National Water Account 2017

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Melbourne: Methods

Summary of methods

Table N4 outlines the methods used to derive the item volumes for the Melbourne region. For a more detailed description of the method, click on the relevant item name in the table.

Table N4 Methods used to derive item volumes

Detail of methods

Water storage product data

Storages

Storage volumes at the start and end of the year were calculated using water level data (metres above Australian Height Datum) collected at each storage. Rating tables established for each storage were used to convert the height measurement to a volume. Storage capacity was calculated using the full supply level provided by the managing authority.

The volume of individual storages was aggregated to present the total storage volume for the region. The uncertainty range for the storage volume was not quantified.

The assumptions made were as follows:

• Storage–volume curves represent specifically surveyed parts of the storage and may not reflect the storage–volume relationship across the entire storage.
• Water storages are subject to sedimentation and other physical changes over time that in turn affect the accuracy of the storage–volume curves.
• Storage capacity may differ from those published by storage operators due to rounding and uncertainty in the true values.

AWRA-R model

AWRA-R is a river network model that represents key hydrological processes and diversions at a daily time step (Dutta et al. 2017; 2015). The model was used in the National Water Account to quantify river fluxes and stores along the river network.

The river system is conceptualised in AWRA-R as a node-link network comprising nodes connected by river reaches. Gauged streamflow data are used where available. For ungauged portions of catchment, the landscape runoff from the AWRA-L model is used (Viney et al. 2015). River processes represented in the AWRA-R model are shown in Figure N1.

Figure N1 Conceptual diagram of AWRA-R reach showing model components (from Dutta et al. 2015)

For further detail on the AWRA-R model, its assumptions and its calibration refer to Dutta et al. (2015; 2017).

Rivers

The volume of water in the river channels at 30 June was estimated by using the daily water balance approach within AWRA-R. The water balance includes inflow at the upstream nodes and outflow at the downstream nodes; contributing catchment runoff, reservoir contribution, irrigation diversion and return; overbank flooding and floodplain return; loss to groundwater; anabranch flow; rainfall; and evaporation.

Overbank flow and flood return

The AWRA-R floodplain module was used to model the volume of overbank flow from the river onto the floodplain, and the return flow from the floodplain back into the river. The module applies a simple storage-based floodplain model to each river reach. The floodplain modelling method is detailed in Dutta et al. (2013).

Groundwater recharge from surface water

Groundwater recharge from surface water could not be estimated for the 2017 Account as the AWRA-R Model needs further improvements and testing to produce necessary outputs for the region.

Climate gridded data and AWRA-L model

Runoff

Runoff to surface water was estimated based on the AWRA-L version 5 model streamflow outputs (Viney et al. 2015).

Using gridded climate data for the Melbourne region (including precipitation, temperature, and solar radiation data), AWRA-L was used to estimate the runoff depth, which was converted to a runoff volume at each grid point within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major reservoirs were excluded from the analysis.

The average runoff depth from the landscape into the surface water was determined as the weighted mean of the relevant grid points within the region boundary. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. Runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages).

Runoff estimates were subject to the assumptions of the AWRA-L model detailed in Viney et al. (2015). Uncertainty estimates were not available for the modelling method. More information on runoff calculations using the AWRA-L model is available in Australian Landscape Water Balance.

Groundwater recharge from landscape

Groundwater recharge from landscape was estimated using the AWRA-L model version 5.0 (Viney et al. 2015). The AWRA-L model is a regionally calibrated water balance model. It estimates daily diffuse deep drainage, which is the free drainage at the bottom of the deep soil layer (6m) for each 5km x 5km stand-alone grid cell across the continent. The average deep drainage was estimated as the weighted mean of the relevant grid-cells within the regions. The estimated deep drainage volume was considered as the groundwater recharge from landscape. The main factors affecting the deep drainage rate at any location are:

1. Rainfall

AWRA-L is driven by daily gridded climate data (including precipitation, solar radiance, and temperature) that were available on a 0.05 degree (approximately 5 km) national grid (Jones, Wang and Fawcett 2009).

2. Soil properties

Rates of drainage through the model's conceptual water stores (0-0.1m, 0.1m-1m, and 1m-6m) are controlled by the estimated saturated hydraulic conductivity across those soil depths. AWRA-L makes use of clay content based pedotransfer functions (Dane and Puckett 1994) to derive conductivities of the various soil layers.

3. Evapotranspiration rates

AWRA-L model divides the landscape into two types of vegetation: deep-rooted and shallow rooted. The fraction covered with vegetation (the same for both types) is further estimated by using the Leaf Area Index (LAI) estimate for each grid, and this value responds dynamically to moisture in the ground. This then affects the rate of evapotranspiration from the unsaturated soil stores and groundwater.

