Melbourne
Water resources and systems

The following set of notes provides consolidated reports for each of the water stores and systems within the Melbourne region during the 2012–13 year. The water stores and systems included in the region are shown in Figure 1.

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


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

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

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


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

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

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

The Statement of Water Assets and Water Liabilities for the surface water store (Table 1) shows that total surface water assets decreased and net water assets increased slightly during the 2012–13 year in the Melbourne region.

Table 1 Statement of Water Assets and Water Liabilities for the surface water store 

Water assets	Volume at 30 June 2013 (ML)	Volume at 30 June 2012 (ML)
1 Surface water
1.1 Storages	582,979	686,630
1.2 Unregulated river	–	–
1.3 Regulated river	–	–
1.5 Inter-region claim on water	794,979	693,212
1.7 Claim on desalinated water	0	0
Total surface water assets	1,377,958	1,379,842
Water liabilities
5 Surface water liability
5.1 Surface water allocation remaining	0	0
5.2 Surface water allocation remaining – urban water system	495,189	588,618
5.4 Inter-region commitment on surface water	0	0
Total surface water liabilities	495,189	588,618
Opening net water assets	791,224	545,796
Change in net water assets	91,545	254,428
Closing net water assets	882,769	791,224

The volume of water in unregulated rivers (line item 1.2) and regulated rivers (line item 1.3) could not be quantified due to a lack of available data.

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 2012–13 year are shown in Figure 3

Figure 3 Location map of the storages within the Melbourne region; the % full volume on 30 June 2013 for each storage is also shown.

The surface water storage volume within the Melbourne region decreased during the 2012–13 year (from 80% to 68%); this was reflected by decreases in storage volumes across the region. With the exceptions of the Maroondah, Upper Yarra, and Melton reservoirs, storage volumes within the Melbourne region were greater than 70% of capacity as at 30 June 2013 (Figure 3).

The year on year increase of 91,545 ML in net water assets is contributed from an increase in inter-region claim by 101,767 ML and decrease in end of the year surface water liability by 93,429 ML. The below average rainfall across the region contributed to relatively lower inflows (line item 9.4), which contributed to decreased surface water allocation announcement (line item 21.2) and increased surface water diversions (line item 17.12). Changes in these two line items resulted in a lower surface water liability at the end of the year and an increase in closing net water assets.

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

Table 2 Statement of Changes in Water Assets and Water Liabilities for the surface water store

Water asset increases	2012–13 volume (ML)	2011–12 volume (ML)
9 Surface water increases
9.1 Precipitation on surface water	35,591	48,406
9.3 Groundwater discharge	–	–
9.4 Runoff to surface water	1,903,420	3,205,287
9.5 Point return from irrigation scheme	63	77
9.9 Discharge from urban water system	26,243	12,161
9.10 Direct discharge by user	–	–
9.15 Increase of inter-region surface water claim on water	188,068	259,701
9.17 Increase of surface water claim on desalinated water	0	0
Total surface water increases	2,153,385	3,525,632
Water liability decreases
13 Surface water liability decreases
13.1 Adjustment and forfeiture of surface water allocation	7,229	10,636
13.2 Adjustment and forfeiture of surface water allocation – urban water system	53,052	45,156
13.4 Adjustment and forfeiture of inter-region commitment on surface water	0	0
Total surface water liability decreases	60,281	55,792
Water asset decreases
17  Surface water decreases
17.1 Evaporation from surface water	47,133	43,621
17.2 River outflow from the region	1,377,155	1,918,282
17.3 Leakage to groundwater	–	–
17.4 Leakage to landscape	–	–
17.6 Surface water diversions – other statutory rights	–	–
17.7 Entitled diversion on non-allocated surface water to users	13,509	17,571
17.17 Decrease of inter-region surface water claim on water	60,064	20,791
17.19 Decrease of surface water claim on desalinated water	0	0
Total surface water decreases	1,497,861	2,000,265
Water liability increases
21 Surface water liability increases
21.1 Surface water allocation announcements	18,302	20,761
21.2 Surface water allocation announcements – urban water system	368,435	425,527
21.4 Increase of inter-region commitment on surface water	3,231	3,040
Total surface water liability increases	389,968	449,328
Balancing item – surface water	234,292	886,403
Change in net water assets	91,545	245,428

