Perth
Water resources and systems

The following set of notes provides consolidated reports for each of the water stores and systems within the Perth region during the 2011–12 year. The water stores and systems included in the region are shown in Figure 1.

For more information about the region, please refer to the General description section of the 'Contextual information'.

Figure 1  Schematic diagram of the water stores and systems within the Perth region

Information on all water flows to and from each water store and system are presented in the following set of notes, including between-store flows and transfers that are not presented in the water accounting statements. The between-store flows and transfers that occur in the region are presented in Figure 2.

The numbers on the diagram refer to the line item numbers in the water store notes. For each between-store flow, there are two line item numbers: one refers to flow out of a water store and the other refers to flow into a water store.

Figure 2  Schematic diagram of between-store flows that occur within the Perth region. Line item numbers are provided next to the flows.

The between-store flows and transfers (Figure 2), which are eliminated from the region's water accounting statements, are shown in italics throughout the following set of notes. The allocation announcement, forfeiture and allocation remaining line items associated with each between-store abstraction (brown arrows in Figure 2) are also shown in italics in the following notes. 

A description of the Perth region's surface water resources is provided in the Surface water section of the 'Contextual information'.

The Statement of Water Assets and Water Liabilities for the surface water store (Table 1) shows that total surface water assets and net water assets increased during the 2011–12 year in the Perth region.

The volume of water in unregulated rivers (line item 1.2), regulated rivers (line item 1.3) and lakes and wetlands (line item 1.4) 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.

The location of each storage within the Perth region and the volume of water in each storage (including dead storage) as a percentage of total storage capacity at the end of the 2011–12 year is shown in Figure 3.

Figure 3  Location map of the storages within the Perth region. The percentage-full volume on 30 June 2012 for each storage is also shown.

The water volume in almost all the storages within the Perth region at the end of the 2011–12 year is greater than that at the start (1 July 2011) (see 1.1 Storages). With the exception of four storages (Drakes Brook, Mundaring, Victoria and Wungong), storage volume within the Perth region was less than 50% of capacity on 30 June 2012 (Figure 3).

The increase in surface water storage during the 2011–12 year is primarily attributed to the increase in inflows into the storages compared to the 2010–11 year. According to the Water Corporation website, inflow into the storages during the 2011–12 year was above 80,000 ML, much greater than the inflow recorded during the 2010–11 year of less than 20,000 ML.

The Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows for the surface water store are provided in Tables 2 and 3 respectively.

A schematic diagram representing all the inflows and outflows associated with the surface water store in the Perth region is provided in Figure 4. The numbers in brackets on the diagram refer to the line item numbers in Table 3.

Figure 4  Schematic diagram of water inflows and outflows for the surface water store within the Perth region during the 2011–12 year. Line item numbers are provided in brackets.

Table 3 shows that surface water storage increased during the 2011–12 year by 49,282 ML. This is a substantial change from the previous year, which saw water storage decrease by 93,705 ML. The primary reason for the increase in water storage during the 2011–12 year is that runoff to surface water (line item 9.4) was more than five times larger than the comparison year (Table 3). The increase in runoff reflects the improved rainfall conditions observed throughout the region during the 2011–12 year (see Climate overview) relative to the record low rainfall recorded during the comparison year (see the 2011 Account).

Despite the increase in water storage during the 2011–12 year, the closing net water assets and water storage have not returned to the 2010–11 year opening balance volumes (Tables 1 and 3 respectively).

Allocation diversions

Most of the allocation diversion from surface water is to the urban water system (line item 17.12), which accounts for approximately 52% of the total allocation diversion.

Figure 5 shows that allocation diversions from storages during the 2011–12 year to individual users, the urban water system and the irrigation scheme have decreased from last year. Allocation diversions for inter-region supply are similar to the 2010–11 year.

Figure 5  Graph of allocation diversions from storages within the Perth region during the 2011–12 year and the 2010–11 comparison year. Line item numbers are given in brackets.

These allocation diversions are all associated with a water access entitlement. When an allocation is announced, a present obligation (water liability) is created on the surface water to deliver water to the users. As there is no carry-over provision in the region, the portion of the announced allocations that were not diverted by the end of the year was forfeited. This was also the case for the inter-region agreement, under which water was transferred out of the region.

