National Water Account 2015

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Region overview

Contextual information

Physical information

Administration

Water rights

Climate and water overview

Water Accounting Statements

Water Assets and Water Liabilities

Changes in Water Assets and Water Liabilities

Water Flows

Notes

Accounting policies

Supporting information

Quantification approach

Reconciliations

Water resources & systems

Water access & use

Future outlook

References

Accountability statement

Perth: Supporting information

Introduction

The following set of notes provides consolidated reports for each of the water stores and systems within the Perth region during the 2014–15 year. The water stores and systems included in the region are shown in Figure N1.

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

Figure N1 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 N2.

Figure N2 Between-store flows that occur within the Perth region

The between-store flows and transfers (Figure N2), which are eliminated from the region's water accounting statements, are shown in italics throughout the following set of notes.

Surface water store

The Perth region's surface water store has four components:

• storages
• regulated rivers
• unregulated rivers
• lakes and wetlands.

A reconciliation of the surface water Statement of Water Assets and Water Liabilities and Statement of Water Flows is provided in Table N2. Further details on assets, liabilities, and flows are provided in the following sections.

Methods used to derive reported volumes can be found in the Quantification approach note.

Assets and liabilities

Physical assets

Physical surface water asset volumes in the Perth region at 30 June 2015 were 202,362 ML, representing the volume of water held in storages.

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 channels 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 2014–15 year is shown in Figure N3.

Figure N3 Percentage-full volume on 30 June 2015 for each storage

Total surface water storage remained relatively constant at 31% during the 2014–15 year, despite 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 (see Surface water inflows below). During the 2014–15 year these transfers were much larger than previous years, minimising the impact of below-average rainfall and streamflow conditions.

Water volume in almost all storages was less than 50% capacity at 30 June 2015, with the exception of three smaller storages (Drakes Brook, Harvey, and Serpentine Pipehead). The volumes of the three largest storages (Canning, Serpentine, and South Dandalup) at 30 June 2015 were each less than 30% capacity.

Total volume of water within each storage at 30 June 2015 compared with the previous year and the 5-year average is given in Figure N4.

Figure N4 Total volume of water in each storage at 30 June 2015 compared with the previous year and the 5-year average

Non-physical assets

There are no non-physical surface water assets in the Perth region.

Liabilities

Surface water liabilities in the Perth region refer to the volume of surface water allocation remaining on licence entitlements at the end of the 2014–15 year for:

• individual users—8,437 ML
• urban system—0 ML
• irrigation scheme—22,877 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 2014–15 year for the Perth region is the unused component of the annual allocation for these licence entitlements. The allocation remaining for each licence entitlement is provided in tables N3–N5.

A more detailed description of surface water allocations and associated water rights in the Perth region is given in the Surface water rights note.

Inflows and outflows

A schematic diagram representing all surface water inflows and outflows is provided in Figure N5.

Figure N5 Water inflows and outflows for the surface water store during the 2014–15 year

Surface water inflows

Inflows to surface water decreased (approximately 18%) compared with the previous year. 

Total inflows to surface water were 939,147 ML and comprised:

• Precipitation—21,238 ML
• Runoff—549,983 ML
• Discharge: groundwater—291,896 ML
• Discharge: urban supply—74,172 ML
• Delivery: inter-region—1,858 ML.

The largest natural water inflow during the 2014–15 year was runoff to surface water and was almost a third less than the previous year, reflecting the reduced rainfall experienced across the region during the year (see Climate and water overview). Similarly, precipitation that fell on the major storages within the region also decreased compared with the previous year. 

The remaining natural surface water inflow is groundwater discharge, which represents the volume of water that discharged from Perth's superficial aquifers to rivers and drains. This volume increased marginally compared with the previous year.

Discharge from the urban water system increased (25%) during the 2014–15 year and is comprised of two components:

• internal water transfers (72,130 ML): a combination of surface water, groundwater and desalinated water transferred from the urban water supply system in order to store winter production of desalination water and groundwater and to balance the storages to prevent overflow and enable summer peak supply capacity; and
• riparian releases (2,042 ML): water discharged from water storages and water mains within Perth's urban water supply system into rivers for environmental benefit (see the Environmental benefit note).

