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

Melbourne: Supporting information

Introduction

The following set of notes provides consolidated reports for each of the water stores and systems within the Melbourne 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 Melbourne 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 Melbourne 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. The allocation announcement, forfeiture, and allocation remaining items associated with each between-store abstraction (red arrows in Figure N2) are also shown in italics in the following notes.

Surface water store

The Melbourne region's surface water store has three components:

• storages
• regulated and unregulated rivers
• surface water claims.

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

Storages

The volume of water held in storages was 522,005 ML at the end of the 2014–15 year. The location of each storage within the Melbourne 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

Thomson Reservoir, which is the largest storage supplying water to the Melbourne metropolitan area, is outside the Melbourne region boundary. Therefore, it is not shown in Figure N3. Water sourced from Thomson Reservoir to the region is treated as an inter-region transfer.

The overall storage volume within the Melbourne region during the 2014–15 year was at 61% capacity and 10% less than the volume of the previous year. Ten storages showed a decrease in volume while the other three showed increases. The majority of the storages are connected through supply networks; therefore, volume fluctuations during the 2014–15 year in individual storages are not a major concern.

Catchment runoff was the main contributor to storage volumes. During the 2014–15 year, 371,800 ML of surface water runoff reached the storages within the region. This volume was just 34% of the total runoff that reached rivers, storages, and wetlands within the region. The runoff volume to storages was 10% less than that observed for the previous year, reflecting the effects of below-average rainfall during the year.

In addition to the storages, inter-region water transfers from Thomson Reservoir and Silver and Wallaby creeks are a major contributor to urban water supply (see Non-physical water assets and Water received under inter-region agreements). If the volume of water transferred from the Thomson Reservoir had not been increased by 13%, total volume of the storages within the region would have been further decreased at end of the 2014–15 year.

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

Regulated and unregulated rivers

The volume of water in regulated and unregulated rivers could not be quantified due to a lack of available data. Regulated and unregulated rivers within the Melbourne region that could not be quantified are as follows (Figure N5):

• Yarra and Bunyip are unregulated rivers.
• The Maribyrnong River is regulated along Jacksons Creek downstream of Rosslynne Reservoir. Deep Creek is unregulated upstream of its confluence with the Maribyrnong River.
• The Werribee River is regulated downstream of the point where releases from Pykes Creek Reservoir enter the river. Tributaries of the Werribee River (the Lerderderg River, Pyrites Creek, and Djerriwarrh Creek) are all unregulated upstream of the storages.

Figure N5 Regulated and unregulated river channels in the Melbourne region

Non-physical assets

Non-physical water assets (surface water claims) in the Melbourne region refer to the remaining volume of surface water that Melbourne retail water authorities were entitled to under bulk entitlements and agreements from inter-region sources at the end of the 2014–15 year. There were claims associated with:

• bulk entitlements for Thomson Reservoir and Silver and Wallaby creeks
• rights to water from Goulburn and Murray systems (water held in Lake Eildon)
• bulk entitlements to desalinated water from the Victorian Desalination Plant.

Inter-region claims for water from all the sources at the beginning and end of the 2014–15 year are summarised in Table N3.

Inter-region claim for water in the Thomson Reservoir

The inter-region claim for water in the Thomson Reservoir (731,148 ML) for Melbourne region's urban water supply at the end of the 2014–15 year represents 95% of the closing balance stated in Table N3. The inter-region claim at end of the year is the allowed volume to be carried over to the following year for the entitlement holders from their unused water. The claim for water in the Thomson Reservoir at the end of the 2014–15 year represents a 6% decrease compared with the previous year (see details). This reflects the delivery of more water for Melbourne region's urban water supply than entitled net inflow share (which is the increase of claim during the year) within the year. The entitled net inflow share was 123,731 ML while the delivery volume was 148,744 ML. It is important to note that for the 2014–15 year the entitled net inflow share recorded a 30% decrease while the delivery volume recorded a 13% increase over that of the previous year's volumes.

