 June 2019. This is the third consecutive increase in storage volume.
- Despite below-average annual rainfall in 2018–19, the increase in storage volume was influenced by high storage inflows following the wettest winter in the region since 2000.
- The increase in storage volume over the last three years is also attributed to lower diversions and increased 'banking' of desalinated water in the storages. The Water Corporation's reduced reliance on surface water in recent years is a result of their adoption of a range of demand management strategies and continued development of climate-independent water sources in response to the long-term decline in storage inflows.
Figure S2 Percentage-full volume on 30 June 2019 for each storage and total daily storage volume from 2013–2019 (inset)
- Storage levels peaked in October 2018 at around 66% full following high winter rainfall. This was the highest storage volume in the region in more than 10 years.
- About 60% of the surface water stored in the region was for urban supply and 40% for irrigation.
- More detailed information on the individual storages within the region is available on the Bureau of Meteorology's Water Storage website.
Groundwater assets
- Volumes of water stored in aquifers cannot be quantified due to a lack of available data; however, information on the total annual change in aquifer storage is available.
Figure S3 Total annual change in aquifer volume in the region at 30 June 2019 compared with the previous 8 years
- The high rainfall during the 2018 winter contributed to an increase in groundwater storage during 2018–19.
- This was the third consecutive year of increasing aquifer storage within the region.
Water liabilities
- Total water liabilities for the Perth region was 151,531 ML, which refers to the volume of allocation remaining on licence entitlements at the end of the 2018–19 year.
| Description | Individual users (ML) | Urban system (ML) | Irrigation scheme (ML) | |
| Opening balance at 1 July 2018 | 8,522 | 0 | 38,141 | |
| add | Allocation | 43,599 | 247,950 | 113,333 |
| less | Allocated diversion | 43,599 | 107,811 | 74,809 |
| less | Adjustment and forfeiture | 0 | 140,139 | 36,031 |
| Closing balance at 30 June 2019 | 8,522 | 0 | 40,634 |
| Description | Individual users (ML) | Urban system (ML) | |
| Opening balance at 1 July 2018 | 102,375 | 0 | |
| add | Allocation | 310,525 | 150,052 |
| less | Allocation extraction | 310,525 | 122,318 |
| less | Adjustment and forfeiture | 0 | 27,734 |
| Closing balance at 30 June 2019 | 102,375 | 0 |
- Many of the water supply licences for the region do not have a water management year that ends on 30 June; therefore, the allocation remaining is the unused component of the annual allocation for these entitlements.
- A more detailed description of water allocations and associated water rights in the Perth region is given in the Water access and use note.
Water flows
- Total surface water and groundwater flows into the region's storages was around 3,245 GL. The key item contributing to surface water and groundwater inflows was runoff and recharge: landscape, which together made up around 79% of the storage inflows.
- Runoff was slightly higher than average (based on modelled data from 1971–2019), largely due to the high 2018 winter rainfall across the region. This was the second successive year of above-average runoff across the region.
- A key item contributing to surface water outflows from the region's storages was river outflow from the region, which made up more than 70% of the total surface water outflows. Although river outflow was below average, it was the highest since 2004–05, which reflects the wet conditions and high runoff across the region during the 2018 winter.
- A detailed description on all the water flows associated with the surface water and groundwater stores, as well as the urban system and the irrigation scheme, is provided in the Water stores note.
Unaccounted-for difference
- The unaccounted–for difference is the volume necessary to reconcile the opening and closing water storage with the total water inflows and outflows reported in the water accounting statements.
| Description | Volume (ML) | |
| Closing water storage at 30 June 2019 | 415,416 | |
| less | Total inflows | 3,782,929 |
| add | Total outflows | 4,019,901 |
| less | Opening water storage at 1 July 2018 | 390,226 |
| Unaccounted-for difference | 262,162 |
- The unaccounted-for difference is mostly attributed to uncertainties associated with the estimates of runoff and river losses (see Methods).
- The opening and closing water storage volumes do not include the groundwater component (only surface water) as groundwater assets could not be quantified. As a result, part of the unaccounted-for difference represents the actual change in aquifer storage. The annual change in aquifer storage in 2018–19 was 52,641 ML (see Groundwater assets), approximately 20% of the unaccounted-for difference.