Table 3 Statement of Water Flows for the surface water store

Water inflows	2012–13 volume (ML)	2011–12 volume (ML)
9 Surface water inflows
9.1 Precipitation on surface water	35,591	48,406
9.3 Groundwater discharge	–	–
9.4 Runoff to surface water	1,903,420	3,205,287
9.5 Point return from irrigation scheme	63	77
9.9 Discharge from urban water system	26,243	12,161
9.10 Direct discharge by user	–	–
9.11 Delivery of water under inter-region agreement to surface water	26,237	2,192
9.13 Delivery of claimed desalinated water to surface water	0	0
Total surface water inflows	1,991,554	3,268,123
Water outflows
17 Surface water outflows
17.1 Evaporation from surface water	47,133	43,621
17.2 River outflow from the region	1,377,155	1,918,282
17.3 Leakage to groundwater	–	–
17.4 Leakage to landscape	–	–
17.6 Surface water diversions – other statutory rights	–	–
17.7 Entitled diversion on non-allocated surface water to users	13,509	17,571
17.11 Entitled diversion of allocated surface water to users	11,073	10,125
17.12 Entitled diversion of allocated surface water to urban water system	408,812	374,198
17.14 Surface water transfer under inter-region commitment	3,231	3,040
Total surface water outflows	1,860,913	2,366,837
Balancing item – surface water	234,292	886,403
Opening water storage	686,630	671,747
Add/(Less): Change in water storage	(103,651)	14,883
Closing water storage	582,979	686,630

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


Figure 4 Schematic diagram of water inflows and outflows for the surface water store within the Melbourne region during the 2012–13 year; line item numbers are provided in brackets

Table 3 shows that surface water storage decreased during the 2012–13 year by 103,651 ML, due to declining inflows to the surface water store compared to the 2011–12 year where water storage increased by 14,883 ML. The decrease in surface water storage volumes across the Melbourne region is primarily attributed to a decrease in runoff to surface water (line item 9.4 Runoff to surface water) which was approximately 59% less than the previous year. The decrease in runoff also reflects the below average rainfall observed thoughout the region during the 2012–13 year (see Climate overview) relative to the above average rainfall conditions experienced during the 2011–12 year (see the 2012 Account).

Surface water allocation diversions

The majority of diversions from surface water are for the urban water system (line item 17.12 Entitled diversion on allocated surface water to urban water system), which accounts for more than 90% of total surface water allocation diversion. Diversions to Melbourne's irrigation districts and private take and use licence holders makes up the remaining surface water diversions.

Figure 5 shows that during the 2012–13 year diversions to the urban water system, irrigation districts, and inter-region supply increased while use by private licence holders decreased compared to the 2011–12 year. Rainfall across the entire Melbourne region was below average for the 2012–13 year; this contrasted with above average rainfall for the 2011–12 year, and probably increased demand and subsequently surface water diversions during the 2012–13 year.

Figure 5 Graph of allocation diversions from storages with the Melbourne region during the 2012–13 year and the 2011–12 year; line item numbers are given in brackets

Diversions to the irrigation districts and the urban water system are associated with a water access entitlement.

For the irrigation districts, 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 for the irrigation district's water shares, the portion of the announced allocations that were not diverted by the end of the year was forfeited.

For the urban water system, the retail water authorities hold several bulk entitlements that entitle them to a share of storage capacity in reservoirs across the region and a share of the Thomson and Eildon reservoirs which are outside the region. This share of storage may increase due to inflows and runoff (line item 21.2) and decrease due to evaporation, operational losses, and spills (line item 13.2).

The entitlement, allocation announcement, and forfeiture for each of these water rights during the 2012–13 year are provided in the Surface water rights section of the 'Water access and use' note.

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.