Inter-region claim for water from the Silver and Wallaby creeks

Unused water allocated under the Silver and Wallaby creeks bulk entitlement is not carried over to the following year. As a result, the opening and closing balance of the claim remained zero for this source.

Inter-region claim on rights to water from Goulburn and Murray systems

The inter-region claim to water from Goulburn and Murray systems (held in Lake Eildon) was 34,475 ML at the end of the 2014–15 year, which is a 10% decrease from that of the previous year. This is due to a 10% decrease in allocated volume from the Goulburn and Murray systems during the 2014–15 year from that recorded for the previous year.

Claim for desalinated water from the Victorian Desalination Plant

The Melbourne metropolitan retail water authorities, City West Water, South East Water, and Yarra Valley Water hold bulk entitlements to desalinated water from the Victorian Desalination Plant. The bulk entitlements allow them to take a combined volume up to 150,000 ML each year on an annual basis during any period of five consecutive years (subject to rules specified in the bulk entitlements). The Victorian Government decides water allocations based on desalinated water order advices provided by water authorities. The Victorian Government has not ordered any water from the desalination plant for 2013–14 and 2014–15 years. As a result, the desalinated claim at end of the 2014–15 year was considered as zero for the 2015 National Water Account (2015 Account).

Liabilities

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

• individual users—5,952 ML
• urban water system—484,266 ML
• inter-region commitment—0 ML.

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

Individual users

The allocation remaining for individual users is provided in Table N4.

Carryover of unused water allocated for high-reliability and low-reliability water shares in the Werribee and Bacchus Marsh irrigation districts was not permitted in the previous years. The Victorian Minister for Water approved carryover provisions for the two irrigation districts on 22 June 2014. As a result, unused water in an allocation account at the end of the water year could be carried over to a limit of 100% of the entitlement volume subject to a 15% evaporation loss.

Urban water system

The allocation remaining for the urban system is provided in Table N5.

Within the Melbourne region, several bulk entitlements are in place that govern the source, volume, and arrangements under which the retail water authorities can divert surface water for urban water supply. Carryover of unused allocations from the reservoirs within Yarra, Tarago, and Bunyip has been made possible under the bulk entitlement order 2014 for the Greater Yarra System–Thomson River Pool.

Bulk entitlements for the regional water sources other than the Greater Yarra System–Thomson River Pool specify an annual (or rolling annual average) volumetric upper limit; however, formal annual allocations are not made. As allocations are not made, carryover of allocations is not possible. However, the water authorities are entitled to a share of storage. This fluctuates annually depending on inflows to (rainfall, runoff) and outflows (evaporation, other losses) from the storages. This share of storage, at the start and end of the year, is recognised as carryover and included in Table N5.

Inter-region commitment

Gippsland Water has a right to water from the Tarago River under Bulk Entitlement (Tarago River–Gippsland Water) Conversion Order 2009. As the bulk entitlement does not provide for a share of storage, carryover of unused water is not permitted. Therefore, the volume reported as the opening and closing balance of the inter-region commitment on surface water was zero (see Table N6).

Inflows and outflows

A schematic diagram representing all the inflows and outflows associated with the surface water store in the Melbourne region is provided in Figure N6.

Figure N6 Water inflows and outflows for the surface water store during the 2014–15 year

Surface water inflows

In 2014–15, Inflows to surface water was 67% of the previous year volume. Total inflows to surface water were 1,315,425 ML during the 2014–15 year and comprised the following:

• Precipitation—31,431 ML
• Runoff—1,105,096 ML
• Delivery: inter-region agreement—150,198 ML
• Discharge: wastewater—28,230 ML
• Discharge: urban system—349 ML
• Point return: irrigation—121 ML.

Natural inflows

The largest water inflow was runoff to surface water (1,105,096 ML) during the 2014–15 year. This volume is about 63% of that of the previous year, which reflects the below average rainfall experienced across the region for the third year in a row (see Climate conditions). The volume is comprised of inflow to rivers and surface water storages. Inflow to surface water storages was 371,800 ML, which is 34% of total runoff volume (see Physical assets).