The entitlement, allocation announcement and forfeiture for each of these water rights during the 2011–12 year are provided in the Surface water rights section of the 'Water access and use' note.

Balancing item

The balancing item volume represents the difference between the measured opening and closing balances of the surface water store, after physical inflows and outflows have been applied. This item is an indication of both the accuracy of the volumes reported and the degree to which the reported water flows represents a complete surface water store balance.

The balancing item is calculated according to Table 4.

The calculation of the water balance on the surface water store yielded a balancing item of 188,567 ML. This is approximately 90% of the total surface water store volume at the end of the 2011–12 year and approximately 20% of the total surface water inflows during the 2011–12 year.

It is likely that the balancing item is primarily attributed to uncertainty associated with the runoff (a large source of surface water increase, line item 9.4) and river outflow from the region (a large source of surface water decrease, line item 17.2). The runoff volume is derived from a rainfall–runoff model and it is reasonable to expect a 10–20% uncertainty around the estimated runoff volume (+/– 130,000 ML). The river outflow to sea is based on measured flow data collected at the most downstream station along a river. There is no adjustment made for the contributing area below the gauging station. As such, outflow to sea is likely to be underestimated by 10–20% (100,000 ML).

A description of the Perth region's groundwater resources is provided in the Groundwater section of the 'Contextual information'.

The aquifers in the Perth region are described in the Groundwater section of the 'Contextual information'. While good information exists on water movement to and from the aquifers (see following section on 'Changes in water store'), no value is available for the volume of water stored in the aquifers. No value could be estimated for the volume of the groundwater asset either, indicated by dashes ('–') in Table 5. The Department of Water is currently working on a method to calculate the storage volume of water in the aquifers necessary to maintain and protect groundwater-dependent ecosystems and intends to use those values to estimate groundwater asset in the future.

A more detailed explanation is provided in line item notes 2.1 Water table aquifer and 2.2 Underlying aquifers.

The Statement of Water Assets and Water Liabilities for the groundwater store is shown in Table 5.

The Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows for the groundwater store are provided in Tables 6 and 7 respectively.

A schematic diagram representing all the inflows and outflows associated with the groundwater store in the Perth region is provided in Figure 6. The numbers in brackets on the diagram refer to the line item numbers in Table 7.

Figure 6  Schematic diagram of water inflows and outflows for the groundwater store within the Perth region during the 2011–12 year. Line item numbers are provided in brackets.

Most of the extractions from the groundwater store are for licensed private use (line item 18.11), which accounts for approximately 56% of the total groundwater extraction. The majority of the allocated private water extractions are for agriculture and parks, gardens and recreational use (see line item note 22.1).

Figure 7 shows that allocation extractions from aquifers during the 2011–12 year to the urban water system have marginally decreased from last year. Estimations of private use and non-allocated extractions are similar to the 2010–11 year. Non-allocated water use refers to unlicensed domestic bores within the Perth region (see line item 18.8).

Figure 7  Graph of extractions from aquifers within the Perth region during the 2011–12 year and the 2010–11 year. Line item numbers are given in brackets.

The allocation extractions are all associated with a water access entitlement. When an allocation is announced, an obligation (water liability) is created on the groundwater to deliver water to the users. As there is no carry-over provision in the region, the portion of the announced allocations that were not diverted by the end of the year was forfeited.

The entitlement, allocation announcement and forfeiture for each of these water rights during the 2011–12 year are provided in the Groundwater rights section of the 'Water access and use' note.

Balancing item

The balancing item volume represents the difference between the measured opening and closing balances of the groundwater store, after physical inflows and outflows have been applied.

The balancing item is calculated according to Table 8.

The calculation of the water balance on the groundwater store yielded a balance of 256,302 ML, approximately 15% of the total groundwater inflows during the 2011–12 year.

Because the volume of water in the groundwater stores within the Perth region could not be quantified, the volume disclosed in the balancing item (256,302 ML) represents the net change in groundwater store throughout the 2011–12 year.