The remaining two transfers of water to the surface water store are delivery of water from outside the region, which represents the water transferred from the Collie River Irrigation District into the Harvey Reservoir, and point return from the irrigation scheme, which represents return flow from the Harvey Water Irrigation Area to rivers. The point return flow was not measured and could not be quantified accurately.

Surface water outflows

Outflows from surface water decreased (approximately 19%) compared with the previous year.

Total outflows from surface water were 618,395 ML and comprised:

• Evaporation—46,052 ML
• Outflow—380,925 ML
• Recharge: groundwater—0 ML
• Leakage: landscape—150 ML
• Allocated diversion: individual users—38,661 ML
• Allocated diversion: urban system—119,649 ML
• Allocated diversion: irrigation— 32,958 ML

The largest natural water outflow is river outflow from the region, which refers to the annual volume of flow to the sea from the Swan, Canning, Murray and Harvey rivers during the 2014–15 year. This outflow is almost a third less than that of the previous year, which is consistent with reduced flow conditions in these rivers compared with the 2013–14 year (see Climate and water overview).

Evaporation from the major storages within the region during the 2014–15 year was considerably higher (almost 40%) than that during the previous year, which may be attributed to the decreased rainfall (and hence cloud cover) that occurred during the 2014–15 year (see Climate and water overview).

The total annual leakage from surface water storages within the Perth region is a very small outflow and varies little annually.

The volume of river and floodplain losses cannot be quantified accurately due to a lack of available data; however, these losses are considered to be relatively large and their omission may have a material impact on the water balance of the surface water store (see Balancing item).

Allocated diversions

Total surface water diverted in the region was 191,268 ML during the 2014–15 year. Allocated diversion to the urban system (119,649 ML) accounts for more than 60% of the total allocated diversion in the Perth region. This volume represents the water diverted from storages to the Perth region's Integrated Water Supply System. The volume of water diverted from each storage to the urban system is shown in Figure N6.

Figure N6 Surface water diversions to the urban system during the 2014–15 year; percentage of allocation diverted is also shown

Allocated diversions to individual users accounts for almost 20% of the total allocated diversion. Licences primarily relate to water use for the mining and industrial sectors (see Surface water rights note).

Allocated diversion to the irrigation scheme accounts for the remaining 20% of the total allocated diversion. This volume represents the water diverted to the Harvey and Waroona irrigation districts within the Harvey Water Irrigation Area.

Surface water balancing item

The surface water balance (Table N6) yielded a balance of –323,288 ML, indicating that inflows are greater than outflows. This is larger than the total surface water store volume at the end of the 2014–15 year and more than a third of the total surface water inflows during the year. The negative 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 cannot be quantified in the account. It is likely that these losses are a relatively large source of surface water decrease throughout the region. For example, the portion of total runoff to surface water (549,983 ML) that occurred upstream of Perth's storages is approximately 250,000 ML. According to the Water Corporation website, actual measured inflow into the storages during the year was approximately 75,000 ML, which represents a 225,000 ML unaccounted-for difference in the region (upstream of reservoirs) during the 2014–15 year. A portion of this difference can be attributed to river losses; the remainder is likely to be an overestimation of the runoff to surface water.

Runoff to surface water, which is a large source of surface water increase, is derived from a rainfall–runoff model and it is reasonable to expect a 10–20% uncertainty around the estimated runoff volume (+/– 110,000 ML).

The balancing item is also attributed to uncertainty associated with river outflow from the region (a large source of surface water decrease). 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 (see Quantification approaches). As such, the outflow to sea is likely to be underestimated by 10–20% (75,000 ML).

Groundwater store

The Perth region's groundwater store has two components:

• water table aquifer
• underlying aquifer.

A reconciliation of the groundwater Statement of Water Assets and Water Liabilities and Statement of Water Flows is provided in Table N7. Further details on assets, liabilities and flows are provided in the following sections.

Methods used to derive reported volumes can be found in the Quantification approach note.

Assets and liabilities

Physical Assets

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 Groundwater inflows and outflows below), no value is available for the volume of water stored in the aquifers as indicated by dashes ('–') in the water accounting statements.