The remaining natural surface water inflow is precipitation on surface water (31,431 ML). The volume of precipitation represents rainfall that fell on the major storages within the region. The other notable natural surface water inflow, the groundwater discharge to surface water, could not be quantified due to lack of data. The Port Phillip and Westernport Catchment Management Authority reported that the main watercourses in the Melbourne region gained flow from groundwater discharge along the majority of their length (Department of Sustainability and Environment 2010a).

Water received under inter-region agreements

The largest surface water source after runoff to surface water was water received under inter-region agreements and comprised of:

• Thomson Reservoir—148,744 ML
• Silver and Wallaby creeks—1,454 ML

Melbourne's water authorities—City West Water, South East Water, Western Water, and Yarra Valley Water—receive water from Thomson Reservoir and Silver and Wallaby creeks under their bulk entitlements.

Water stored in Thomson Reservoir is delivered to Upper Yarra Reservoir for distribution to Melbourne's water supply system. The volume of water received from the Thomson Reservoir during the 2014–15 year was 13% higher than that delivered during the previous year. The increase is largely due to below-average inflows to the surface water storages within the Melbourne region in 2014–15 and increased water availability in the Thomson Reservoir.

Water is diverted from Silver and Wallaby creeks and delivered to Tooroorung Reservoir and Yan Yean Reservoir for distribution to Melbourne's water supply system. The volume received during the 2014–15 year was 77% of the previous year's volume.

Water stored in Lake Eildon (i.e. rights to water from Goulburn and Murray systems) can be delivered to Sugarloaf Reservoir via the North–South pipeline for distribution to Melbourne's water supply system. Water was not diverted during the 2014–15 year to the Melbourne's water supply system. As directed by the Victorian Government, water stored in Lake Eildon can only be delivered in times of critical human need or when needed for local fire-fighting (Melbourne Water 2014).

The other inflows

Discharge: wastewater (28,230 ML) during the 2014–15 year comprised of treated wastewater and recycled water discharged from wastewater treatment plants and recycled water plants to streams and wetlands. The largest of these discharges was 18,129 ML of recycled water provided from the Melbourne Water Western Treatment Plant as environmental flows to the Ramsar-listed Lake Borrie wetlands.

Discharge: urban system (349 ML) was the excess nonpotable water released to Glenfern service basin and Rosslynne Reservoir by Western Water.

Point return: irrigation (121 ML) was the return flows from Bacchus Marsh Irrigation District into the Lerderderg River that then flows into Melton Reservoir on the Werribee River.

Although the Victorian Desalination Plant was operational the Victorian Government did not order any desalinated for delivery during the 2014–15 year. Consequently, desalinated water is yet to be supplied under bulk entitlements since commissioning of the desalination plant.

Surface water outflows

Outflows from surface water decreased by 15% during the 2014–15 year from that of previous year. Total outflows from surface water were 1,353,177 ML during the 2014–15 year and comprised the following:

• Evaporation—54,829 ML
• Outflow—854,717 ML
• Transfer: inter-region—3,106 ML
• Non-allocated diversion: individual users—12,550 ML
• Allocated diversion: urban water system—419,917 ML
• Allocated diversion: individual users—8,058 ML

The largest water outflow is river outflow from the region (854,717 ML), which refers to the annual volume of flow to the sea from the Bunyip, Yarra, Maribyrnong, and Werribee rivers during the 2014–15 year. This outflow is approximately 67% of the river outflow during the previous year, reflecting below-average rainfall during the 2014–15 year (see Climate and water overview).

Evaporation from surface water during the 2014–15 year was 54,829 ML, which represents evaporation from the major storages within the region. Total evaporation during the 2014–15 year was approximately 24% higher than the volume from the previous year.

Transfer: inter-region (3,106 ML) represents the volume of water diverted by Gippsland Water under the Tarago River–Gippsland Water bulk entitlement, and comprised of:

• Labertouche Creek  at Labertouche Weir—0 ML
• Pederson Weir / Westernport Pipeline—2,926 ML
• Tarago Reservoir (Neerim South)—180 ML.

