Water resources and systems
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 and net water assets increased during the 2011–12 year in the Melbourne region.
|Water assets||Volume at 30 June 2012 (ML)||Volume at 30 June 2011 (ML)|
|1 Surface water|
|1.2 Unregulated river||–||–
|1.3 Regulated river||–
|1.5 Inter–region claim on water||693,212||456,494|
|1.7 Claim on desalinated water||0||0|
|Total surface water assets||1,379,842||1,128,241|
|5 Surface water liability|
|5.1 Surface water allocation remaining||0||0|
|5.2 Surface water allocation remaining – urban water system||588,618||582,445|
|5.4 Inter–region commitment on surface water||0||0|
|Total surface water liabilities||588,618||582,445|
|Opening net water assets||545,796||625,183|
|Change in net water assets||254,428||(79,387)|
|Closing net water assets||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 2011–12 year, is shown in Figure 3.
Figure 3 Location map of the storages within the Melbourne region. The % full volume on 30 June 2012 for each storage is also shown.
The surface water storage volume within the Melbourne region increased marginally during the 2011–12 year (from 78% to 80%); this was reflected by small increases or decreases in storage volumes across the region. With the exception of Tarago Reservoir, storage volume within the Melbourne region was greater than 70% of capacity as at 30 June 2012 (Figure 3).
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.
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 2011–12 year. Line item numbers are provided in brackets.
Table 3 shows that surface water storage increased during the 2011–12 year by 14,883 ML, despite declining inflows to the surface water store compared to the 2010–11 year. The maintenance of high surface water storage volumes across the Melbourne region is primarily attributed to the continuation of above average rainfall conditions across the region (refer to the Climate overview section of the 'Contextual information') and the fact that inflows were still considerably greater than outflows during the 2011–12 year.
Surface water diversions
The majority of diversions from surface water are for the urban water system (line item 17.12), which accounts for more than 90% of total surface water diversions. Diversions to Melbourne's irrigation districts and private take and use licence holders makes up the remaining surface water diversions.
During the 2011–12 year diversions to the urban water system, irrigation districts and private licence holders increased compared to the 2010–11 year. Although rainfall across the entire Melbourne region was above average for the 2011–12 year, this contrasted with very much above average rainfall for the 2010–11 year, likely reducing demand and subsequently surface water diversions during the 2010–11 year.
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. This share of storage may increase due to inflows and runoff (line item 21.2) and decrease due to evaporation, operational losses and spills (13.2).
The entitlement, allocation announcement and forfeiture for each of these water rights during the 2011–12 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.
|Opening balance (30 June 2011)||671,747
|add||Total surface water inflows (see Table 3)||2,960,071|
|less||Total Surface water outflows (see Table 3)||2,365,406|
|less||Closing balance (30 June 2012)||686,630|
|Balancing item – surface water store||579,782|
The calculation of the water balance on the surface water store yielded a balancing item of 579,782 ML. This is approximately 85% of the total surface water store volume at the end of the 2011–12 year and around 20% of the total surface water inflows during the 2011–12 year.
It is likely that the balancing item is primarily attributed to errors associated with the rainfall runoff, 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 (+/– 580,000 ML). 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 remained constant during the 2011–12 year (Table 4). This was due to the quantification approach 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.
|Water assets||Volume at 30 June 2012
|Volume at 30 June 2011
|2.1 Water table aquifer||21,873||21,873|
|2.2 Underlying aquifers||16,565||16,565|
|Total groundwater assets||38,438||38,438|
|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||0||0|
|Closing net water assets||38,438||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.
|Water asset increases||2011–12 volume
|10 Groundwater increases|
|10.1 Groundwater inflow from outside region||0||0|
|10.2 Groundwater inflow from outside region at coast||280||1,113|
|10.3 Recharge from landscape||552,266||877,331|
|10.4 Recharge from surface water||–
|10.6 Leakage from urban water system||28,352||25,715
|Total groundwater increases||580,898||904,159
|Water liability decreases|
|14 Groundwater liability decreases|
|14.1 Adjustment and forfeiture of groundwater allocation||28,353||29,186|
|14.2 Adjustment and forfeiture of groundwater allocation – urban water system||412||401|
|Total groundwater liability decreases||28,765||29,587|
|Water asset decreases|
|18 Groundwater decreases|
|18.1 Groundwater outflow to outside region||0||0|
|18.2 Groundwater outflow to outside region at coast||121,259||129,345|
|18.3 Discharge to landscape||42,546||16,516|
|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||163,805||145,861|
|Water liability increases|
|22 Groundwater liability increases|
|22.1 Groundwater allocation announcements||41,526||39,370|
|22.2 Groundwater allocation announcements – urban water system||412||413|
|Total groundwater liability increases||41,938||39,783|
|Balancing item – groundwater||403,920||748,102|
|Change in net water assets||0||0|
A schematic diagram representing all the inflows and outflows associated with the groundwater store in the Melbourne region is provided in Figure 5. The inflow and outflow volumes for the groundwater store during the 2011–12 year are given in Table 7. The numbers in brackets on the diagram refer to the line item numbers in Table 7.