The limitations associated with this approach are:

• Free drainage at a depth of 6m is potential recharge, and may be quite different in magnitude to the net volume that reaches the water table (allowing for transpiration), and is not appropriate to be used for confined aquifers.
• The average deep drainage was determined as the weighted mean of the relevant grid-cells within the regions. Estimates of recharge for 5km by 5km grid-cells may not be of the same order of magnitude as any point estimate made within the grid.
• Lateral flow from grid cell to grid cell, which could affect groundwater levels are not considered.

The AWRA-L annual estimates of deep drainage tends to be lower than previously used WAVES model outputs (~10% of previous value). This is due to the consideration of more recent vegetation and soil type datasets, and a calibrated water balance approach in the AWRA-L model. AWRA-L results lie comfortably within acceptable ranges and are of comparable magnitude and spatial distribution to the long term mean annual recharge estimates recently produced by CSIRO (Shi, Crosbie and Vaze 2015).

Climate gridded data and AWRA-L and AWRA-R models

Precipitation and evaporation on/from surface water and river sections

Rainfall and evaporation into/from storages and rivers were calculated using climate data from the Bureau of Meteorology interpolated to 0.05 degree (5 km) national grids (Jones et al. 2009). Calculations for rivers were carried out on a daily time step using the AWRA-R model. Calculations for storages were done on a monthly time step. Annual totals were summed from the daily or monthly values.

Climate data for each water body at each time step were estimated from the proportionally weighted average of grid-cells that intersected the water body. Evaporation was estimated using Morton's shallow lake formulation (Morton 1983). Rainfall and evaporation volumes were then estimated by multiplying the surface area of each waterbody by the weighted average rainfall and evaporation respectively. The average daily surface area of rivers was estimated using the AWRA-R model and the average monthly surface area of the storages was calculated from daily storage levels and capacity tables.

Outflow

To estimate the total surface water outflow from the Melbourne region, the AWRA-R and AWRA-L models were used to simulate daily flows at the end of Bunyip, Yarra, Maribyrnong, and Werribee rivers flowing to Port Phillip Bay and Western Port Bay. These flows included:

• observed flows at the most downstream gauges (or simulated flow if observed was not available) routed to the end of the river in AWRA-R
• residual catchment inflows estimated from the AWRA-L model for ungauged portions of catchments below the most downstream gauge.

Annual Reports of water authorities

Claims: inter-region and increase in claims: inter-region

Remaining volume of water entitled at 30 June 2017 under inter-region claims was calculated as shown in Table N5.

Rights to remaining entitled water at Thomson Reservoir, Silver and Wallaby creeks, and Lake Eildon (water entitled from the Goulburn and Murray systems) were the inter-region claims for the Melbourne region. The Melbourne retail water authorities' share of storage volume in the Thomson Reservoir and Lake Eildon at the end of the 2016–17 year was derived from the share of storage opening volume, inflows, diversions, and outflows or losses from the reservoir. Remaining volume at end of the year is not carried over to the following year from entitled water from Silver and Wallaby creeks. As a result, the opening and closing balances of the claim remained at zero for this source.

 Increase in claims: desalinated water

Three water utilities: City West Water, South East water and Yarra Valley Water are entitled to water from the Wonthaggi Desalination Plant. As information extracted from their annual reports, City West Water (12,199 ML), South East Water (16,470 ML) and Yarra Valley Water (17,547 ML) had 46,216 ML allocation from the desalination plant for the 2016–17 year. 

Surface water allocation remaining: urban water system / Urban claims: surface water

The allocation remaining at 30 June 2017 was calculated as shown in Table N6.

The share of storages remaining for retail and regional water authorities at the end of the year was recognised as allocation remaining for the urban water system.

The uncertainty estimate was not quantified for allocation remaining for the urban water system.

Allocation: inter-region

The volumes allocated under Tarago River–Gippsland Water bulk entitlement and drought contingency allocation for Gippsland Water from Tarago Reservoir were extracted from the Gippsland Water annual report 2016–17. As formal annual allocations are not made, entitled volumes were treated as allocations.

Adjustment and forfeiture: inter-region

The forfeiture volume for drought contingency allocation from Tarago Reservoir and allocations made for the Tarago River–Gippsland Water bulk entitlement for Gippsland Water was allocation less diverted volume because carryover provisions were not available.

Transfer: inter-region

The volume of surface water diverted from the Melbourne region to Gippsland Water authority area (which is beyond the region boundary) was extracted from the Gippsland Water annual report 2016–17. A portion of the volume diverted by Gippsland Water was supplied by Melbourne Water which manages the Tarago Reservoir. This volume is published in the Melbourne Water annual report 2016–17.

Reporting partner internal databases

Surface water allocation: urban system / Increase in urban claims: surface water

Surface water allocation volume to the urban water system includes:

• increases to the share of storage through inflows such as rainfall and runoff for those bulk entitlements that include a share of storage
• increases in the entitlement equivalent to the delivery volume for those bulk entitlements that do not include a share of storage.