Gippsland Water did not divert water under its drought contingency allocation from Tarago Reservoir.

Entitled water diversions from the surface water store are described in Entitled surface water diversions below.

The volume of surface water discharged to groundwater and diversions made under other statutory rights (i.e. the private right to abstract and use water for domestic and stock use without an entitlement) could not be quantified accurately due to a lack of available data.

Entitled surface water diversions

Non-allocated diversions

Diversions made in unregulated streams and unregulated reaches of regulated streams are considered as non-allocated surface water diversions. Non-allocated water diversions for the 2014–15 year (12,550 ML) were slightly less than the volume diverted during the previous year.

Non-allocated surface water diversions represent the volume of water diverted under take-and-use licences issued for irrigation, stock and domestic, commercial and industrial, and other purposes. Take-and-use licences specify a maximum entitlement volume but this does not represent a surface water liability. Take-and-use licences exist for unregulated sections of the Bunyip, Maribyrnong, Werribee, and Yarra rivers as well as regulated sections of the Maribyrnong and Werribee rivers. Diversions made for irrigation purposes accounted for 85% of non-allocated diversions during the 2014–15 year.

Allocated diversions

Allocated surface water diversions within the Melbourne region occur for the following two purposes:

• allocated diversions to the urban water supply system under the various bulk entitlements held by Melbourne's retail water authorities
• allocated diversions for individual users.

Most allocated surface water diversions in the Melbourne region are to the urban water system. During the 2014–15 year, 419,917 ML of surface water was diverted from storages to Melbourne's urban water supply system. The volume accounted for about 98% of the total allocation diversion (Figure N7).

Figure N7 Volumes received by water authorities under surface water allocation diversion to the urban water systems during the 2014–15 year

The volume reported includes:

• potable and nonpotable water supplied by Melbourne Water to City West Water, South East Water, and Yarra Valley Water  from the Yarra, Tarago, and Bunyip rivers, and water received under inter-region agreements
• potable water supplied by Melbourne Water to Western Water from the Melbourne Headworks system
• nonpotable water supplied by Melbourne Water to Southern Rural Water from the Tarago and Bunyip rivers
• surface water diverted from the Maribyrnong (Western Water) and Werribee river (Western Water and Central Highlands Water) basins under various bulk entitlements.

Surface water from the Maribyrnong and Werribee river basins for Western Water is primarily diverted from storages managed by Southern Rural Water. There are also diversions made from smaller storages operated by Western Water.

The Water access and use note provides more information on relevant bulk entitlements.

Entitled diversion of allocated surface water for individual users during the 2014–15 year was 8,058 ML, which accounts for about 2% of the total allocation diversion. These diversions mainly relate to irrigation water use by high-reliability and low-reliability entitlement holders in Bacchus Marsh and Werribee irrigation districts. The volume is 18% less than the volume diverted during the previous year, reflecting limited water availability during the 2014–15 year in Pykes Creek, Merrimu Reservoir, and Melton Reservoir from which the water releases are made.

Surface water balancing item

The calculation of the surface water balance (Table N7) on the surface water store yielded a balancing item of 22,195 ML. The balancing item volume was approximately 4% of the total surface water store volume at the end of the 2014–15 year and around 2% of the total surface water inflows during the 2014–15 year.

It is likely that the balancing item volume is primarily attributed to errors associated with the rainfall runoff volumes (Runoff to surface water), a large source of surface water increase. The rainfall runoff volume is derived from a rainfall–runoff model, and it is reasonable to expect a 10–20% uncertainty around the modelled runoff volume. As a consequence, the modelled runoff is likely to be an underestimate of the volume that actually entered the surface water store.

The portion of the catchment that is ungauged is 38%. River outflow from this ungauged catchment was calculated using the ratio of the mean average flows and included in the 2015 Account (see Outflow in the 'Quantification approaches' note for details). There are uncertainties associated with the calculation method and the gauged flows used in the calculation. Overestimated river outflow from the region resulting from these uncertainties could be another contributing factor to the balancing item. 