Table 4 Balancing item for the surface water store for the 2012–13 year

Account	Volume (ML)
Opening balance at 1 July 2012	686,630
Total surface water inflows (see Table 3)	1,991,554
Total surface water outflows (see Table 3)	(1,860,913)
Closing balance at 30 June 2013	(582,979)
Balancing item—surface water store	234,992

The calculation of the water balance on the surface water store yielded a balancing item of 234,992 ML. This is approximately 40% of the total surface water store volume at the end of the 2012–13 year and around 12% of the total surface water inflows during the 2012–13 year.

It is likely that the balancing item is primarily attributed to errors associated with the rainfall runoff volumes, a large source of surface water increase (line item 9.4). 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 overestimate of the volume that actually entered the surface water store.

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

The volume of groundwater assets changed slightly during the 2012–13 year (Table 5). This was due to the changes in permissible consumptive volume due to applying a re-estimation of the groundwater assets (refer to line items 2.1 Water table aquifer and 2.2 Underlying aquifers). 

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

Table 5 Statement of Water Assets and Water Liabilities for the groundwater store

Water assets	Volume at 30 June 2013 (ML)	Volume at 30 June 2012 (ML)
2 Groundwater
2.1 Water table aquifer	22,054	21,873
2.2 Underlying aquifers	16,565	16,565
Total groundwater assets	38,619	38,438
Water liabilities
6 Groundwater liability
6.1 Groundwater allocation remaining	0	0
6.2 Groundwater allocation remaining – urban water system	0	0
Total groundwater liabilities	0	0
Opening net water assets	38,438	38,438
Change in net water assets	181	0
Closing net water assets	38,619	38,438

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

Table 6 Statement of Changes in Water Assets and Water Liabilities for the groundwater store

Water asset increases	2012–13 volume (ML)	2011–12 volume (ML)
10 Groundwater increases
10.1 Groundwater inflow from outside region	0	0
10.2 Groundwater inflow from outside region at coast	31,076	280
10.3 Recharge from landscape	382,674	620,167
10.4 Recharge from surface water	–	–
10.6 Leakage from urban water system	31,529	28,352
Total groundwater increases	445,279	648,799
Water liability decreases
14 Groundwater liability decreases
14.1 Adjustment and forfeiture of groundwater allocation	26,167	28,353
14.2 Adjustment and forfeiture of groundwater allocation – urban water system	347	412
Total groundwater liability decreases	26,514	28,765
Water asset decreases
18 Groundwater decreases
18.1 Groundwater outflow to outside region	0	0
18.2 Groundwater outflow to outside region at coast	136,022	121,259
18.3 Discharge to landscape	18,379	10,077
18.4 Discharge to surface water	–	–
18.7 Groundwater extractions – other statutory rights	–	–
18.8 Entitled extraction of non-allocated groundwater to users	–	–
Total groundwater decreases	154,401	131,336
Water liability increases
22 Groundwater liability increases
22.1 Groundwater allocation announcements	41,579	41,526
22.2 Groundwater allocation announcements – urban water system	413	412
Total groundwater liability increases	41,992	41,938
Balancing item – groundwater	275,219	504,290
Change in net water assets	181	0

Table 7 Statement of Water Flows for the groundwater store

Water inflows	2012–13 volume (ML)	2011–12 volume (ML)
10 Groundwater inflows
10.1 Groundwater inflow from outside region	0	0
10.2 Groundwater inflow from outside region at coast	31,076	280
10.3 Recharge from landscape	382,674	620,167
10.4 Recharge from surface water	–	–
10.6 Leakage from urban water system	31,529	28,352
Total groundwater inflows	445,279	648,799
Water outflows
18 Groundwater outflows
18.1 Groundwater outflow to outside region	0	0
18.2 Groundwater outflow to outside region at coast	136,022	121,259
18.3 Discharge to landscape	18,379	10,077
18.4 Discharge to surface water	–	–
18.7 Groundwater extractions – other statutory rights	–	–
18.8 Entitled extraction of non-allocated groundwater to users	–	–
18.11 Entitled extraction of allocated groundwater to users	15,412	13,173
18.12 Entitled extraction of allocated groundwater to urban water system	66	0
Total groundwater outflows	169,879	144,509
Balancing item – groundwater	275,219	504,290
Opening water storage	38,438	38,438
Add/(Less): Change in water storage	181	0
Closing water storage	38,619	38,438

Improved estimation method has resulted in change to the reported volume for line items 10.2 and 18.2 during the 2011–12 reporting period. Further details are provided in the relevant line item notes.