Uncertainty estimates were not quantified for the provided volumes.

Surface water adjustment and forfeiture: urban system / Decrease in urban claims: surface water

Adjustment and forfeiture of surface water allocation to the urban water system was calculated as specified in Table N6 taking into account opening and closing balances, allocation announcement and entitled diversions.

The unaccountable losses used in the calculation process were estimated by the Bureau. All the other adjustments, forfeitures, and losses used in the calculation process were as received from Melbourne Water and Western Water.

The reasons for unaccountable losses are as follows:

• The Yarra basin bulk entitlement harvest volume is calculated using the metered inflow to Sugarloaf Reservoir (from the Yarra River, Maroondah aqueduct, and the north-south pipeline); however, the actual volume diverted for urban water supply (for consumption) is the outflow from Sugarloaf Reservoir.
• Similarly, the Yarra basin bulk entitlement harvest volume is calculated using the metered inflow to Silvan Reservoir; however, the actual volume diverted for urban water supply (for consumption) is the outflow from various reservoirs that can be supplied from Silvan Reservoir (e.g. Cardinia and Greenvale reservoirs).
• In years when storages are refilling (e.g., post-drought), the inflow to a reservoir may exceed its outflow. Similarly, in dry years when storages are being drawn down, the outflow may exceed the inflow.

The uncertainty estimate for adjustments and forfeiture volumes was not quantified.

Decrease in claims: inter-region

Decreases to inter-region claims held by the Melbourne region were calculated for entitled water from the Thomson Reservoir, Silver and Wallaby creeks and Lake Eildon.

Adjustment and forfeiture volume for the Thomson Reservoir comprised of evaporation and adjustments for environmental flows. Evaporation of the Melbourne retailers' (i.e. City West Water, Yarra Valley Water, and South East Water) share of storage in Thomson Reservoir was determined using the pan evaporation method.

Carryover provisions were not available and diversion was treated as the claim; therefore, there was no forfeiture of claims for entitled water from Silver and Wallaby creeks during the 2016–17 year.

Adjustment and forfeiture volume for the Lake Eildon (water entitled from the Goulburn and Murray systems) comprised of evaporation, the volume traded from entitled water, water provided for firefighting purposes and unaccounted losses. In the allocation management process, 5% of the water held in Lake Eildon for the Melbourne region is subtracted on 1 July each year to account for annual evaporation.

Uncertainty estimates for the provided adjustment and forfeiture volumes were not quantified.

Delivery: inter-region agreements

The total volume of water delivered to the Melbourne region under inter-region claims during the 2016–17 year was the sum of the volumes received from the Thomson Reservoir, Silver and Wallaby creeks, and Lake Eildon.

The volume of water delivered from Thomson Reservoir was measured by the depth of water flowing over the spillway chute at the inflow point to Upper Yarra Reservoir (which behaves as an ogee weir) using a Mindata 2100P level sensor. The water depth was converted to flow using a stage–discharge relationship. Estimated uncertainty related to the accuracy of measurements is +/– 2%.

The volume of water delivered from the Silver and Wallaby creeks to the Toorourrong and Yan Yean reservoirs occurs via Clearwater Channel. Clearwater Channel also diverts water from the Plenty River (in the Melbourne region) to Yan Yean Reservoir. The total volume of water transferred along the Clearwater Channel was metered after the point where water enters from Plenty River. Based on historical records of catchment runoff, the flow along Clearwater Channel was estimated to comprise 64% of the water delivered from the Silver and Wallaby catchments, and 36% of water diverted from the Plenty River. The volume of water transferred-in from the Silver and Wallaby catchments was derived by multiplying the Clearwater Channel meter reading by a factor of 0.64. Estimated uncertainty related to the accuracy of measurements is +/– 1%.

The volume of water delivered from Lake Eildon was measured at the North–South pipeline pump station. There are six pumps at the pump station that are divided evenly into two pump-well groups. The total volume of water diverted from the Goulburn River was calculated as the sum of the volume pumped from the two wells, measured using a Siemens Sitrans SONOKIT dual-beam ultrasonic flow meter. Estimated uncertainty related to the accuracy of measurements is +/– 1%.

Decrease in claims: desalinated water

As there were no carryover provisions, allocations (increase in claims) less delivered volume was treated as decrease in claims for desalinated water. 

Delivery: desalinated water

As information extracted from their annual reports, three water utilities: City West Water (12,199 ML), South East Water (16,470 ML) and Yarra Valley Water (17,547 ML) received 46,216 ML for the 2016–17 year from the Wonthaggi Desalination Plant. Received volume was metered and delivered at Cardinia Reservoir.

Wastewater system

The storage volume of the wastewater system includes treated wastewater stored in the region’s wastewater system via tanks and open lagoon systems.