Groundwater store

The Melbourne 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 N8. 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 Melbourne region are described in the Water resources section of the 'Contextual information'. The groundwater asset volume for the Melbourne region represents permissible consumptive volumes for water table aquifers (22,054 ML) and underlying aquifers (16,565 ML) in groundwater management areas (GMA) and water supply protection areas (WSPA) (Figure N8).

Figure N8 Permissible consumptive volumes: water table and underlying aquifers in the Melbourne region

The groundwater asset volume remained constant during the 2014–15 year as there was no change in licences or the permissible consumptive volume.

The groundwater asset volume does not include the permissible consumptive volume (2,015 ML) for Kinglake GMA as the majority of this GMA (which is included in Upper Goulburn GMA from July 2013) lies outside the boundary of the Melbourne region. 

The volume presented does not include groundwater in unincorporated areas outside of groundwater management units. Unincorporated areas have not been evaluated from a groundwater resource perspective and do not have established permissible consumptive volumes. The allocation limit for unincorporated areas in the southern half of Victoria including the Melbourne region was 71,542 ML.

Non-physical assets

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

Liabilities

Groundwater liabilities in the Melbourne region for the 2014–15 year (0 ML) refer to the volume of groundwater allocation remaining on licence entitlements at the end of the year for individual users and the urban water system.

Carryover rules for groundwater entitlements throughout the Melbourne region do not allow unused water to be carried over at the end of the year. Therefore, any unused water is forfeited, resulting in a zero carryover volume. Details of reconciliation of groundwater carryover, allocations, extraction, and forfeiture are provided in tables N9-N10.

A more detailed description of Groundwater allocations and associated water rights in the Melbourne 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 Melbourne region is provided in Figure N9.

Figure N9 Water inflows and outflows for Melbourne's groundwater store during the 2014–15 year

Groundwater inflows

Inflows to groundwater decreased by 12% from that of the previous year. Total inflow to groundwater was 352,111 ML and comprised of:

• Inter-region coastal inflow—33,665 ML
• Recharge: landscape—287,949 ML
• Leakage: urban system—30,497 ML.

The largest groundwater inflow is recharge from landscape (287,949 ML), which accounts over 80% of the total groundwater inflow. The volume of recharge during the 2014–15 year was about 85% of that recorded for the previous year, which reflects the below average rainfall experienced across the region for the third year in a row (see Climate conditions). Total volume of rainfall over the entire landscape of the Melbourne region was 8,159,200 ML during the 2014–15 year (derived by multiplying the area-averaged rainfall of 696 mm by the area of the region [11,723 km²]), which means that 3.5% of the total annual rainfall over the Melbourne region contributed to groundwater recharge.

Inter-region coastal inflow (33,665 ML) during the 2014–15 year was relatively similar to the previous year's volume. This flow represents seawater movement from outside the region towards the coastal aquifers. It includes fresh and saline groundwater flows at the coastline through major sedimentary and basalt aquifers. The volume for the 2014–15 year indicates that there was seawater movement towards the coastal aquifers during the year. 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.

Groundwater inflow from outside the region (other than at coast) during the 2014–15 year was 0 ML. The volume represents groundwater flow across the Melbourne region boundary for the following processes:

• through fractured rock along the northern and northeastern boundary
• through major sedimentary and basalt aquifers at the western and eastern boundary.

Leakage from the urban water system (30,497 ML) during the 2014–15 year (Figure N10) was marginally lower than the previous year's volume.

Figure N10 Leakage from the urban water system

Recharge from surface water could not be quantified for the region due to a lack of available data.

