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 2012–13 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 decreased during the 2012–13 year in the Perth region.

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

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 2012–13 year is shown in Figure 3.

Figure 3  Location map of the storages within the Perth region; the percentage-full volume at 30 June 2013 for each storage is also shown

The overall storage volume within the Perth region decreased during the 2012–13 year from 30% to 26% capacity. The water volume in almost all the storages decreased during the year (see line item 1.1 Storages). With the exception of three storages (Drakes Brook, Mundaring, and Serpentine Pipehead), storage volume was less than 50% of capacity at 30 June 2013. The percentage-full volume of the four largest storages (Canning, North Dandalup, Serpentine, and South Dandalup) at 30 June 2013 was between 10% and 23%.

The decrease in surface water storage during the 2012–13 year is primarily attributed to the decrease in inflows into the storages compared to the 2011–12 year. According to the Water Corporation website, inflow into the storages during the 2012–13 year was less than 30,000 ML, much less than the inflow recorded during the previous year of more than 80,000 ML. 

Storage volumes also include a portion of groundwater and desalinated water transferred from the urban water supply system. These transfers mainly occur in order to store winter production of desalinated water and groundwater and to prepare for summer peak demand (see line item 9.9 Discharge from urban water system).

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 2012–13 year; line item numbers are provided in brackets

Table 3 shows that surface water storage decreased during the 2012–13 year by 26,040 ML. This is a substantial change from the previous year, where water storage increased by 49,282 ML. The primary reason for the decrease in water storage is that runoff to surface water (line item 9.4 Runoff to surface water) was approximately half the previous year (Table 3). The decrease in runoff reflects the below average rainfall observed throughout the region during the 2012–13 year (see Climate overview) relative to the average rainfall conditions experienced during the 2011–12 year (see the 2012 Account).

Allocation diversions

Most of the allocation diversion from surface water is to the urban water system (line item 17.12 Entitled diversion of allocated surface water to urban water system), which accounts for approximately 54% of the total allocation diversion.

Figure 5 shows that allocation diversions from storages during the 2012–13 year to individual users, the urban water system and for inter-region supply have slightly increased from the previous year. Allocation diversions to the irrigation scheme have decreased from the previous year.

Figure 5  Graph of allocation diversions from storages within the Perth region during the 2012–13 year and the 2011–12 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 are not diverted by the end of the licence water year is 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 2012–13 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 250,555 ML. This is larger than the total surface water store volume at the end of the 2012–13 year and approximately 40% of the total surface water inflows during the year. The positive balancing item indicates that either the inflows are too high or the outflows are too low.

The balancing item is primarily attributed to the fact that river and floodplain losses (line item 17.10 River and floodplain leakage, evaporation and errors) cannot be quantified in the account. It is likely that these losses are a relatively large source of surface water decrease throughout the region.

The balancing item is also attributed to uncertainty associated with the runoff (a large source of surface water increase, line item 9.4 Runoff to surface water) and river outflow from the region (a large source of surface water decrease, line item 17.2 River outflow from the region). 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 (+/– 70,000 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% (40,000 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, as indicated by dashes ('–') in Table 5.

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 2012–13 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 Entitled extraction of allocated groundwater to users), 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 22.1 Groundwater allocation announcements).

Figure 7 shows that allocation extractions from aquifers during the 2012–13 year to the urban water system have decreased by approximately 12% from the previous year. Estimations of private use have slightly decreased and non-allocated extractions have slightly increased from the 2011–12 year. Non-allocated water use refers to unlicensed domestic bores within the Perth region (see line item 18.8 Entitled extraction of non-allocated groundwater to users).

Figure 7  Graph of extractions from aquifers within the Perth region during the 2012–13 year and the 2011–12 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 are not diverted by the end of the licence water year is forfeited.

The entitlement, allocation announcement, and forfeiture for each of these water rights during the 2012–13 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 –180,342 ML, approximately 14% of the total groundwater inflows during the 2012–13 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 of –180,342 ML represents the net change in groundwater store throughout the 2012–13 year.

During the 2012–13 year, total groundwater outflow was similar to the 2011–12 year; however, total groundwater inflows were much less than in the 2011–12 year; in particular, recharge from landscape (line item 10.3 Recharge from landscape) was approximately 456,000 ML less than that reported during the previous year. This large decrease in recharge reflects the below average rainfall observed throughout the region during the 2012–13 year (see Climate overview) compared to the average rainfall conditions experienced during the previous year (see the 2012 Account).

Therefore, the balancing item value, which represents a net change in groundwater storage during the 2012–13 year, is mainly due to a lower 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'.

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 2012–13 year in the Perth region.

The volume of water within the urban wastewater system (line item 3.2 Wastewater system) and the recycled water system (line item 3.3 Recycled water system) 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 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 2012–13 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 2012–13 year the first stage of the Southern Seawater Desalination Plant was able to operate at the plant's capacity for the entire year for the first time. More than 47,000 ML of desalinated water was produced for the Integrated Water Supply Scheme from the plant during the year (see line item 11.6 Delivery of desalinated water to urban water system). As a result, total supply of desalinated water to the urban water system increased by more than 20% from the previous year (Figure 9). Desalinated water made up almost 30% of the urban supply water during the 2012–13 year.

The total water supply to the urban water system during the 2012–13 year of 327,184 ML was similar to the previous year (318,189 ML). Surface water supply to the urban water system slightly increased during the 2012–13 year from last year; groundwater supply decreased (Figure 9). 

Figure 9  Sources of water for the Perth region's urban water supply system during the 2012–13 year and the 2011–12 year; line item numbers are given in brackets

Discharge from the urban wastewater collection system

Most of wastewater from the urban water system is treated and discharged into the sea (line item 19.7 Wastewater discharge outside of region). Treated wastewater is also discharged into the groundwater store via injection wells (see line item 19.9 Managed aquifer recharge) and via infiltration ponds (see line item 19.10 Discharge from urban water system to groundwater). During the 2012–13 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 448 ML. This is approximately 7% of the total urban water system store volume at the end of the 2012–13 year and less than 0.1% of the total urban water system inflows during the year.

The irrigation scheme within the Perth region is discussed in the Other water resources and systems section of the 'Contextual information'.

The Statement of Water Assets and Water Liabilities for the irrigation scheme is provided in Table 13. The total water assets for the irrigation scheme at 30 June 2013 has increased from the end of the previous year. During the 2012–13 year the Harvey Water Irrigation Scheme changed their licence water year anniversary date from 30 June to 30 September. As a result, no forfeitures occurred during the 2012–13 year and the irrigation claim on surface water (line item 4.5 Irrigation scheme claim on surface water) was greater than 0 ML.

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 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 2012–13 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. 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 irrigation scheme store balance.

The balancing item is calculated according to Table 16.

The calculation of the water balance on the irrigation scheme store yielded a balancing item of –709 ML. This is larger than the total irrigation scheme store volume at the end of the 2012–13 year and approximately 3% of the total irrigation scheme inflows during the year. The negative balancing item indicates that either the inflows (line item 12.9 Entitled diversion of allocated surface water to irrigation scheme) are too low or the outflows (line item 20.4 Delivery to irrigation scheme users) are too high.