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 unconfined aquifer necessary to maintain and protect groundwater-dependent ecosystems and the saltwater interface (the 'extractable limit' water level surface).

Preliminary results have shown that there are some areas of the Gnangara system where the aquifer levels are above this extractable limit and other areas where the aquifer levels are below this limit. Water above the extractable limit is considered to be a water asset. In areas where the water table is below the extractable limit, the gap between the water table and the extractable limit is considered to be a water liability to the environment (i.e., water is owed back to the water table to raise it to the extractable limit).

The Department of Water is working on a similar method for the confined aquifers in the Gnangara groundwater system, but it will be some time before these methods can be applied to the other groundwater aquifers in the Perth region (including Stakehill and Rockingham water table aquifers).

It should be noted that groundwater levels in the Jandakot groundwater system have already been prescribed to maintain and protect groundwater-dependent ecosystems; however, at the time of preparing this water accounting report the Department of Water had not commenced work to determine the volumes of water asset and water liability in the Jandakot system.

Non-physical assets

There are no non-physical groundwater assets in the region.

Liabilities

Groundwater liabilities in the Perth region refer to the volume of groundwater allocation remaining on licence entitlements at the end of the 2014–15 year for:

• individual users—109,093 ML
• urban water system—0 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 2014–15 year for the Perth region is the unused component of the annual allocation for these licence entitlements. Details are provided in tables N8–N9.

A more detailed description of surface water allocations and associated water rights in the Perth region is given in the Groundwater rights note.

Inflows and outflows

A schematic diagram representing all the inflows and outflows associated with the groundwater store in the Perth region is provided in Figure N7.

Figure N7 Water inflows and outflows for the groundwater store during the 2014–15 year

Groundwater inflows

Inflows to groundwater decreased by 32% compared with the previous year.

Total outflows from groundwater were 1,372,693 ML and comprised:

• Inter-region inflow—45,218 ML
• Inter-region coastal inflow—0 ML
• Recharge: landscape—1,304,530 ML
• Recharge: surface water—0 ML
• Leakage: urban system—21,155 ML
• Managed aquifer recharge: urban system—198 ML
• Discharge: urban system—1,592 ML

The largest water inflow, recharge from landscape, accounts for more than 95% of the groundwater inflows. The volume of recharge during the 2014–15 year is substantially (30%) less than the previous year, reflecting the decrease in rainfall observed across the region during the 2014–15 year (see Climate and water overview). Total volume of rainfall over the entire landscape of the Perth region was 12,989,784 ML during the 2014–15 year (derived by multiplying the area-averaged rainfall of 614 mm [see Climate and water overview] by the area of the region [21,156 km²]), which means that approximately 10% of the total annual rainfall over the Perth region contributed to groundwater recharge.

Groundwater inflow from outside the region during the 2014–15 year was approximately 15% less than the previous year, which also likely reflects the decrease in rainfall across the region. Groundwater inflow from outside the region at the coast was 0 ML, indicating there was no seawater intrusion into Perth's aquifers. It should be noted that the interface between the aquifer and the ocean moves throughout the year. During the summer months, when levels in the water table aquifers are relatively low, the saltwater interface moves towards the coastline; however, net flow occurs from the aquifers into the ocean.

Recharge from surface water during the 2014–15 year is consistent with previous years. This indicates that no groundwater recharge from rivers, drains, and storages occurs within the region.

The volume of managed aquifer recharge from the urban system was almost 70% less than the previous year. This volume represents groundwater replenishment from the Beenyup Wastewater Treatment Plant (water from the Advanced Water Recycling Plant). In this case, treated wastewater is physically pumped into the aquifer. Following the successful completion of a 3-year trial, the trial facility closed at the end of 2014 to facilitate construction of a full-scale scheme. This accounts for the notable decrease in managed aquifer recharge in 2014-15 compared with the previous year. Further information on the groundwater replenishment scheme can be found in the Major water reforms section of the 'Climate and water overview'.

Discharge from the urban system was consistent with previous years and  represents the volume of treated wastewater infiltrated to Perth's aquifers from the Kwinana Wastewater Treatment Plant. The infiltrated wastewater assists in maintaining environmental values at the Spectacles wetlands on the Swan Coastal Plain. The groundwater in this region is also extracted and recycled by Alcoa as part of the overall allocation of groundwater in the region.