At the Western Treatment Plant, an estimate of wastewater storage volume was based on surveys conducted by Melbourne Water between 1980 and 2000 to determine the approximate volume of each lagoon.

At the Eastern Treatment Plant, an estimate of wastewater storage volume was determined using level gauging in effluent-holding basins. Levels were interpreted as volumes using stage–storage relationships. Stage–storage relationships were determined from surveys of the effluent holding basins. Effluent-holding basins at the Eastern Treatment Plant that held wastewater at the time of measurement were Forebay, SEHD, and EHB6.

The total volume of wastewater currently stored in the Western Treatment Plant was assumed to be the same as that held on 1 July 2011 and 30 June 2012. Since the surveys were completed in 2000, the volume of sludge in the process has varied considerably and there have been significant changes in the operation of the lagoons that undertake the bulk of the wastewater treatment.

At the Eastern Treatment Plant, the content of liquid in aeration tanks was approximately 50% primary effluent and 50% activated sludge. Raw sewage and primary and secondary effluent channel volumes have not been included because the volumes are insignificant.

There are a number of other wastewater treatment plants in the region; however the volumes of their stored treated wastewater was not included as they were not considered material to the Account.

The uncertainty estimate was +/–40%.

Metered and estimated data provided by water authorities

Discharge: wastewater / Wastewater discharge: surface water

The volume of wastewater and recycled water discharged to surface water is metered and includes:

• disposal of treated wastewater to rivers and other surface water
• discharge of recycled water for environmental purpose

Treated wastewater disposal to rivers and streams which are estuarine in nature, or subject to tidal impacts, are not reported in this volume but instead reported as discharge to sea.

For the reported volume, the uncertainty estimate was +/–10%.

Discharge: urban system / Supply system discharge: surface water

The supply system discharge: surface water volume is metered and includes return of excess water from the urban water supply system back to surface water / reservoirs for balancing.

For the reported volume, the uncertainty estimate was +/–10%.

Leakage: urban water system /Leakage: groundwater

This volume now includes volumes that were historically reported under "leakage: landscape" which accounted for pipe bursts. The real losses component of non-revenue water reported as leakage: groundwater is based on both avoidable and unavoidable losses (including pipe network background leaks, pipe leaks and bursts, tank and service reservoir leakage, and overflows) and is calculated using the following equation:

Real losses = Non-revenue water – (Apparent losses + Unmetered authorised consumption).

The volume of apparent losses comprises two components: unauthorised consumption (e.g., water theft); and customer meter under-registration (e.g., meter inaccuracies).

Leakage in the wastewater system is not included here. Where non-revenue water real losses are reported as a combined volume for pipe bursts and background leakage, they are also reported in this volume, which may overestimate the volume.

The uncertainty estimate was not quantified for the volume provided by Melbourne Water. For the other volumes, the uncertainty estimate was +/–20%.

Supply system delivery: inter-region

The Supply system delivery: inter-region volume is water received from inter-regional sources and is based on metered information at the distribution infrastructure.

Water received from Ballarat supply line and provided to Ballan customers by Central Highlands Water has been treated as inter-region supply to the Melbourne region's urban supply system. The uncertainty was estimated to be 0% to +/–5% based on meter specifications.

Wastewater collected

The wastewater collected volume is estimated using the aggregated metered inflow to wastewater treatment plants and sewer mining plants within the region:

• minus any recirculation such as treated wastewater volume that was reported as discharge back to sewer in the region, to avoid double counting.

Given wastewater volumes are typically measured at the treatment plants (and not at customer connections), the collected wastewater volume includes any variation due to (a) ingress of stormwater; (b) infiltration of groundwater; and (c) unreported wastewater overflows to stormwater.

Where inflow meter readings are not available, outflow meter readings have been used, which could underestimate the volume because it assumes no losses during wastewater treatment.

This volume does not include wastewater collected for individual or community wastewater management systems.  

Melbourne Water has a suite of bulk wastewater meters at their two major wastewater treatment plants, the Western Treatment Plant and the Eastern Treatment Plant. Melbourne Water uses formulas to apportion the wastewater volumes generated in the service areas of the three water retailers (City West Water, South East Water, and Yarra Valley Water). These apportioned volumes are reported by City West Water, South East Water, and Yarra Valley Water and were used in the 2017 Account for calculating the collected wastewater in the region.

The uncertainty was estimated to be +/–20% based on meter specifications.

Supply system delivery: urban users

The 'Supply system delivery: urban users' volume is urban consumption of water and was derived from:

• customer meters
• billing meters
• estimated non-revenue water volumes.

Urban consumption consists of residential, commercial, industrial, municipal use, and small scale agriculture / irrigation uses.

Information for the volume was received from Central Highlands Water, City West Water, Melbourne Water, South East Water, Western Water, and Yarra Valley Water.

The volume delivered to non-urban users (supply to irrigation schemes and the environment) was not included in the reported volume.

