Perth: Statement details
Well below-average rainfall during the 2015–16 year, particularly during July–December, contributed to very low annual streamflows across the region. Surface water storage dropped from 31% to 26% full and aquifer storage decreased for the second successive year.
Water assets
Surface water assets
The surface water asset volume in the Perth region at 30 June 2016 was 167,165 ML, which represents the volume of water held in storages. The location of each storage within the region and the volume of water in each storage as a percentage of total storage capacity at the end of the year is shown in Figure S1.
The volume of water in rivers, lakes and wetlands could not be quantified due to a lack of available data; however, the volume of water held in rivers and lakes is considered to be relatively small compared to the volume held in storages.
Figure S1 Percentage-full volume on 30 June 2016 for each storage
Total surface water storage was reduced to 26% capacity during the 2015–16 year, reflecting the below-average rainfall and streamflow conditions within the region. Operational requirements for the Perth region's Integrated Water Supply Scheme may have mitigated the effects of drier climatic conditions experienced during the year. Storage volumes include a portion of groundwater and desalinated water transferred from the urban system. These transfers mainly occur to store winter production of desalinated water and groundwater and to prepare for summer peak demand. During the 2015–16 year, these transfers were much larger than previous years, minimising the impact of below-average rainfall and streamflow conditions.
Total surface water storage volume at 30 June 2016 compared with the previous six years is given in Figure S2.
Figure S2 Total storage volume in the region at 30 June 2016 compared with the previous 6 years
Groundwater assets
The aquifers in the Perth region are described in the Groundwater section of the 'Region description'. While information exists on water movement to and from the aquifers (see Groundwater store note), volumes of water stored in the aquifers could not be quantified.
Information on the total annual change in aquifer storage volume is available, which provides an indication of whether groundwater levels in the region's aquifers are increasing or decreasing. Figure S3 shows the annual change in aquifer storage since 2011. Aquifer storage during the 2015–16 year decreased for the second successive year, reflecting the relatively poor rainfall experienced across the region over the last two years.
Figure S3 Total annual change in aquifer volume in the region at 30 June 2016 compared with the previous five years
The Gnangara groundwater system is the largest and most important groundwater resource in the region. The Western Australian Department of Water is currently working on a method to determine the water level surface in the aquifers necessary to maintain and protect groundwater-dependent ecosystems and the saltwater interface (the 'extractable limit' water level surface). Areas of the Gnangara system where the aquifer levels are above this extractable limit are considered to be a water asset; areas where the aquifer levels are below this limit are considered to be a water liability to the environment (i.e. water is owed back to the aquifer to raise it to the extractable limit).
Water liabilities
Water liabilities in the Perth region refer to the volume of allocation remaining on licence entitlements at the end of the 2015–16 year for:
- surface water: individual users—8,520 ML
- groundwater: individual users—107,255 ML.
Most of the water supply licences for the region do not have a water management year that ends on 30 June. Therefore, the volume of allocation remaining at the end of the 2015–16 year for the Perth region is the unused component of the annual allocation for these licence entitlements (tables S1–S2). The allocation (water liability increases), allocated abstraction (water outflow), and the adjustment and forfeiture (water liability decreases) for each licence entitlement is also provided.
A more detailed description of water allocations and associated water rights in the Perth region is given in the Water rights note.
| Volume ML | |
| Opening balance at 1 July 2015 | 8,437 |
| Allocation | 38,842 |
| Allocated diversion | (38,709) |
| Adjustment and forfeiture | (50) |
| Closing balance at 30 June 2016 | 8,520 |
| Volume ML | |
| Opening balance at 1 July 2015 | 109,093 |
| Allocated | 306,671 |
| Allocation extraction | (299,754) |
| Adjustment and forfeiture | (8,755) |
| Closing balance at 30 June 2016 | 107,255 |
Water asset increases
Total water asset increases for the region (1,865,375 ML) comprised:
- surface water increases—373,338 ML
- groundwater increases—1,212,581 ML
- urban water system increases—279,456 ML.
The key water asset increases for the region were runoff (surface water increase) and recharge: landscape (groundwater increase), which made up approximately 20% and 60% of the water increases, respectively. Both these flows are climate-dependent and were less than the previous year, reflecting the reduced rainfall experienced across the region (see Climate and water).
Urban water system increases primarily represents wastewater collected and delivery: desalinated water. The volume of wastewater collected changes little from year to year; desalinated water has increased over the last 5 years (see Urban water store note).
Water asset decreases
Total water asset decreases for the region (1,386,120 ML) comprised:
- surface water decreases—206,658 ML
- groundwater decreases—735,954 ML
- urban water system decreases—411,311 ML
- irrigation scheme decreases—32,197 ML.
The key water asset decreases for the region were outflow (surface water decrease), discharge: landscape, and inter-region coastal outflow (groundwater decreases). Combined, these flows made up around 60% of the total water asset decreases. These flows are very climate-dependent and were less than the previous year which, similar to water asset increases, reflects the reduced rainfall experienced across the region (see Climate and water).
Urban water system decreases primarily relate to supply system delivery: urban users; irrigation scheme decreases represent the delivery of water to scheme customers. The delivery of water from these systems are less climate-dependent than surface water and groundwater decreases, and generally change little from year to year.
Water flows
Total water inflows to the region (1,865,375 ML) was equal to the volume of Water asset increases above.
Total water outflows from the region (1,724,583 ML) was higher than the volume of Water asset decreases (1,386,120 ML) because the volume of water outflows also includes the flows corresponding to the accrual transactions; that is, the allocated water diversions and extractions (see the Water liabilities section above).
A detailed description on all the water inflows associated with the surface water store, groundwater store, urban water system and irrigation scheme are provided in the Water stores note.
Unaccounted-for difference
The volume recognised in the water accounting statements (175,989 ML) represents the total unaccounted-for difference for the Perth region for the 2015–16 year.
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 Table S3.
| Volume ML | |
| Opening water storage balance at 1 July 2015 | 202,362 |
| Total inflows | 1,865,375 |
| Total outflows | (1,724,583) |
| Closing water storage balance at 30 June 2016 | (167,165) |
| Unaccounted-for difference | (175,989) |
The unaccounted–for difference can also be calculated by adding 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 Table S4.
| Volume ML | |
| Surface water store | (371,105) |
| Groundwater store | 193,098 |
| Urban water system | 454 |
| Irrigation scheme | 1,564 |
| Unaccounted-for difference | (175,989) |
Table S4 shows that the unaccounted-for difference volume is primarily explained by the balancing item of the surface water and groundwater store. The surface water balancing item is attributed to uncertainty associated with runoff and river outflow, and the volume of river and floodplain losses not being able to be quantified (see Surface water store note).
The groundwater assets of the Perth region could not be quantified (see Groundwater store note). As a result, the calculation of the unaccounted-for difference in the first table above does not include opening or closing groundwater storage. Therefore, the groundwater balancing item volume of 193,098 ML represents the difference between known groundwater inflows and known groundwater outflows during the 2015–16 year, as well as any actual unaccounted-for differences when reconciling the terms.