Leakage from the urban system during the 2014–15 year is relatively similar to the previous year. Leakage from the irrigation scheme (– ML) was not measured and could not be quantified accurately.

Groundwater outflows

Outflows from groundwater also decreased (18%) compared with the previous year.

Outflows from groundwater were 1,518,521 ML and comprised:

• Inter-region outflow—23,439 ML
• Inter-region coastal outflow—341,881 ML
• Discharge: landscape—378,114 ML
• Discharge: surface water—291,896 ML
• Non-allocated extraction: individual users—79,610 ML
• Allocated extraction: individual users—281,453 ML
• Allocated extraction: urban system—122,128 ML

The largest water outflow from Perth's aquifers is discharge to landscape and was approximately 40% less than the previous year. Decreases were also observed in groundwater outflow from the region (inter-region outflow), including at the coast, which were approximately 20% less than the previous year. These decreases probably reflect the decrease in rainfall experienced across the region during the year (see Climate and water overview). 

Discharge to surface water was the only natural outflow to increase in the 2014–15 year; however, this increase was relatively small (3%).

Non-allocated extractions increased by 9% compared with the previous year. This volume represents water extracted from domestic non-licensed bores within the Perth region. 

Allocated extractions

Most of the allocated extraction of groundwater in the Perth region is for individual use, accounting for almost 70% of the total allocated groundwater extraction. Licences primarily relate to water use for agricultural, local irrigation, and industrial purposes. The remaining 30% of allocated groundwater is extracted from each bore field to the Perth region's Integrated Water Supply Scheme, as shown in Figure N8.

Figure N8 Groundwater extractions to the urban water system for the Perth region during the 2014–15 year; percentage of allocation extracted is also shown

Groundwater balancing item

The groundwater balance (Table N10) yielded a balance of 145,828 ML, approximately 11% of the total groundwater inflows during the 2014–15 year. This volume actually represents the net change in groundwater store throughout the 2014–15 year.

During the 2014–15 year, total groundwater inflow was less than in the 2013–14 year; in particular, recharge from the landscape was approximately 633,402 ML less than that reported during the previous year. This large decrease in recharge reflects the decreased rainfall conditions observed throughout the region during the 2014–15 year (see Climate and water overview) compared with the average rainfall conditions experienced during the previous year.

Therefore, the net change in groundwater storage during the 2014–15 year (represented by the balancing item volume) is mainly due to a smaller recharge from the landscape during the year.

Urban water

Perth's urban water system has three core components:

• water supply: raw water transfers, water treatment, and potable water supply
• wastewater: collections, transfers and treatment
• recycled water supply.

A reconciliation of the urban water Statement of Water Assets and Water Liabilities and Statement of Water Flows is provided in Table N11. Further details on assets, liabilities, and flows are provided in the following sections. 

Methods used to derive reported volumes can be found in the Quantification approach note.

Assets and liabilities

Physical assets

Physical assets at 30 June 2015 were 7,102 ML and included water held in the distribution pipe network (5,561 ML) and service reservoirs pipes (1,541 ML). The volume of water within the wastewater and recycled water systems could not be quantified due to a lack of available data.

Nonphysical assets

Nonphysical assets represent the following claims on water:

• Claims: surface water—0 ML
• Claims: groundwater—0 ML

These claims are equivalent to the volume of surface water and groundwater allocation remaining for urban water supply licence entitlements at the end of the 2014–15 year. Details are provided in tables N12 and N13. As there are no carryover provisions for these entitlements, unused water was forfeited and claims at year end were zero.

Further information on all entitlements and allocation arrangements are detailed in the Water rights section.

Liabilities

There are no urban water liabilities in the region.

Inflows and outflows

A schematic diagram representing all urban inflows and outflows is provided in Figure N9.

Figure N9 Water inflows and outflows for the urban water system during the 2014–15 year

A detailed breakdown of the inflows and outflows of the urban water system components are provided in the urban water balance diagram.