The following assumptions have been made in calculating the volume:

• Western Water’s data for water use was based on metered readings with the assumption that 85% of the total water supplied was for residential purposes and 15% for commercial, industrial, and municipal use. The volume was calculated by deducting the water loss component associated with residential supply (assumed to be 12.67% of total supply).

For the reported volume, the uncertainty estimate was +/–10%.

Supply system delivery: irrigation

The 'Supply system delivery: irrigation' volume is the metered volume of water supplied for use in Irrigation schemes.

The volume supplied to the Werribee Irrigation District and other river diverters was reported by Melbourne Water on behalf of Southern Rural Water.

Uncertainty information for this volume was not quantified.

Supply system transfer inter-region

The 'Supply system transfer: inter-region' volume measures the transfer of water to outside of the region. The volumes are based on metered information at the distribution infrastructure. The volume includes Western Water's transfer from Mount Macedon to Woodend via its water supply infrastructure and Melbourne Water's transfer from Cowies Hill Reservoir to Barwon Water. Uncertainty estimate was not quantified for the volume.

Other supply system decreases

The 'Other supply system decreases' volume was assumed to be the non-revenue water associated with apparent losses and the remaining non-revenue water from the urban water supply system (if not reported in 'Leakage: landscape' and 'Leakage: groundwater' respectively).

The non-revenue volume was calculated based on physical observations of bursts events. Real losses reported related to pipe leakage is reported in Leakage: groundwater. Where pipe bursts and background leakages are provided as a combined volume, for simplification this is reported as Leakage: groundwater because the leakage volume to landscape cannot be separated. City West Water and Western Water used this approximation.

The uncertainty estimate was +/–20% for the provided volume.

Remaining non-revenue water was estimated using:

• difference based on a water balance between metered volumes of water sourced and supplied to customers
• difference between metered supply into the urban water supply system and metered volume of water consumed (revenue water) and subtracting real losses
• modelling software of network real losses (leakages and busts) and apparent losses (unauthorised/authorised unbilled use)
• time to repair leaks
• difference between inlet meter and outlet meter of treatment plants for treatment losses.

South East Water included infrastructure leakage, pipe burst, thefts, and leak allowances in the non-revenue water volume.

For Melbourne Water, water supply losses were not included and water lost from aqueducts was not included because these are located upstream of the seasonal storages.

No water losses were reported and therefore losses in the system are likely to be underestimated.

The uncertainty for the provided total volume was estimated to be +/–40%.

Discharge: sea

The 'Discharge: sea' volume is the metered volume of disposals from the wastewater system and recycled water system to the sea, estuaries, inlets, and portions of rivers and streams with tidal impacts (which are considered outside of the region). This includes flows to Port Phillip Bay and Bass Strait. City West Water, Melbourne Water, and South East Water provided discharged volume to the 2017 Account.

For Melbourne Water, the volume was based on a mass balance calculation of all wastewater collected, treated, stored, and recycled at its Western Treatment Plant and the Eastern Treatment Plant. The uncertainty estimate was not quantified for this volume.

Volumes from City West Water and South East Water were based on metered data. The uncertainty for both of them was estimated as +/– 5%.

Where metered disposal data was not available, the volume was estimated based on the difference between metered inflow to a wastewater treatment plant and metered volume of recycled water used.

Discharge: landscape

The 'Discharge: landscape' volume is the metered treated wastewater discharge to landscape and/or infiltration ponds, where the primary purpose is disposal of the effluent rather than using the effluent for irrigation purposes. Also included in this volume is any other managed treated wastewater discharges not reported as discharges to surface water or sea.

Information on discharge of treated wastewater to landscape was received from South East Water and Yarra Valley Water for the 2017 Account. Discharges were made from Aurora and Whittlesea wastewater treatment plants.

Where metered disposal data was not available, the volume was estimated based on the difference between metered inflow to a wastewater treatment plant and metered volume of recycled water used. This volume may include discharges to landscape that make their way into the groundwater.

The volume provided in the 2017 Account has been estimated and the uncertainty estimate was not quantified.

Recycled water delivery: urban users

The 'Recycled water delivery: urban users' consists of residential, commercial, industrial, municipal, onsite (water and wastewater treatment plant) use, and small scale agriculture / irrigation uses.

The volume excludes recycled water re-circulated within the wastewater treatment process.

Information for the provided volume was received from Central Highlands Water, City West Water, Melbourne Water, South East Water, Western Water, and Yarra Valley Water.

The volume was derived from customer meters, billing meters, and onsite re-use water meters.
The volume delivered to non-urban users (supply to irrigation schemes and the environment) is not included in the reported volume.

 The following assumptions have been made in calculating the volume:

• Part of the volume used on-site for wastewater treatment processes re-enters the system. Therefore, the volumes may not be accurate at some plants.