Figure 5 Schematic diagram of water inflows and outflows for the groundwater store within the Melbourne region during the 2011–12 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 2010–11 year. This increase is consistent with the increase observed in surface water diversions, primarily attributed to the fact that the 2010–11 year was a very wet year, reducing 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 2011–12 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.
|Opening balance (30 June 2011)||38,438|
|add||Total groundwater inflows (see Table 7)||580,898|
|less||Total groundwater outflows (see Table 3)||176,978|
|less||Closing balance (30 June 2012)||38,438|
|Balancing item – groundwater store||403,920|
The calculation of the water balance on the groundwater store yielded a balance difference of 403,920 ML, approximately 70% of the total groundwater inflows during the 2011–12 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 are not responsive to 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 than to the opening and closing balance of the groundwater asset.
The volume reported as the balancing item for the groundwater asset is considerably less than that reported for the 2010–11 comparative year (741,083 ML). During the 2010–11 year, total groundwater inflows were very large. This large increase in recharge reflected the well above average rainfall conditions observed throughout the region in the 2010–11 year. While above average rainfall conditions persisted during the 2011–12 year, these conditions were substantially drier than the 2010–11 year.
The change in aquifer storage was calculated using groundwater levels for the water table aquifer within the sedimentary area identified in Figure 6. The groundwater levels were estimated using all bores within the region, assuming that all hydrogeological layers are hydraulically inter-connected.
Figure 6 Map of water table aquifer within the sedimentary area used to calculate aquifer storage
The change in storage for water table aquifer in the Melbourne region during the 2011–12 year was estimated at 46,692 ML. This increase in storage was considerably less than that of the 2010–12 year (Table 9).
Change in storage 2011–12 (ML)
Change in storage 2010–11 (ML)
Clipped sedimentary areas
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.
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.
Bureau of Meteorology.
Change in extractable storage is estimated using a simple geographic information system (GIS) approach based on measured groundwater levels and aquifer properties. Firstly, groundwater levels are estimated at the start (1 July 2011) and the end (30 June 2012) of the 2011–12 year. This is achieved by considering all groundwater level measurements between March 2011 to October 2011 and March 2012 to October 2012, 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 are then spatially interpolated to grids using the ArcGIS Topo-to-Raster tool. The interpolated groundwater level surfaces at the start and the end of financial year and the interpolated change in groundwater level at each bore are then used to calculate the change in volume between them within the sedimentary area. Finally, these volumes are 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. The average of these volumes is reported.
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 the ArcGIS Topo-to-Raster tool. 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.
Urban water system
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:
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 changed little during the 2011–12 year in the Melbourne region.
|Water assets||Volume at 30 June 2012
|Volume at 30 June 2011
|3 Urban water system|
|3.1 Urban water supply system||–
|3.2 Wastewater system||25,202||24,219|
|3.3 Recycled water system||–
|3.7 Urban claim on surface water||588,618||582,445|
|3.8 Urban claim on groundwater||0||0|
|Total urban water system assets||613,820||606,664|
|Opening net water assets||606,664||24,737
|Change in net water assets||7,156
|Closing net water assets||613,820||606,664|
A volume was not presented in the water accounting statements for urban water supply systems. Water used to supply the urban system is stored in surface water storages and presented as 1.1 Storages.
The recycled water supply system could not be quantified accurately due to a lack of available data for the 2011–12 year. Given that this system is smaller than the wastewater supply system, the volume of water within the recycled water supply would have been relatively small and its exclusion from the water accounting statements is not considered to be material.
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.
A schematic diagram representing all the inflows and outflows associated with the urban water system in the Melbourne region is provided in Figure 7. The inflow and outflow volumes for the urban water system during the 2011–12 year are given in Table 12. The numbers in brackets on the diagram refer to the line item numbers in Table 12.
Figure 7 Schematic diagram of water inflows and outflows for the urban water system within the Melbourne region during the 2011–12 year. Line item numbers are provided in brackets.
The urban system receives most of its water from diversion of surface water (11.12 Entitled diversion of allocated surface water to urban water system) and collection of wastewater (11.4 Wastewater collected). 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.
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.
|Opening balance (30 June 2011)||24,219|
|add||Total urban water system inflows (see Table 12)||730,897|
|less||Total urban water system outflows (see Table 12)||729,956|
|less||Closing balance (30 June 2012)||25,202|
|Balancing item – urban water system||(42)|
The calculation of the water balance on the urban water system yielded a negative balance item of 42 ML, representing a very marginal proportion of the total urban water system store volume and the total urban water system inflows during the 2011–12 year.