Groundwater outflows

Outflows from groundwater increased marginally from that of the previous year. Total outflow from groundwater was 169,761 ML and comprised of:

• Inter-region coastal outflow—139,716 ML
• Discharge: landscape—10,918 ML
• Allocated extraction: individual user—19,041 ML
• Allocated extraction: urban system—86 ML

The largest groundwater outflow from the Melbourne region during the 2014–15 year was outflow at the coastline through major sedimentary and basalt aquifers. The volume, 139,716 ML, increased marginally from the previous year. This outflow may or may not specifically represent groundwater discharge to the sea.

Groundwater discharge to landscape (10,918 ML) also increased marginally during the 2014–15 year over that of the previous year. The volume represents groundwater discharge via evapotranspiration from shallow water tables. Most of the groundwater discharge occurs from areas with shallow water tables in the south, southeast, and southwest of the region.

Details of other groundwater outflows are as follows:

• Inter-region outflow (other than at coast) during the 2014–15 year was 0 ML.
• Allocated extractions for individual user from the groundwater store are described in Allocation extractions below.
• Discharge to surface water, non-allocated entitled extractions, and extractions made under other statutory rights could not be quantified for the region.

Allocated extractions

Most of the entitled extraction of allocated groundwater in the Melbourne region is for individual use. During the 2014–15 year, 19,041 ML of groundwater was taken by individual users, which accounts for almost 99% of the total allocated groundwater extraction. The volume represents metered extraction in groundwater management units and unincorporated areas, and estimated stock and domestic extraction in groundwater management units. The volume of water extracted from each groundwater management unit is shown in Figure N11.

Figure N11 Groundwater extractions for individual use within the Melbourne region during the 2014–15 year; percentage of allocation extracted is also shown

The total volume of groundwater extracted is about 31% of the total allocated groundwater for the groundwater management units and unincorporated areas shown in Figure N11. The total volume extracted does not include possible stock and domestic extraction in unincorporated areas due to lack of data; therefore, the volume provided is an underestimation.

The total volume of allocated extraction of groundwater to the urban system (86 ML) was made from Lancefield GMA (Bore Number 3) and an unincorporated area (Glenfern Road Bore in Romsey) by Western Water during the 2014–15 year.

Groundwater balancing item

The groundwater balance (Table N11) yielded a balance of 182,350 ML, approximately 52% of the total groundwater inflows during the 2014–15 year. The volume for the 2014–15 year is less than the volume reported (238,959 ML) for the 2013–14 year.

The volume reported as the balancing item is the difference between total inflows and total outflows because the groundwater asset in the Melbourne region is equivalent to the legal extractable limit and does not reflect fluctuation of groundwater levels over time. Groundwater assets for the region do not reflect groundwater storage changes resulting from water table fluctuations. Therefore, estimated inflows and outflows were compared to the change of water stored in the aquifers to establish whether the change in the storage reflects the balancing item (inflow–outflow difference). The change in aquifer storage was calculated using groundwater levels for the water table aquifers within the sedimentary area identified in Figure N12. The groundwater levels were estimated using all bores within the region, assuming that all hydrogeological layers were hydraulically inter-connected.

Figure N12 Water table aquifer areas used to calculate change in aquifer storage

The change in storage for water table aquifer volume in the Melbourne region during the 2014–15 year was estimated as (21,994) ML, which is a decrease in storage. Decrease in the storage aligns with reduced recharge volume (see Inflows); however, it does not explain the balancing item, which is an unaccounted outflow. Therefore, it is reasonable to assume that the balancing item is the result of other unaccounted items (e.g., discharge to surface water), underestimation of discharge to landscape and overestimation of recharge from landscape. Discharge to / recharge from landscape was estimated using the WAVES model, which has parameters approximated to the whole region (see Quantification approaches).

Urban water system

Urban water assets

Melbourne's urban water system has three core components:

• Water supply: water treatment, potable and nonpotable water supply, and water transfers
• 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 N12. 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 28,740 ML and included water held in treatment plant lagoons in the wastewater system. The volume comprised water in:

• the Western Treatment Plant lagoons—27,725 ML
• storages and holding basins in the Eastern Treatment Plant—1,015 ML.