For the reported volume, the uncertainty estimate was +/–10%.

Recycled water delivery: irrigation

The 'Recycled water delivery: irrigation' volume is the metered volume of recycled water supplied for use in irrigation schemes.

Melbourne Water supplied recycled water to the Werribee Irrigation District from the Western Treatment Plant and to the Eastern Irrigation Scheme from the Eastern Treatment Plant.

Melbourne Water's recycled water supply from the Western Treatment Plant to Southern Rural Water for the Werribee Irrigation District was measured with magnetic flow meters at the interface of two authorities. The Western Treatment Plant logs data daily and reports quarterly. Magnetic flow meters are assumed to have an accuracy of +/–2%. Uncertainty information for the remainder of the reported volume was not quantified.

Other wastewater decreases

The volume reported in the 2017 Account represents the sum of the following components:

• losses from wastewater treatment system not reported as evaporation
• known losses from the wastewater collection system
• known egress or exfiltration from the wastewater collection system occurring before metered inflow to wastewater treatment plants.

The losses from the wastewater system were estimated based on metered data, or estimated based on observations.

Wastewater overflows or spills were estimated based on observation or monitoring of the sewer network. This may have occurred at emergency relief systems built into the network or uncontrolled points at manholes and network leaks.

The uncertainty for the provided volume was not quantified.

Allocated diversion: urban system / Allocated diversion: surface water

The 'Allocated diversion: urban system / Allocated diversion: surface water' refers to allocated diversions from surface water sources to water treatment plants under the various bulk entitlements held by Melbourne's retail and regional water authorities. Non-allocated diversions (mostly unregulated) are not included in the volume.

The volume was calculated from the volume of surface water diverted (metered at the source) and inflow to water treatment plants. The volume reported does not include water delivered by Melbourne Water to Gippsland Water.

Meters used to measure the volumes have an accuracy of +/–5% in accordance with the manufacturers' annual bulk meter tests.

Allocated extraction: urban system / Allocated extraction: groundwater

The 'Allocated extraction: groundwater' volume was calculated from the volume of groundwater extracted (metered at the source) and inflow to water treatment plants.

Where metered inflows to water treatment plants are not available, these volumes are assumed to equal the metered outflow volume (it is assumed no water losses occurred during the treatment process).

The uncertainty estimate for the volume was estimated to be +/– 5%, in accordance with manufacturers' annual bulk meter test.

Victorian Water Register

Surface water allocation: individual users

Yearly allocations were calculated by multiplying entitlement volumes with relevant percentages of announced allocations. The percentages of announced allocations for high-reliability and low-reliability water shares were 15% and 0% respectively.

Surface water adjustment and forfeiture: individual users

Adjustment and forfeiture of surface water allocation to individual users was calculated taking into account opening and closing balances, allocation announcement, and entitled diversions.

Groundwater allocation: individual users

Groundwater allocations in the Melbourne region were calculated using the following methods:

• Licensed entitlement: allocation was equal to the sum of licensed entitlement volumes for each groundwater management unit recorded in the Victorian Water Register.
• Stock and domestic entitlements (for individual users): allocation was equal to the number of stock and domestic bores in groundwater management units (recorded in the Victorian Water Register) multiplied by the 1.5 ML entitlement per bore except for Nepean Groundwater Management Area for which allocation was estimated as 1 ML per bore.

The uncertainty estimate was not quantified for these estimations.

Groundwater allocation: urban system / Increase in urban claims: groundwater

Groundwater allocation was equal to the sum of licensed entitlement volumes for each groundwater management unit recorded in the Victorian Water Register. The uncertainty estimate was not quantified for these estimations.

Non-allocated diversion: individual users

The non-allocated diversion volume is a combination of metered diversion data and estimates of non-metered diversions. Southern Rural Water meters surface water diversions for take-and-use licences in the Maribyrnong, Werribee, and Bunyip catchments. Melbourne Water meter surface water diversions for take-and-use licences in the Maribyrnong and Yarra catchments. Diversion data is submitted by those two authorities to the Victorian Water Register.

Where metered data were not available, the diversion was estimated using the following assumptions:

• the diversion volume was assumed to be equal to zero for licences in the Yarra catchment (as per Melbourne water assumptions)
• the diversion volume was assumed to be equal to the licensed entitlement volume for licences in the Bunyip, Maribyrnong, and Werribee catchments (as per Southern Rural Water assumptions).

The following assumptions were made to estimate unmetered diversions:

• The assumption that the volume diverted under take-and-use licences in the Bunyip, Maribyrnong, and Werribee catchments is equal to the entitlement volume is likely to overestimate the actual diversion.
• The assumption that the volume diverted under take-and-use licences in the Yarra catchment is equal to zero is likely to underestimate the actual diversion.

Metered diversions have an uncertainty of +/– 10% due to known errors of meters. The uncertainty estimate for unmetered estimates of diversion was not quantified.