A schematic diagram representing all the inflows and outflows associated with the groundwater store in the Melbourne region is provided in Figure 6. The inflow and outflow volumes for the groundwater store during the 2012–13 year are given in Table 7. The numbers in brackets on the diagram refer to the line item numbers in Table 7.


Figure 6 Schematic diagram of water inflows and outflows for the groundwater store within the Melbourne region during the 2012–13 year; Line item numbers are provided in brackets

Allocations and extractions

Most of the extractions from the groundwater store were for licensed private use (18.11 Groundwater allocation extraction). In general, extractions from groundwater increased marginally compared to the 2012–13 year. This increase is consistent with the increase observed in surface water diversions, primarily attributed to the fact that the 2012–13 year was a relatively dry year, increasing demand on water resources for all water users.

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 was not diverted by the end of the year was forfeited.

The entitlement, allocation announcement, and forfeiture for these water rights during the 2012–13 year are  provided in the Groundwater rights section of the 'Water access and use' note.

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

The balancing item is calculated according to Table 8.

Table 8 Balancing item for the groundwater store for the year 2012–13 year 

Account	Volume (ML)
Opening balance at 1 July 2012	38,438
Total groundwater inflows (see Table 7)	445,279
Total groundwater outflows (see Table 7)	(169,879)
Closing balance at 30 June 2013	(38,619)
Balancing item—groundwater store	275,219

The calculation of the water balance on the groundwater store yielded a balance difference of 275,219 ML, approximately 62% of the total groundwater inflows during the 2012–13 year. 

The groundwater asset in the Melbourne region is equivalent to the legal extractable limit and does not reflect fluctuation of groundwater levels over time. Therefore, groundwater assets for the region do not reflect groundwater storage changes resulting from water table fluctuations. As a result, estimated inflows and outflows are more appropriately compared to the change of water stored in the aquifers and the balancing item reflects these changes.

The volume reported as the balancing item for the groundwater asset is considerably less than that reported for the 2011–12 comparative year (504,290 ML). During the 2011–12 year, total groundwater inflows were relatively larger. This larger increase in recharge reflected the above average rainfall conditions observed throughout the region in the 2011–12 year. Below average rainfall conditions were observed during the 2012–13 year, resulting relatively lower recharge volumes in the 2012–13 year.

The change in aquifer storage was calculated using groundwater levels for the water table aquifer within the sedimentary area identified in Figure 7. The groundwater levels were estimated using all bores within the region, assuming that all hydrogeological layers are hydraulically inter-connected.

Figure 7 Map of water table aquifer areas used to calculate aquifer storage

The change in storage for water table aquifer volume in the Melbourne region during the 2012–13 year was estimated at -92,522 ML, which is a decline in storage within the financial year.  During the year 2011-12, an increase in storage was noted (Table 9).

Table 9  Change in aquifer storage volume estimated for 2012–13 and 2011–12 years

Management area	Change in storage 2012–13 (ML)	Change in storage 2011–12 (ML)
Clipped major sedimentary/basalts aquifers	-92,522	31,901

Because the aquifers included in the calculation of the change in aquifer storage represent only a fraction of the groundwater resources in the Melbourne region, the value in Table 9 only partially reflects the difference between inflows and outflows shown in Table 7.

Data source

Port Phillip Catchment Management Authority: groundwater model (Department of Sustainability and Environment 2010b), hydraulic conductivity and aquifer thickness; Victorian Department of Environment and Primary Industries: bore locations, groundwater level data and aquifer attribution.

Method

Change in extractable storage was estimated using a simple geographic information system (GIS) approach based on measured groundwater levels and aquifer properties. Firstly, groundwater levels were estimated at the start (1 July 2012) and the end (30 June 2013) of the 2012–13 year. This was achieved by considering all groundwater level measurements between March 2012 to October 2012 and March 2013 to October 2013, respectively, and using the measurements closest in time to interpolate the start and end levels.