Water supply inflows

Total inflows to the water supply system were 363,156 ML and are sourced from:

• Allocated diversion: surface water—119,649 ML
• Allocated extraction: groundwater—122,128 ML
• Delivery: desalinated water—119,457 ML.
• Supply system delivery: inter-region—1,922 ML

The volume of allocated diversion from surface water is partially made up of groundwater as well as desalinated water produced in the winter and then transferred to storages to enable summer peak supply capacity. Therefore, net surface water diversion to Perth's Integrated Water Supply Scheme (47,519 ML) is the difference between the total diversion and the volume of internal transfers (72,130 ML), See the Water supply outflows section below.

Net inflows to the water supply system during the 2014–15 year were 291,026 ML, a slight increase compared with the previous year.

Groundwater  and desalinated water made up almost the same proportion of urban water supply during the 2014–15 year, 42% and 41% of inflows respectively  (Figure N10). Net surface water diversion made up 16% of urban water supply, which was similar to the previous year (Figure N10).

Inter-region inflows made up less than 1% of urban water supply, which is similar to the previous few years. This delivery refers to surface water diversion from the Mundaring Reservoir, which is not considered a surface water asset in the Perth region as it primarily supplies water to the Goldfields and Agricultural region outside the Perth region boundary.

Further information on supply system inflows is available in the Supply inflows table.

Figure N10 Water sources used in Perth's urban supply system during the 2014–15 year compared with the previous year and the 5-year average

Wastewater and recycled water inflows

Total wastewater collected and delivered to Perth's nine wastewater treatment plants was 140,771 ML during the 2014–15. The volume of wastewater collected at each treatment plant is given in Figure N11.

Figure N11 Wastewater inflows and outflows at each wastewater treatment plant

Further information on wastewater and recycled water system inflows is available in the Wastewater flows table.

Water supply outflows

Total outflows from the water supply system increased by approximately 5% compared with the previous year. Total outflows were 362,676 ML and comprised the following:

• Supply system delivery: urban users—232,537 ML
• Supply system transfer: inter-region—24,796 ML
• Supply system discharge: surface water—74,172 ML
• Leakage: groundwater—21,155 ML
• Other supply system decreases—10,016 ML

Figure N12 Outflows from the water supply system

Further information on supply system outflows is available in the Supply outflows table.

A comparison of water supply outflows with the previous year is provided in Figure N12. Delivery to urban users remained steady compared with the previous year, despite increased inflows to the water supply system during the 2014–15 year (see Water supply inflows above). The increase in inflows is offset by the increase (25%) in discharge to surface water, likely to be due to operational requirements. This volume comprises two components:

• internal water transfers (72,130 ML) for the purposes of storage and enabling summer peak supply capacity; and
• riparian releases (2,042 ML).

Transfers from Perth's urban water system outside the region made up 7% of supply system outflows and comprised transfers to the Goldfields and Agricultural region (23,565 ML) and Western Australia's southwest region (1,231 ML).

Losses from the urban supply system were similar to the previous year. This included 'Leakage to groundwater' and 'Other supply system decreases'. The volume of 'Other supply system decreases' comprised operational losses and all other non-revenue water from the urban water supply system, such as consumer meter inaccuracies and unauthorised consumption.

Wastewater and recycled water system outflows

Total outflows from the wastewater and recycled water system decreased minimally compared with the previous year. Total outflows were 140,812 ML and comprised the following:

• Recycled water delivery: urban users—8,035 ML
• Discharge: landscape—5,962 ML
• Discharge: sea—125,025 ML
• Managed aquifer recharge—198 ML
• Discharge: groundwater—1,592 ML.

Figure N13 Outflows from the wastewater and recycled water systems

Further information on wastewater and recycled water outflows is available in the Wastewater flows table.

The majority of treated wastewater is disposed of through discharge to sea (89%). A small portion is disposed of to landscape (4%) and the remaining portion (7%) of treated wastewater is recycled for use, primarily delivered to urban users (6%). Total aquifer recharge, including 'Managed aquifer recharge' and 'Discharge: groundwater' account for 1% of treated wastewater produced.