Allocated diversion: individual users

Diversion volume is the sum of diverted volume for high-reliability water shares and low-reliability water shares and the volume supplied by Southern Rural Water under agreements. Southern Rural Water meters these diversions in the Melbourne region. Diversion volumes for supply by agreements for the 2017 Account were obtained directly from Southern Rural Water. Metered diversions have an uncertainty of +/– 5% due to known errors of meters.

Allocated extractions: individual users

Entitled extraction of allocated groundwater to individual users presented for the Melbourne region is a combination of metered extraction data and estimates of non-metered extractions.

Metered extraction volumes were obtained from the Victorian Water Register for all groundwater management units for the 2016–17 year. Southern Rural Water provides its metered groundwater extraction data to the Victorian Water Register.

Stock and domestic entitlements are not currently metered in Victoria. Extraction was estimated by multiplying a usage factor between 0 and 2 ML by the number of stock and domestic bores in the region as follows:

• a factor of zero estimates that no extraction occurred
• a factor of two estimates that the full entitlement was extracted.

During the 2016–17 year, a usage factor of 1.5 was applied to all stock and domestic entitlements, with the exception of Nepean groundwater management area (GMA) where a usage factor of 1 was applied.

The following assumptions were made in estimating entitled extraction of allocated groundwater to individual users:

• Not all groundwater bores are metered. Bores that were not metered were not used or did not fall within the threshold of Southern Rural Water's metering program. The number of non-metered bores was estimated to be 10% to 20% of the total bores in the area. The volume of extraction from bores that are not metered was not considered to have a material impact on this water accounting report.
• A broad assumption was made to estimate stock and domestic extraction. It is unlikely that all extraction matches the assumption. In a wet year, the estimate is likely to overestimate the volume of stock and domestic extraction.

Metered data used in estimating entitled extraction has an uncertainty of +/–5% based on meter specifications. Estimated data has an uncertainty of +/–100% based on the broad assumptions applied.

Werribee and Bacchus Marsh system allocation model

Surface water allocation remaining: individual users

The allocation remaining at 30 June 2017 was calculated as shown in Table N7.

Surface water allocation remaining for individual users calculated according to Table N7 aligns with the calculation method specified in the Werribee and Bacchus Marsh allocation model. Details of the allocation model are available on the Southern Rural Water website. The uncertainty estimate was not quantified for allocation remaining for individual users.

Southern Rural Water: Irrigation Planning Module Generation 2 database

Point return: irrigation

Irrigation water returns from the Bacchus Marsh Irrigation District were metered at each outfall point that returns water to the Lerderderg River. This data was collated by Southern Rural Water and provided to the Victorian Department of Environment, Land, Water and Planning. The uncertainty estimate was not quantified for the provided volume.

Groundwater catchment statements

Aquifers

The Victorian Department of Environment, Land, Water and Planning determined the volume of aquifers in groundwater management units to set a permissible consumptive volume for each aquifer.

Permissible consumptive volumes for the aquifers of groundwater management units were determined as follows:

• Cut Paw Paw GMA: the permissible consumptive volume is based on the through-flow method.
• Koo Wee Rup water supply protection area (WSPA): the permissible consumptive volume is based on numerical modelling and observed responses to pumping.

• Frankston GMA: permissible consumptive volume was determined using rainfall recharge, hydrograph fluctuation, through-flow, and aquifer storage.

• Lancefield GMA: permissible consumptive volume was determined on the rainfall recharge method.

• Merrimu GMA: permissible consumptive volume was derived from a rounded calculation of rainfall recharge.
• Moorabbin GMA, Nepean GMA, Wandin Yallock WSPA, and Deutgam WSPA: permissible consumptive volumes were based on the total volumes of existing licences issued for these groundwater management units.

The following assumptions and limitations apply to the estimated groundwater asset volume:

• Permissible consumptive volumes represent the groundwater asset within groundwater management units in the Melbourne region.  An estimate of the size of the groundwater asset in the unincorporated areas outside the groundwater management units has not been made, making it difficult to estimate the total groundwater resource for the Melbourne region.
• Where the bulk of extraction is from depths greater than 50m, groundwater management units are considered underlying aquifers.
• Frankston, Moorabbin, Nepean, Wandin Yallock, and Lancefield groundwater management units and Koo Wee Rup WSPA include all geological formations.
• Frankston GMA and Lancefield GMA: the permissible consumptive volume estimation assumed rainfall infiltration factors of 5%.
• Merrimu and Deutgam groundwater management units include all geological formations from 0m–30m below the surface. Rainfall infiltration factors of 10% in the north, 7.5% in the central area, and 5% in the south were assumed in estimating permissible consumptive volume for Merrimu GMA.
• Cut Paw Paw GMA includes all geological formations more than 50m below the surface. The permissible consumptive volume is based on the assumption that through-flow is equal to the safe groundwater extraction volume.