The estimated groundwater levels on the start and end dates were then spatially interpolated to grids using kriging with external drift and the 9" Digital Elevation Model as an external driver, taking into account the effect of the coastline, following the methodology presented in Peterson et al. (2011). The interpolated groundwater level surfaces at the start and the end of financial year were then used to calculate the change in volume between them within the sedimentary area. Finally, these volumes were multiplied by appropriate specific yield values to convert to a change in groundwater storage and masked for areas within 10 km of a groundwater observation bore.

Uncertainty

The uncertainty estimate was not quantified.

The uncertainty in the field-measured data (example: groundwater levels, specific yield) was not specified and hence the impacts of such uncertainty on the change in storage is not estimated.

The change in storage estimations were based on interpolated groundwater level grids produced using kriging with external drift and the 9" Digital Elevation Model as an external driver following the methodology presented in Peterson et al. (2011). Use of other interpolation methods may impact the values of the groundwater level grids and hence the estimated values for change in groundwater storage.

Approximation, assumptions, caveats, and limitations

The groundwater levels were estimated by assuming that all the hydrogeological layers (excluding basement) within the Port Phillip and Western Port groundwater model region are hydraulically inter-connected. This assumption facilitates the interpolation of a groundwater potential surface from groundwater level measurements as these measurements were limited in number.

Groundwater levels are assumed equal to zero metres Australian Height Datum (mAHD) at the coastline.

Change in groundwater storage was not calculated for confined aquifers. The annual change in storage was considered to be negligible for confined aquifers due to their very low storativity, which is much lower than the specific yield of unconfined aquifers (Freeze and Cherry 1979). Upon lowering of water levels in such aquifers, they remain fully saturated so that no dewatering occurs. The water diverted is volumetrically equivalent to the volumetric expansion of the water and contraction of the pore space.

Annual change in storage in fractured bedrock is considered negligible as these systems typically have a low specific yield. Furthermore, groundwater extraction in fractured rock areas is limited in volume.

The spatially varying specific yield values considered in the Port Phillip and Western Port groundwater model are used in these estimations.

A 10-km buffer around bores was used to acknowledge the spatial variability of groundwater levels and the density of  the data available. Outside the buffer the storage is considered constant through the year to reflect the no data availability.

The urban water system within the Melbourne region is discussed in the Other water resources and systems section of the Contextual information.

Volumetric information relating to the urban system is provided by the operators listed below. More information on the urban water system in the Melbourne region is available from the following websites:

• Melbourne Water
• City West Water
• South East Water
• Yarra Valley Water
• Western Water

The Statement of Water Assets and Water Liabilities for the urban water system (Table 10) shows that the volume of water in the urban water system was reduced by 89,499 ML during the 2012–13 year in the Melbourne region.

Table 10 Statement of Water Assets and Water Liabilities for the urban water system

Water assets	Volume at 30 June 2013 (ML)	Volume at 30 June 2012 (ML)
3 Urban water system
3.1 Urban water supply system	–	–
3.2 Wastewater system	29,132	25,202
3.3 Recycled water system	–	–
3.7 Urban claim on surface water	495,189	588,618
3.8 Urban claim on groundwater	0	0
Total urban water system assets	524,321	613,820
Opening net water assets	613,820	606,664
Change in net water assets	(89,499)	7,156
Closing net water assets	524,321	613,820

A volume was not presented in the water accounting statements for urban water supply system. Surface water storages which supply water to the urban water system have been considered with the surface water storage and presented as line item 1.1 Storages.  They are not included in line item 3.1. Urban water service reservoirs have not been quantified for line item 3.1. The recycled water supply system could not be quantified accurately for line item 3.3 due to a lack of available data for the 2012–13 year.

The Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows for the urban water system are provided in Table 11 and Table 12, respectively.