These volumes changes little from year to year. The volume of water within the urban water supply system (distribution pipe network and service reservoirs), the recycled water system, and some of the lagoons receiving wastewater could not be quantified due to a lack of available data. In addition, there are a number of smaller wastewater treatment plants operated by urban water authorities in the region; however, their volumes could not be quantified due to a lack of data and are not considered material to the 2015 Account.

Non-physical assets

Nonphysical assets represent the following claims on water in the Melbourne region:

• Claims: surface water—484,266 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 N13 and N14.

Remaining volumes in the Thomson Reservoir and Lake Eildon were not included in Table N13 because they were treated as inter-region claims to surface water store (see surface water: non-physical water assets), .

As carryover rules for groundwater entitlements throughout the Melbourne region do not allow unused water to be carried over at the end of the year, any unused water was forfeited resulting in a carryover volume of 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 Melbourne region.

Inflows and outflows

A schematic diagram representing all the inflows and outflows associated with the urban water system in the Melbourne region is provided in Figure N13.

Figure N13 Water inflows and outflows for Melbourne's 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 420,391 ML and were sourced from:

• Allocated diversion: surface water—419,917 ML
• Allocated extraction: groundwater—86 ML
• Supply system delivery: inter-region—388 ML.

Figure N14 compares volumes received from different sources for the most recent five years. The total volume received in the 2014–15 year is consistent with the previous year's volume and reveals a 6% increase from the average volume for the most recent five years.

Surface water volumes shown in Figure N14 are purely from within-region sources. Water received under inter-region agreements including Thomson supply is shown in Figure N14 as inter-region inflow.

Figure N14 Water sources and total volume sourced for the Melbourne's urban water system during the 2014–15 year compared with the previous year and the 5-year average

Surface water is the main source for the Melbourne region's urban water supply system. Allocated diversion: surface water (419,917 ML) during the 2014–15 year includes water received under inter-region agreements (150,198 ML) to the surface water store for the urban water system from the Thomson Reservoir, and Silver and Wallaby creeks. Therefore, the net surface water diversion to the urban water supply system from the sources within the Melbourne region was 269,719 ML, assuming that all water received under inter-region agreements was used.

In the 2014-15 year, water diversions from the region's surface water sources decreased 4% while inter-region supply increased 12% from that of the previous year.

Allocated extraction: groundwater (86 ML) represents Western Water's extractions from Lancefield Bore 3 and Glenfern Road bore.

Supply system delivery: inter-region (388 ML) during the 2014–15 year was made by Central Highlands Water for Ballan water supply (included in inter-region inflow in Figure N14).

Desalinated water was not utilised in the Melbourne region's urban water supply system during the 2014–15 year.

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

Wastewater and recycled water inflows

Total wastewater collected during the 2014–15 year was 336,494 ML, a 4% decrease from the previous year. The volume of wastewater collected by each water authority is given in Figure N15.

Figure N15 Wastewater collected by each water authority during the 2014–15 year

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

Water supply outflows

Total outflows from the water supply system during the 2014–15 year remained consistent with that of the previous year. Total outflows were 420,369 ML and comprised the following:

• Supply system delivery: urban users—368,451 ML
• Supply system delivery: irrigation—67 ML
• Supply system transfer: inter-region—247 ML
• Supply system discharge: surface water—349 ML
• Leakage: groundwater—30,497 ML
• Leakage: landscape—5,013 ML
• Other supply system decreases—15,745 ML

Figure N16 Outflows from the water supply system

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

The largest water outflow from Melbourne's urban water supply system was the delivery to urban water users. It accounted for about 88% of the total outflow from the supply system. During the 2014–15 year, urban water authorities delivered 368,451 ML comprising 368,140 ML of potable water and 311 ML of nonpotable water from the urban water supply system to urban water users.

During the 2014–15 year, 67 ML of nonpotable water was provided by Melbourne Water to Southern Rural Water (water taken from Tarago River for Southern Rural Water for supply to farmers). During the same year, 247 ML of water was transferred outside of the region from the urban water supply system. This transfer was made by Western Water to supply nonpotable water from Mount Macedon to Woodend via Western Water's water supply infrastructure.

