Perth
Resources and Systems

The purpose of this note is to provide a consolidated report on the surface water store within the region during the 2010–11 year. Information on all water flows to and from the surface water store are presented here, including between-store flows and transfers that are not presented in the water accounting statements.

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

Table 1 shows that the total surface water store decreased during the 2010–11 year in the Perth region.

The volume of water in rivers and lakes and wetlands could not be quantified due to a lack of available data. However, it is considered that the volume of water held in rivers and lakes is 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 2010–11 year, is shown in Figure 1.

Figure 1. Location map of the storages within the Perth region. The % full volume on 30 June 2011 for each storage is also shown.

The water volume in almost all the storages within the Perth region at the end of the 2010–11 year is less than that at the start (1 July 2010) (see 1.1 Storages). Except for two storages (Mundaring and Serpentine Pipehead), storage volume within the Perth region was less than 50% of capacity on 30 June 2011 (Figure 1).

The large decrease in surface water storage during the 2010–11 year is primarily attributed to the record low inflows into the storages during the year (see Water Corporation website). Inflow into the storages during the 2010–11 year was below 20,000 ML, much less than the inflow recorded during the 2009–10 year of approximately 163,000 ML. The record low inflow reflects the below-average rainfall conditions observed throughout the region (see Climate overview).

Surface water inflows and outflows

A schematic diagram representing all the inflows and outflows associated with the surface water store in the Perth region is provided in Figure 2. The inflow and outflow volumes for the surface water store during the 2010–11 year are given in Table 2. The numbers on the diagram refer to the line item numbers in Table 2.

Figure 2. Schematic diagram of water inflows (blue arrows) and outflows (red arrows) for the surface water store within the Perth region. Solid arrows indicate water transfers; dotted arrows indicate natural water movement; waved arrows indicate leakage. Line item numbers are provided next to the flows.

Details of the inter-region transfers of water into and out of the region are given in the specific line item note (9.7 and 17.14, 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 53% of the total allocation diversion.

Figure 3 shows that allocation diversions from storages during the 2010–11 year to the urban water system and the irrigation scheme have decreased from last year. Allocation diversions for private use and inter-region supply are similar to the 2009–10 year. (Details on water abstraction for private use are provided in line item 17.11.)

Figure 3. Graph of allocation diversions from storages within the Perth region during the 2010–11 year and the 2009–10 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 2010–11 year are provided in the Water rights, entitlements, allocations and restrictions note, in the 'Surface water rights' table.

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 3.

The calculation of the water balance on the surface water store yielded a balancing item of 25,611 ML. This is approximately 16% of the total surface water store volume at the end of the 2010–11 year and less than 10% of the total surface water inflows during the 2010–11 year.

It is likely that the balancing item is primarily attributed to errors 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 (+/– 25,000 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% (+/– 23,000 ML).

Storage inflows and outflows

In this section, water flow into and out of surface water storages used for urban water supply are presented. All but four of the 15 storages within the Perth region are used for urban water supply to the Perth metropolitan area (see Table P2).

The inflows and outflows into and out of these urban water supply storages are shown in Table 4. These volumes are different from the flows reported in the water accounting statements and in Table 2 because they represent only a subset of the total surface water store.

The runoff into storages line item refers to inflows into a storage via river flow or direct landscape runoff. The inflow into each storage is calculated using a water balance approach, based on all the other measured or estimated volumes in Table 4, as described in 41.3 Runoff into storages.

The purpose of this note is to provide a consolidated report on the groundwater store within the region during the 2010–11 year. Information on all water flows to and from the groundwater store are presented here, including between-store flows and transfers that are not presented in the water accounting statements.

A description of the Perth region's groundwater resources are provided in the Contextual information.

The aquifers in the Perth region are described in the Contextual information. While good information exists on water movement to and from the aquifers (see following section on Water flows), 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 and 2.2.

A schematic diagram representing all the inflows and outflows associated with the groundwater store in the Perth region is provided in Figure 4. The inflow and outflow volumes for the groundwater store during the 2010–11 year are given in Table 6. The numbers on the diagram refer to the line item numbers in Table 6.

