Melbourne: Water stores
Below average rainfall and soil moisture conditions contributed to low runoff and a slight change in storage. Urban water supply increased with a significant increase in inter-region supply. Inter-region claims decreased due to the reduced share of inflows to the Melbourne region from the Thomson Reservoir. Groundwater extractions increased due to reduced surface water availability.
Surface water store
The volume of water in the Melbourne region's surface water store decreased marginally during the 2015–16 year from 522,005 ML at 1 July 2015 to 515,682 ML at 30 June 2016 (Table S9).
| 2016 ML |
2015 ML |
|
| Opening surface water store | 522,005 | 581,952 |
| Inflows | 1,221,354 | 1,315,360 |
| Outflows | (1,308,903) | (1,353,177) |
| Balancing item | (81,226) | (22,130) |
| Closing surface water store | 515,682 | 522,005 |
A schematic diagram representing all surface water inflows and outflows during the 2015–16 year is provided in Figure S4.
Figure S4 Water inflows and outflows for the surface water store during the 2015–16 year
Surface water inflows
The largest natural water inflow in the region was runoff, which was 17% less than the previous year due to reduced streamflow resulting from below average rainfall (see Climate and water). The volume is comprised of inflow to rivers and surface water storages. Inflow to surface water storages was 287,210 ML, which is 31% of the total runoff volume. Similarly, precipitation for the 2015–16 year includes 11,658 ML on river reaches within the region. The remaining natural surface water inflow, discharge: groundwater, 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 some flow from groundwater discharge along the majority of their length (Department of Sustainability and Environment 2010).
The largest water transfer in the region (delivery: inter-region agreement) comprised transfers from Thomson Reservoir (247,098 ML) and transfers from Silver and Wallaby Creeks (1,059 ML). The delivery of water from Thomson Reservoir increased by approximately 65% from the previous year. The remaining three transfers of water to the surface water store were discharge: wastewater, discharge: urban supply and point return: irrigation scheme.
Surface water outflows
The largest natural outflow in the region was river outflow, which was 19% less than the previous year. This reflected the reduced streamflow across the region during the year (see Climate and water).
Evaporation also decreased from the previous year, which may be attributed to the decreased storage volumes and hence surface area during the first 10 months of the year (see Climate and water). This year the volume also includes evaporation from major rivers in the region. The remaining two natural surface water outflows were recharge: groundwater and river and floodplain losses. River and floodplain losses could not be quantified accurately due to a lack of available data.
Transfer: inter-region was 3,540 ML. This refers to water diverted by Gippsland Water under the Tarago River–Gippsland Water bulk entitlement (Gippsland Water Annual Report 2015-16).
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 diversion: individual users (14,670 ML) was 15% more than the volume diverted during the previous year. The majority of these non-allocated diversions during the 2015–16 year were for irrigation purposes.
Allocated diversions
Allocated surface water diversions within the Melbourne region comprise:
- allocated diversion: urban system under the various bulk entitlements held by Melbourne's retail water authorities
- allocated diversion: individual users.
Most allocated surface water diversions in the Melbourne region are to the urban water system. During the 2015–16 year, 445,845 ML of surface water was diverted from storages (including inter-region transfers) to Melbourne's urban water supply system. This volume accounted for about 99% of the total allocation diversion within the region. The remaining 1% was for allocated diversion to irrigators (Figure S5).
Figure S5 Volume received by water authorities under surface water allocation diversion to the urban water system during the 2015–16 year
The volume reported includes:
- 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
- water supplied by Melbourne Water to Western Water from the Melbourne Headworks system
- 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 (Western Water and Central Highlands Water) river 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 2015–16 year was 5,004 ML, which accounts for about 1% of the total allocation diversion. These diversions mainly relate to irrigation water use by high-reliability and low-reliability entitlement holders in the Bacchus Marsh and Werribee irrigation districts. The volume is 38% less than the volume diverted during the previous year, reflecting limited water availability during the 2015–16 year in Pykes Creek, Merrimu, and Melton reservoirs from which the water releases are made.
Surface water balancing item
The surface water balance (Table S9) yielded a balance of –81,226 ML. This is approximately 16% of the total surface water inflows and 19% of the total surface water outflows during the 2015–16 year. The negative balancing item indicates that either the inflows are too low or the outflows are too high.
It is also likely a portion of the balancing item is attributed to uncertainties associated with the runoff estimation (a large source of surface water increase). Runoff is estimated from a rainfall–runoff model (see Methods) and it is reasonable to expect a relatively high uncertainity around this volume.
Groundwater store
The volume of water in the Melbourne region's groundwater store remained unchanged during the 2015–16 year (Table S10).
| 2016 ML |
2015 ML |
|
| Opening groundwater store | 38,619 | 38,619 |
| Inflows | 344,397 | 407,315 |
| Outflows | (161,536) | (169,761) |
| Balancing item | (182,861) | (237,554) |
| Closing groundwater store | 38,619 | 38,619 |
A schematic diagram representing all groundwater inflows and outflows during the 2015–16 year is provided in Figure S6.
Figure S6 Water inflows and outflows for the groundwater store during the 2015–16 year
Groundwater inflows
The larger groundwater inflows were recharge: landscape and recharge: rivers, which both account for 80% of the total groundwater inflows. Recharge to landscape decreased by 31% from the previous year, reflecting the drier conditions across the region (see Climate and water).