During the 2014–15 year, 349 ML of water was discharged from the urban water supply system to surface water. This volume comprised excess nonpotable water released to Glenfern Service Basin (65 ML) and Rosslynne Reservoir (284 ML) by Western Water.

The loss from the urban water supply system was 51,255 ML for the 2014–15 year. This volume included 3,035 ML retained in service reservoirs before reaching retail water authorities. The loss from the supply system was equal to approximately 12% of inflows to the supply system for the 2014–15 year. The same percentage loss was recorded for each of the previous two years. Background leakage to groundwater decreased while both leakage to landscape due to pipe bursts and other non-revenue water losses marginally increased in the 2014–15 year from that of the previous year's volumes.

Wastewater and recycled water system outflows

Total outflows from the wastewater and recycled water system during the 2014–15 year, 335,822 ML, decreased by 4% compared to the previous year. Total outflows comprised the following (see also Figure N17):

• Discharge: sea—261,293 ML
• Wastewater discharge: surface water—28,230 ML
• Discharge: landscape—131 ML
• Recycled water delivery: urban users—22,634 ML
• Recycled water delivery: irrigation—7,567 ML
• Evaporation—15,896 NL
• Other wastewater decreases—71 ML.

Figure N17 Outflows from the wastewater and recycled water systems

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

Recirculation of treated wastewater and recycled water occurs in the system for additional treatment and/or re-use on site. The outflows in Figure N17 exclude any recirculated volumes.

The largest outflow from the wastewater system is treated wastewater discharged into the sea (261,293 ML) by the following water authorities:

• City West Water—4,821 ML
• Melbourne Water—248,549 ML
• South East Water—7,923 ML.

In addition to this volume, Melbourne Water, South East Water, Western Water and Yarra Valley Water discharged 28,230 ML of treated wastewater and recycled water to the streams and wetlands (see Figure N17) during the 2014–15 year. Melbourne Water's delivery of recycled water from the Western Treatment Plant to Lake Borrie wetlands as an environmental flow accounted for 64% of the volume.

South East Water and Yarra Valley Water discharged 131 ML of treated wastewater to the landscape during the 2014–15 year.

All urban water authorities supplied recycled water to urban users during the 2014–15 year. The volume, 22,634 ML, slightly increased from the previous year and excluded recycled water volumes supplied to irrigation schemes and the environment. There was 7,567 ML volume of Class A recycled water supplied to irrigation schemes as follows during the 2014–15 year (see also Figure N17):

• Melbourne Water from its Western Treatment Plant to Werribee irrigation district—3,938 ML
• Trility Pty Ltd (formally managed by Water Infrastructure Group) from the Eastern Irrigation Scheme to its customers—3,629 ML.

The recycled water volume provided to Werribee irrigation district increased by more than 80% compared with the previous year as a result of limited surface water availability. Overall surface water allocations to irrigation customers in the Werribee catchment decreased by 33% in 2014–15 from the 2013–14 year due to below average inflows. There was no change in the recycled water supplied by the Eastern Irrigation Scheme to its customers between the two years.

About 90% of evaporation from the wastewater system (15,896 ML) shown in Figure N17 was from Melbourne Water's Western Treatment Plant. Information for some of the small wastewater treatment plants could not be quantified due to a lack of data. These losses were considered to be relatively small and their omission does not have a material impact on the water balance of the urban water system.

Urban water system balancing item

The urban water balance (Table N15) yielded a balance item of 1,669 ML. This is approximately 6% of the total urban water system store volume at the end of the 2014–15 year and 0.2% 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.

Unaccounted-for difference

The volume recognised in the water accounting statements (206,214 ML) represents the total unaccounted-for difference for the Melbourne 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 (Table N16).

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 N17.

Table N17 shows that the unaccounted-for difference volume is largely explained by the surface water and groundwater store balancing item volumes. Details of the balancing items of these two stores are available in Surface water balancing item and Groundwater balancing item.