Figure 4. Schematic diagram of water inflows (blue arrows) and outflows (red arrows) for the groundwater store within the Perth region. Solid arrows indicate water transfers; dotted arrows indicate natural water movement; waved arrows indicate leakage. Line item numbers are provided next to the flows.

Allocations and extractions

Most of the extractions from the groundwater store are for licensed private use (line item 18.11), which accounts for approximately 55% of the total groundwater extraction. Almost 75% of the private water extractions are for agriculture and parks, gardens and recreational use (see line item note 18.11).

Figure 5 shows that allocation extractions from aquifers during the 2010–11 year to the urban water system have increased from last year. Estimations of private use and non-allocated extractions are similar to the 2009–10 year. Non-allocated water use refers to unlicensed domestic bores within the Perth region (see line item 18.8).

Figure 5. Graph of extractions from aquifers within the Perth region during the 2010–11 year and the 2009–10 comparison 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 2010–11 year are provided in the Water rights, entitlements, allocations and restrictions note, in the 'Groundwater rights' table.

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 7.

The calculation of the water balance on the groundwater store yielded a balance of –433,972 ML, approximately 42% of the total groundwater inflows during the 2010–11 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 (–433,972 ML) represents the net change in groundwater store throughout the 2010–11 year.

During the 2010–11 year, total groundwater outflow was about 375,000 ML less than the 2009–10 year. However, total groundwater inflows were much less than the 2009–10 year; in particular, recharge from landscape (line item 10.3) was approximately 810,000 ML less than that reported during the 2009–10 year. This large decrease in recharge reflects the below-average rainfall conditions observed throughout the region during the 2010–11 year (see Climate overview).

Therefore, the balancing item value, which represents a net change in groundwater storage during the 2010–11 year, is mainly due to a low recharge from the landscape during the year.

The purpose of this note is to provide a consolidated report on the urban water system within the region during the 2010–11 year. Information on all water flows to and from the urban water system are presented here, including between-store flows and transfers that are not presented in the water accounting statements.

The urban water system within the Perth region is discussed in the Contextual information. For more information on the system, refer to the Water Corporation website.

Table 1 shows that the volume of water in the urban water system changed little during the 2010–11 year in the Perth region.

The volume of water within the urban wastewater system cannot be quantified due to a lack of available data. However, given that this system is smaller than the urban water supply system, the volume of water within this system is likely to be relatively small.

A schematic diagram representing all the inflows and outflows associated with the urban water system in the Perth region is provided in Figure 6. The inflow and outflow volumes for the urban water system during the 2010–11 year are given in Table 9. The numbers on the diagram refer to the line item numbers in Table 9.

Figure 6. Schematic diagram of water inflows (blue arrows) and outflows (red arrows) for the urban water system within the Perth region. Solid arrows indicate water transfers; dotted arrows indicate natural water movement; waved arrows indicate leakage. Line item numbers are provided next to the flows.

Water sources, allocations and diversions

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

• surface water
• groundwater
• desalinated water

Figure 7 shows that most of the water was obtained from groundwater during the 2010–11 year. The previous year, most of the urban supply water was sourced from surface water. Lower than average rainfall during the 2009–10 year (see the 2010 Account) resulted in lower storage levels across the region compared with previous years. As a consequence, less surface water was available for urban water supply during the 2010–11 year.

Figure 7. Sources of water for the Perth region's urban water supply system. Line item numbers are given in brackets.

Desalinated water made up 16% of the urban supply water during the 2010–11 year, similar to the comparison year.

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 2010–11 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 10.

During the 2010–11 year, urban water system users and garden bore users were on a sprinkler roster.

Whilst on the sprinkler roster, urban water users in the Perth region can use their sprinklers two days per week. The sprinkler days are determined by the last digit of the user's street number and sprinklers must be used before 9am and after 6pm.

This note complements the System's interactions with the region note. It gives the following additional information:

• split of the urban water flows according to the sub-systems they affect by water type, source and quality
• flows between sub-systems, which are not reported in the System's interactions with the region note as they occur within the urban water system itself
• split of the total urban water use into several components.