During the 2011–12 year, total groundwater outflow was similar to the comparison year. However, total groundwater inflows were much greater than in the 2010–11 year; in particular, recharge from landscape (line item 10.3) was approximately 685,000 ML greater than that reported during the 2010–11 year. This large increase in recharge reflects the improved rainfall conditions observed throughout the region during the 2011–12 year (see Climate overview) compared to the record low rainfall recorded during the comparison year (see the 2011 Account).

Therefore, the balancing item value, which represents a net change in groundwater storage during the 2011–12 year, is mainly due to a high recharge from the landscape during the year.

The urban water system within the Perth region is discussed in the Other water resources and systems section of the 'Contextual information'. For more information on the system, refer to the Water Corporation website.

The Statement of Water Assets and Water Liabilities for the urban water system (Table 9) shows that the volume of water in the urban water system changed little during the 2011–12 year in the Perth region.

The volume of water within the urban wastewater system (line item 3.2) and the recycled water system (line item 3.3) cannot be quantified due to a lack of available data. However, given that these systems are smaller than the urban water supply system, the volume of water within these systems is likely to be relatively small.

The Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows for the urban water system are provided in Tables 10 and 11 respectively.

A schematic diagram representing all the inflows and outflows associated with the urban water system in the Perth region is provided in Figure 8. The inflow and outflow volumes for the urban water system during the 2011–12 year are given in Table 11. The numbers in brackets on the diagram refer to the line item numbers in Table 11.

Figure 8  Schematic diagram of water inflows and outflows for the urban water system within the Perth region during the 2011–12 year. Line item numbers are provided in brackets.

There are three main sources of water for the urban water supply system:

• surface water
• groundwater
• desalinated water.

During the 2011–12 year the Southern Seawater Desalination Plant commenced operation within the Perth region. This increased the supply of desalinated water to the urban water system by more than 50% from the comparison year (Figure 9). Desalinated water made up 25% of the urban supply water during the 2011–12 year.

Surface water and groundwater supply to the urban water system decreased during the 2011–12 year from last year (Figure 9). This is likely to be due to the increased contribution of desalinated water into the system.

Figure 9  Sources of water for the Perth region's urban water supply system during the 2011–12 year and the 2010–11 comparison year. Line item numbers are given in brackets.

Discharge from the urban wastewater collection system

Most of the treated wastewater from the urban wastewater collection system is discharged into the sea (line item 19.7). Treated wastewater is also discharged into the groundwater store via injection wells (see line item 19.9) and via infiltration ponds (see line item 19.10). During the 2011–12 year, the total volume of aquifer recharge was equivalent to approximately 2% of the total treated wastewater discharged from treatment plants within the region.

Balancing item

The balancing item volume represents the difference between the measured opening and closing balances of the urban water system, after physical inflows and outflows have been applied. This item is an indication of both the accuracy of the volumes reported and the degree to which the reported water flows represents a complete urban water system store balance.

The balancing item is calculated according to Table 12.

The calculation of the water balance on the urban water system yielded a balance item of 2,451 ML. This is approximately one-third of the total urban water system store volume at the end of the 2011–12 year and less than 1% of the total urban water system inflows during the 2011–12 year.

The irrigation scheme within the Perth region is discussed in the Other water resources and systems section of the 'Contextual information'. For more information on the irrigation scheme, refer to the Harvey Water website.

The Statement of Water Assets and Water Liabilities for the irrigation scheme (Table 13) shows that the volume of water in the irrigation scheme changed little during the 2011–12 year in the Perth region.

The Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows for the irrigation scheme are provided in Tables 14 and 15 respectively.

A schematic diagram representing all the inflows and outflows associated with the irrigation scheme in the Perth region is provided in Figure 10. The inflow and outflow volumes for the irrigation scheme during the 2011–12 year are given in Table 15. The numbers in brackets on the diagram refer to the line item numbers in Table 15.

Figure 10  Schematic diagram of water inflows and outflows for the irrigation scheme within the Perth region during the 2011–12 year. Line item numbers are provided in brackets.

This volume represents the difference between the measured opening and closing balances of the irrigation scheme, after physical inflows and outflows have been applied.

The balancing item of zero is calculated according to Table 16.