Inter-region coastal inflow is less climate-dependent and decreased marginally compared with the previous year. Consistent with previous years, inter-region inflow was 0 ML because the region boundary is a groundwater divide.
Leakage: urban system was slightly higher than the previous year.
Groundwater outflows
The largest groundwater outflow in the region was inter-region coastal outflow at the coastline through major sedimentary and basalt aquifers. This outflow decreased marginally from that of the previous year.
The other groundwater outflow was allocated extraction: individual users (24,603 ML), which accounts for almost 99% of the total allocated groundwater extraction. The volume of water extracted from each groundwater management unit is shown in Figure S7. The remaining groundwater outflows were allocated extraction: urban system (102 ML) and other groundwater decreases (12,270 ML) (see Methods).
Discharge: rivers, discharge: landscape, non-allocated extraction: individual users and extractions: statutory rights could not be quantified for the region due to a lack of available data.
Figure S7 Groundwater extractions for individual use within the Melbourne region during the 2015–16 year; percentage of allocation extracted is also shown
The total volume of groundwater extracted was about 35% of the total allocated groundwater for the groundwater management units and unincorporated areas shown in Figure S7. The total volume extracted did not include all possible stock and domestic extraction in unincorporated areas due to lack of complete data; therefore, the volume provided could be an underestimation.
Groundwater balancing item
The groundwater balance (Table S10) yielded a balance of 182,861 ML, approximately 53% of the total groundwater inflows during the 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 hence 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 with the change in water stored in the aquifers to establish whether the change in the storage reflects the balancing item (inflow–outflow difference) (see Methods).
Urban water system
The volume of water in the Melbourne region's urban water system did not change much during the reporting year (Table S11).
| 2016 ML |
2015 ML |
|
| Opening urban water system | 28,740 | 29,715 |
| Inflows | 797,013 | 756,885 |
| Outflows | (792,587) | (756,126) |
| Balancing item | (3,338) | (1,734) |
| Closing urban water system | 29,828 | 28,740 |
A schematic diagram representing all urban inflows and outflows during the 2015–16 year is provided in Figure S8.
Figure S8 Water inflows and outflows for the urban water system during the 2015–16 year
A detailed breakdown of the inflows and outflows of the urban water system components are provided in the urban water balance diagram for the region.
Water supply inflows
Total inflows to the water supply system were 446,325 ML, up by 6% from the previous year (Figure S9). Water received under inter-region agreements including Thomson Reservoir supply is shown in Figure S9 as inter-region inflow.
Figure S9 Water sources and total volume sourced for the Melbourne's urban water system during the 2015–16 year compared with the previous 6 years
Surface water is the main source for the Melbourne region's urban water supply system. Allocated diversion: surface water (445,845 ML) includes water received under inter-region agreements (248,157 ML) to the surface water store from the Thomson Reservoir, and Silver and Wallaby creeks. Therefore, the net surface water diversion to the urban water supply system from sources within the Melbourne region was 197,688 ML. Water diversions from the region's surface water sources decreased 27% while inter-region surface water supply increased 65% from the previous year.
Allocated extraction: groundwater (102 ML) and supply system delivery: inter-region (378 ML) were similar to the previous year.
Desalinated water was not utilised in the Melbourne region's urban water supply system. An order for 50,000 ML was placed with the Victorian Desalination Plant for the 2016–17 reporting year to meet Melbourne's water needs.
Further information on supply system inflows is available in the supply inflows table.
Wastewater inflows
Wastewater collected and delivered to Melbourne's wastewater treatment plants was 350,688 ML during the 2015–16 year, a 4% increase from the previous 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 (446,325 ML) comprised the following (Figure S10):
Figure S10 Outflows from the water supply system
The majority of water supply outflow (390,311 ML) was supply system delivery: urban users. During the 2015–16 year, the delivery was 6% higher than the previous year. This was due to an increase in demand, which aligns with drier conditions during the reporting year. Other water transfers from the urban supply system included supply system delivery: irrigation and supply system transfer: inter-region.
Approximately 12% of the total outflow from the urban supply system was related to losses (Figure S10). The losses, which comprised leakage: groundwater and other supply system decreases, were slightly more than the previous year. Supply system discharge: surface water was similar to the previous year.
Further information on supply system outflows is available in the supply outflows table.
Wastewater and recycled water system outflows
Total outflows from the wastewater and recycled water system were 346,262 ML comprised the following (Figure S11):
Figure S11 Outflows from the wastewater and recycled water systems
Approximately 12% of the treated wastewater was recycled, which comprises recycled water delivery: urban use and recycled water delivery: irrigation (Figure S11). Approximately 6% of the total outflow from the wastewater system was related to losses (other wastewater and recycled water decreases).
Most of the remaining treated wastewater comprises discharge: sea and, to a much lesser extent, discharge: surface water.
Recirculation of treated wastewater and recycled water that occurs in the system may be subjected to additional treatment and/or re-use on site. The reported urban system outflows exclude any such recirculated volumes in the system.
Further information on wastewater and recycled water outflows is available in the Wastewater outflows table.
Urban water system balancing item
The urban water balance (Table S11) yielded a balance item of 3,338 ML. This is 0.4% of the total urban water system inflows during the 2015–16 year. This percentage is low due to most of the urban system data provided by water utilities being metered. The misbalance may be due to a number of factors, such as metering inaccuracies, unaccounted losses and inaccuracies in the estimation of certain volumes.