The urban water system is made up of three sub-systems, which serve the urban water users:

• the urban water supply system
• the wastewater collection system
• the recycled water system.

The main urban water users are residential, commercial, industrial and municipal consumers.

The urban water supply system receives water from various sources, including surface water, groundwater and marine desalinated water. It treats and distributes water to urban water users. Part of the water is used outdoors (e.g. garden use) while the rest is collected into the wastewater collection system. Wastewater is treated and then discharged to surface water, groundwater, the sea or the landscape, or delivered into the recycled water system. The recycled water system, in turn, distributes treated water to users.

Tables 11 to 13 show the balance of each sub-system, including its inflows, outflows, balancing item and % error of the balancing item compared to the total of the inflows. The balancing items are calculated as the difference between the inflows and the outflows, considering that the changes in storage are not material. The values of the balancing items reflect measurement and data handling errors and/or missing data.

Table 14 shows a breakdown of the total urban water use per use sector and type of water (potable and recycled).

Some volumes in Tables 11 to 14 refer to line items that are reported in the water accounting statements and/or the System's interactions with the region note. The correspondence can either be:

• 'Line item A': meaning a one-to-one correspondence of the volume in the table with the line item A
• 'Part of line item A': meaning that the volume in the table is a part of the line item A
• 'Line item A + line item B': meaning that the volume in the table is the sum of line items A and B
• 'Not applicable': meaning that there is no correspondence to a line item

Supporting information and quantifications approaches for each volume shown in Tables 11 to 14 can be accessed via links on the line item numbers.

The total volume of water supplied to users of 237,954 ML (Table 14) is only slightly different to that presented in the water accounting statements of 238,273 ML (see line item 19.4) and in Table 11. This small difference is likely to reflect small errors in measurement and missing data.

Figure 8 represents the inflows and outflows reported in Tables 11 to 14 that occur between and within urban sub-systems and between urban systems and urban users. Flows that occur in the urban water system but could not be quantified are faded out.

Figure 8. Schematic diagram of water inflows and outflows for the urban sub-systems. Flow volumes and the corresponding line item numbers are provided next to the arrows and are shown in [ ] or with reference to the appropriate table number. Faded out arrows indicate water flows that could not be quantified.

There are four unquantified inflow and outflow components in Figure 8. Insufficient data were available to quantify these volumes however they are important components of the urban water balance.

Individually abstracted water includes water abstracted from surface water, groundwater or rainwater by individual users. That is, water diverted from a stream, pumped from a domestic garden bore or sourced from a rainwater harvesting system.

The unquantified stormwater component represents stormwater that is claimed and then used by urban water consumers such as that which is collected, treated and used in municipal stormwater schemes or projects.

The outflow of water to Landscape/outdoor represents the unquantified volume of water that urban users apply or discharge to the landscape. This includes garden, park and sports field watering, car washing and paved surface cleaning.

The unquantified line item [11.7] indicates groundwater or stormwater ingress into the stormwater system.

The purpose of this note is to provide a consolidated report on the irrigation scheme within the region during the 2010–11 year. Information on all water flows to and from the irrigation scheme are presented here, including between-store flows and transfers that are not presented in the water accounting statements.

The irrigation scheme within the Perth region is discussed in the Contextual information. For more information on the irrigation scheme, refer to the Harvey Water website.

The volume of water in the irrigation scheme is 370 ML (see line item 4.1). This volume did not change during the 2010–11 year.

A schematic diagram representing all the inflows and outflows associated with the irrigation scheme in the Perth region is provided in Figure 9. The inflow and outflow volumes for the irrigation scheme during the 2010–11 year are given in Table 15. The numbers on the diagram refer to the line item numbers in Table 15.

Figure 9. Schematic diagram of water inflows (blue arrows) and outflows (red arrows) for the irrigation scheme within the Perth region. Solid arrows indicate water transfers; waved arrows indicate leakage. Line item numbers are provided next to the flows.

Balancing item

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.