The volume of managed aquifer recharge was almost 70% less than the previous year. This volume represents groundwater replenishment from the Beenyup Wastewater Treatment Plant (water from the Advanced Water Recycling Plant). In this case, treated wastewater is physically pumped into the aquifer. Following the successful completion of a 3-year trial, the trial facility closed at the end of 2014 to facilitate construction of a full-scale scheme. This accounts for the notable decrease in managed aquifer recharge in 2014-15 compared with the previous year. Further information on the groundwater replenishment scheme can be found in the Major water reforms section of the 'Climate and water overview'.

The volume of discharge from the urban system was consistent with previous years and represents the volume of treated wastewater infiltrated to Perth's aquifers from the Kwinana Wastewater Treatment Plant. The infiltrated wastewater assists in maintaining environmental values at the Spectacles wetlands on the Swan Coastal Plain. The groundwater in this region is also extracted and recycled by Alcoa as part of the overall allocation of groundwater in the region.

Total wastewater outflows from Perth's wastewater and recycled water system are given in Figure N13. Outflows from each treatment plant are given in Figure N11.

There is likely to be some evaporation losses from the wastewater system but these could not be quantified due to a lack of available data. These losses were considered to be relatively small and their omission should not have a material impact on the water balance of the urban water system.

Recirculation of treated wastewater and recycled water that occurs in the system may be subjected to additional treatment and/or re-use on site. The reported urban system outflows exclude any such recirculated volumes in the system.

Urban water system balancing item

The urban water balance (Table N14) yielded a balance item of –339 ML. This is approximately 5% of the total urban water system store volume at the end of the 2014–15 year and less than 0.1% of the total urban water system inflows during the year.

The misbalance may be due to a number of factors, such as metering inaccuracies, unaccounted losses, and inaccuracies in the estimation of certain volumes.

Irrigation scheme

The Perth region's irrigation scheme represents the Harvey and Waroona irrigation districts within the Harvey Water Irrigation Area.

A reconciliation of the irrigation scheme Statement of Water Assets and Water Liabilities and Statement of Water Flows is provided in Table N15. Further details on assets, liabilities, and flows are provided in the following sections.

Methods used to derive reported volumes can be found in the Quantification approach note.

Assets and liabilities

Physical assets

Physical assets at 30 June 2015 were 390 ML, which is the same as for the previous year. Unless the pipe network is augmented or partly decommissioned, the volume of water in the irrigation supply system remains unchanged.

Non-physical assets

Non-physical assets (22,877ML) refer to the volume of surface water allocation remaining for irrigation scheme licence entitlements at the end of the 2014–15 year. The allocation remaining for the irrigation scheme's entitlement to surface water is provided in Table N16.

More detail on the irrigation scheme water abstraction and associated water rights is given in the Surface water rights note.

The licensed water management year for surface water supply to the Harvey Water Irrigation Area is 1 October–30 September. The above table indicates that there was 22,877 ML of water allocation remaining for the remainder of the 2014–15 irrigation season (which ends on 30 September 2014).

Inflows and outflows

A schematic diagram representing all the inflows and outflows associated with the irrigation scheme in the Perth region is provided in Figure N14.

Figure N14 Water inflows and outflows for the irrigation scheme during the 2014–15 year

The volume of allocated surface water (32,958 ML) diverted to the irrigation scheme during the 2014–15 year was marginally less than the previous year.

The volume of water delivered to Harvey Water irrigation scheme users within the Perth region during the 2014–15 year was 31,426 ML, which was marginally less than the previous year.

The remaining two outflows of leakage to groundwater (– ML) and point return from the irrigation scheme (– ML), which represents return flow from the Harvey Water Irrigation Area to rivers, were not measured and could not be quantified accurately.

Irrigation scheme balancing item

The irrigation scheme water balance (Table N17) yielded a balancing item of –1,532 ML, which is approximately 3% of the total irrigation scheme inflows for the year.

Unaccounted-for difference

The volume recognised in the water accounting statements (179,331 ML) represents the total unaccounted-for difference for the Perth region for the 2014–15 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 N18.

The unaccounted–for difference can also be calculated by summing 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 N19.

Table N19 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 note).

The groundwater assets of the Perth region could not be quantified (see Groundwater 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 145,828 ML represents the difference between known groundwater inflows and known groundwater outflows during the 2014–15 year (see Groundwater note), and any actual unaccounted-for differences when reconciling the terms.