National Water Account 2019

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Melbourne: Methods

Summary of methods

There were five key methods for establishing item volumes in the 2019 Account. Click the down arrow in the table below to view the list of items derived by each method type. For detailed information about each method scroll down this page or click on the links in the table.

Detail of methods

AWRA models

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)

Rivers

The volume of water in the river channels at 30 June was estimated by using the daily water balance approach within the AWRA-R model. The water balance includes inflow at the upstream nodes and outflow at the downstream nodes; contributing catchment runoff, diversions for consumptive use; overbank flooding and floodplain return; rainfall; evaporation; and losses.

Precipitation and evaporation

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 were done on a daily time step, with the annual totals summed from the daily 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 daily surface area of storages was calculated from daily storage levels and capacity tables.

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).

Outflow

To estimate the total surface water outflow from the National Water Account region, the AWRA-R and AWRA-L models were used to simulate daily flows at the end of each river flowing out of the region. These flows included:

Runoff

Runoff to surface water was estimated using the modelled runoff from the AWRA-R model. Runoff within AWRA-R is in turn derived from landscape runoff modelled in the AWRA-L model, with a scaling factor applied within AWRA-R during the calibration process.

The AWRA-L model is a daily grid-based water balance model that is conceptualised as a small unimpaired catchment (Viney et al. 2015). It simulates the flow of water through the landscape from rainfall entering the grid cell through the vegetation and soil, and then out of the grid cell through evapotranspiration, runoff or deep drainage to the groundwater. Its inputs include gridded climate, soil, vegetation and topographic data. For more information see the Bureau of Meteorology's Australian Landscape Water Balance webpage.

Groundwater recharge: 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:

• 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 et al. 2009).
• 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.
• 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 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 et al. 2015).

Water storage product data

Storages

Storage volumes at the start and end of the year were obtained from the Bureau's Water storage product. Storage volumes are calculated using water level data (metres above Australian Height Datum) collected at each storage. Capacity tables established for each storage were used to convert the height measurement to a volume.

The volume of individual storages was aggregated to present the total volume for the line item as detailed in the supporting information table. The uncertainty range for the storage volume is +/–5%.

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.
• Storages are subject to sedimentation and other physical changes over time that in turn affect the accuracy of the storage–volume curves.

Metered and estimated data provided by water authorities

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 that 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%.

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 reservoirs for balancing.

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

Leakage: groundwater

The 'Leakage: groundwater' volume is assumed to be the non-revenue water associated with real losses, specifically due to background pipe leakage from the urban water supply system.

The real losses 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).

The limitations with this approach were:

• Leakage in the wastewater system is not reported and therefore the total leakage to groundwater is likely to be underestimated.
• Where non-revenue water real losses are reported as a combined volume for pipe bursts and background leakage, with no breakdown, this was reported in Leakage: groundwater, which may overestimate the volume.

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

Supply system delivery: inter-region

The 'Supply system delivery: inter-region' volume refers to 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 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 refers to the urban consumption of water and is 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. The volume delivered to non-urban users (supply to irrigation schemes and the environment) is not included.

Volumes provided by Western Water were 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 represents the transfer of water to outside of the region.

The volume is based on metered information at the distribution infrastructure. It 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.

The uncertainty estimate was not quantified for this 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: groundwater').

The non-revenue volume was calculated based on physical observations of bursts events. Real losses related to pipe leakage are reported in 'Leakage: groundwater'. Where pipe bursts and background leakages are provided as a combined volume, for simplification, this is reported as 'Leakage: groundwater'.

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.

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

Discharge: sea

The 'Discharge: sea' volume is metered and refers to 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.

Volumes provided by Melbourne Water were 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 were 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.

Discharges were made from Aurora, Wallan and Whittlesea wastewater treatment plants. Where metered disposal data were 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 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.

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.

It should be noted that 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 'Other wastewater decreases' volume 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

The 'Allocated diversion: urban system' 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

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.

Reporting partner databases and annual reports

Allocation remaining

The water allocation remaining at the end of the 2018–19 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June 2019 is calculated as shown in the table below.

In terms of surface water supply for the urban system, however, the above table is not applied. Instead, the allocation remaining is equal to the share of storages remaining for retail and regional water authorities at the end of the year.

Adjustment and forfeiture

For groundwater supply to individual users, as well as surface water transfers outside the region, the portion of water allocation that has not been abstracted at the end of the year is forfeited (i.e., there is no carryover of entitlements). Therefore, forfeiture is calculated as the total annual allocation for each licence minus the allocation abstraction during the year.

Adjustment and forfeitures related to surface water supply to individual users include deductions made for evaporation, as well as mandatory forfeitures at end of the year. Adjustment and forfeiture is derived based on associated variables such as opening and closing balances, allocation, and entitled diversions, as shown in Table N4 (see allocation remaining above).

In terms of surface water supply for the urban system, adjustment and forfeiture (or unaccountable losses) is also calculated using a variation of Table N4. 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.

Claims: inter-region

The remaining volume of water entitled under an inter-region claim at 30 June 2019 was calculated as shown in the table below.

Inter-region claims refer to the rights to remaining entitled water at Silver and Wallaby creeks, and Lake Eildon (water entitled from the Goulburn and Murray systems).

The Melbourne retail water authorities' share of storage volume in Lake Eildon at the end of the 2018–19 year was derived from the share of storage opening volume, inflows, diversions, and outflows or losses from the reservoir. The volume of entitled water remaining from Silver and Wallaby creeks, however, is not carried over to the following year and so the closing balance of the claim is zero for this source.

Increase in claims: inter-region

The increase in claim on water sourced from outside the region refers to the total volume of entitled water from Lake Eildon and Silver and Wallaby creeks.

For water supply from the Silver and Wallaby creeks, there is no entitlement to share storage capacity for entitled water in these creeks. Therefore, the increase in claim is equal to the volume of water delivered during the year.

Increase in claims for water held in Lake Eildon is based on allocation announcements for the Goulburn River and the River Murray. The announcements are based on audited water savings achieved from modernisation projects in northern Victoria the previous year.

Decrease in claims: inter-region

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

There was no forfeiture of claims for entitled water from Silver and Wallaby creeks as carryover provisions were not available and diversion was treated as the claim.

Adjustment and forfeiture volume for Lake Eildon (water entitled from the Goulburn and Murray systems) comprised 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 is equal to the sum of the volumes received from Lake Eildon and Silver and Wallaby creeks.

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%.

Delivery: desalinated water

The volume of desalinated water delivered to the three water utilities (City West Water, South East water and Yarra Valley Water) from the Wonthaggi Desalination Plant was based on metered data reported in the utilities' annual reports.

The desalinated water is delivered to Cardinia Reservoir where it is then diverted by the utilities for urban supply.

Allocation: inter-region

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

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 based on the Gippsland Water Annual Report. A portion of the volume diverted by Gippsland Water was supplied by Melbourne Water which manages the Tarago Reservoir. Information on this volume is available in the Melbourne Water Annual Report.

Surface water allocation: urban system

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 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.

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

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 were obtained directly from Southern Rural Water.

Metered diversions have an uncertainty of +/– 5% due to known errors of meters.

Allocated extraction: 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 reporting 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 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.

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%.

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 and modelling

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.

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.