Table 11 Statement of Changes in Water Assets and Water Liabilities for the urban water system

Water asset increases	2012–13 volume (ML)	2011–12 volume (ML)
11 Urban water system increases
11.4 Wastewater collected	341,462	356,462
11.18 Increase of urban claim on surface water	368,435	425,527
11.19 Increase of urban claim on groundwater	413	412
11.21 Other urban water system increases	161	237
Total urban water system increases	710,471	782,638
Water asset decreases
19 Urban water system decreases
19.1 Evaporation from urban water system	17,031	18,622
19.2 Leakage to landscape	4,398	3,354
19.3 Leakage to groundwater	31,529	28,352
19.4 Delivery to urban water system users	381,256	353,309
19.5 Discharge from urban water system to surface water	26,243	12,161
19.6 Discharge from urban water system to irrigation scheme	8,076	3,089
19.7 Wastewater discharge outside of region	268,134	299,614
19.8 Other wastewater discharge	0	8
19.11 Transfer of water outside of region	250	146
19.17 Adjustment and forfeiture of urban claim on surface water	53,052	45,156
19.18 Adjustment and forfeiture of urban claim on groundwater	347	412
19.20 Other urban water system decreases	11,638	11,301
Total urban water system decreases	801,954	775,524
Balancing item – urban water system	(1984)	(42)
Change in net water assets	(89,499)	7,156

Table 12. Statement of Water Flows for the urban water system

Water inflows	2012–13 volume (ML)	2011–12 volume (ML)
11 Urban water system inflows
11.4 Wastewater collected	341,462	356,462
11.12 Entitled diversion of allocated surface water to urban water system	408,812	374,198
11.13 Entitled extraction of allocated groundwater to urban water system	66	0
11.21 Other urban water system increases	161	237
Total urban water system inflows	750,501	730,897
Water outflows
19 Urban water system outflows
19.1 Evaporation from urban water system	17,031	18,622
19.2 Leakage to landscape	4,398	3,354
19.3 Leakage to groundwater	31,529	28,352
19.4 Delivery to urban water system users	381,256	353,309
19.5 Discharge from urban water system to surface water	26,243	12,161
19.6 Discharge from urban water system to irrigation scheme	8,076	3,089
19.7 Wastewater discharge outside of region	268,134	299,614
19.8 Other wastewater discharge	0	8
19.11 Transfer of water outside of region	250	146
19.20 Other urban water system decreases	11,638	11,301
Total urban water system outflows	748,555	729,956
Balancing item – urban water system	(1,984)	(42)
Opening water storage	25,202	24,219
Add/(Less): Change in water storage	3,930	983
Closing water storage	29,132	25,202

A schematic diagram representing all the inflows and outflows associated with the urban water system in the Melbourne region is provided in Figure 8. The inflow and outflow volumes for the urban water system during the 2012–13 year are given in Table 12. The numbers in brackets on the diagram refer to the line item numbers in Table 12.


Figure 8. Schematic diagram of water inflows and outflows for the urban water system within the Melbourne region during the 2012-13 year; line item numbers are provided in brackets

The urban system receives most of its water from diversion of surface water (line item 11.12 Entitled diversion of allocated surface water to urban water system). Information about the water access entitlements and water allocations under which surface water and groundwater are abstracted for the urban water system is given in the Water rights, entitlements, allocations and restrictions note.

The total water supply to the urban water system during the 2012–13 year of 408,878 ML which increased from the previous reporting year which was 374,198 ML. Surface water supply to the urban water system increased by approximately 9% for 2012–13 reporting year from previous reporting year but groundwater supply changes were insignificant. This increase in use could be due to several factors like increase in demand due to population growth and the reduced rainfall conditions during the 2012–13 year compared to the 2011–12 year.

Most of the treated wastewater is discharged into the sea (19.7 Wastewater discharge outside of region). The volume reported in the water accounting statements is the volume of wastewater discharged from the Melbourne region to Port Phillip Bay and Bass Strait.

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 13.

Table 13 Balancing item for the urban water system for the 2012–13 year

Account	Volume (ML)
Opening balance at 1 July 2012	25,202
Total urban water system inflows (see Table 12)	750,501
Total urban water system outflows (see Table 12)	(748,555)
Closing balance at 30 June 2013	(29,132)
Balancing item—urban water system	(1,984)

The calculation of the water balance on the urban water system yielded a negative balance item of (1,984) ML. This is appproximately 8 % of the total urban system store volume at the end of the 2012–13 year and around 0.25 % of of total urban water system inflows during the 2012–13 year.