Following uncertainty information applies to estimated groundwater asset volumes in aquifers:

• Cut Paw Paw GMA: the permissible consumptive volume has a low confidence rating due to uncertainty in aquifer parameters and potentiometry.
• Frankston GMA: permissible consumptive volume has a low-confidence rating due to uncertainty in the derivation of the infiltration factor, hydrograph, through-flow calculations, and the extent of the recharge area.

• Lancefield GMA: permissible consumptive volume has a low-confidence rating due to uncertainty in rainfall infiltration data, recharge processes, and distribution.

• Merrimu GMA: permissible consumptive volume has a low-confidence rating due to uncertainty in rainfall infiltration data, recharge processes, and distribution.

Bureau of Meteorology: groundwater modelling

Inter-region inflow and outflow and inter-region coastal inflow and outflow

The Bureau estimated groundwater flow from/to outside the Melbourne region using data from the Port Phillip Catchment Management Authority's groundwater model (Department of Sustainability and Environment 2010) and the following information:

• hydraulic conductivity and aquifer thickness (Port Phillip Catchment Management Authority)
• bore locations, groundwater level data, and aquifer attribution (Department of Environment, Land, Water and Planning).

Groundwater flow was calculated using a simple geographic information system (GIS) approach based on Darcy's law. Groundwater levels were interpolated for each season from reduced groundwater levels measured at monitoring bores using kriging with external drift and the 9" digital elevation model as an external driver taking into account the effect of the coastline on groundwater levels following the methodology presented in Peterson et.al. (2011). Seasonal groundwater flow grids were derived from groundwater level grids, aquifer thickness, and hydraulic conductivity using a modification of the ArcGIS Darcy Velocity tool. Groundwater flow across selected flow boundaries (Figure N2) was then calculated using a simple GIS analysis, and seasonal values were aggregated to the reporting period.

Figure N2 Lateral groundwater flow boundaries in the Melbourne region

Groundwater flow across the Melbourne region boundaries other than at the coast was assumed as negligible on an annual basis based on the fact that the northern and northeast boundaries represent a groundwater divide. Groundwater flow along the landward boundary of the region is thought to be minimal (Department of Sustainability and Environment 2010). A small amount of cross-boundary flow is likely in the Kinglake area due to the concentration of abstractions from the Kinglake GMA immediately to the north of the Port Phillip catchment management authority boundary.

The following assumptions were made in the estimation of groundwater flows:

• Regional flow was estimated for layers 1–5 of the Port Phillip groundwater model (Department of Sustainability and Environment 2010), which represent the sedimentary and basalt aquifers with the exclusion of the basement (Layer 6). These productive aquifers are considered to be the most hydraulically conductive units while the flow in other units is assumed to be insignificant.
• Consistent with the groundwater model report (Department of Sustainability and Environment 2010), groundwater flow in the Nepean Peninsula was calculated only for Layer 1. It was modelled as a fresh water lens in the highly permeable unconfined aquifer using the same GIS tool.
• Groundwater levels were estimated by assuming that all five hydrogeological layers within the Port Phillip groundwater model region are hydraulically interconnected. This assumption facilitated the interpolation of a groundwater potential surface from groundwater level measurements, as these measurements were limited in number. Groundwater levels were also assumed equal to 0 meters AHD at the coastline. These assumptions were used to generate seasonal groundwater level surfaces across the sedimentary area.
• The through-flow boundaries considered in the estimations are indicated in Figure N18. Flow across the remaining boundaries was assumed negligible on an annual basis because aquifer properties such as hydraulic conductivity limit flow (e.g., fractured rock basement). The northern and northeast boundaries represent a groundwater divide with no through-flow, or groundwater flow was approximately parallel to the boundary (e.g., western boundary).
• The regional flow estimations were based on the 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). The use of different interpolation methods may impact the values of the groundwater level grids and hence the estimated regional flow.

The uncertainty in the field measured data (e.g., groundwater levels, hydraulic conductivity) was unspecified and unknown; the impact of such uncertainty on the groundwater flow was not estimated. Groundwater flow was estimated for a simplified boundary constructed from a series of line segments. The uncertainty surrounding this simplification was not analysed.

Other groundwater increases/decreases

The change in aquifer storage was calculated using groundwater levels for the water table aquifers within the sedimentary area identified in Figure N3. The groundwater levels were estimated using all bores within the region, assuming that all hydrogeological layers were hydraulically inter-connected.

Figure N3 Water table aquifer areas used to calculate change in aquifer storage

Bureau of Meteorology: internal calculations

Adjustment and forfeiture: individual users and urban system / Decrease in urban claims: groundwater

As carryover of unused water allocation to the following year is not allowed, adjustment and forfeiture volumes were equal to the announced allocation minus extractions during the 2016–17 year.