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National Water Account 2018

Sydney: Methods

Cataract Dam, New South Wales. Source: WaterNSW

Summary of methods

There were four key methods for establishing item volumes in the 2018 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.


Methods approach

AWRA-R model and WaterNSW

Water storage product data, WaterNSW and Utilities

Water resourcing licence database, water sharing plan and meter readings

Metered data provided by water authorities


Detail of methods

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)



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.


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).


Leakage: groundwater

The AWRA-R river and groundwater interaction module was used to model the volume of leakage: groundwater. The module simulates seepage based on the potential infiltration rate from the river, the available storage in the underlying aquifer and the aquifer's discharge rate. The equations used in the module are based on Doble et al. (2014).


Precipitation and evaporation

Data source: Bureau of Meteorology

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 daily surface area of storages was calculated from daily storage levels and capacity tables.

Data source: WaterNSW

WaterNSW provided precipitation data for the following storages: Avon Reservoir, Blue Mountains reservoirs, Cataract Reservoir, Cordeaux Reservoir, Fitzroy Falls Reservoir, Lake Burragorang (Warragamba Dam), Lake Yarrunga (Tallowa Dam), Nepean Reservoir, Prospect Reservoir, Wingecarribee Reservoir and Woronora Reservoir.

Rainfall received over the entire catchment area was calculated by interpolating rainfall values from point gauges (primarily tipping-bucket rain gauges). Rainfall volumes over storages were calculated by multiplying relevant surface areas and interpolated rainfall values. It was assumed that catchment average rainfall falls on the surface of a lake.

WaterNSW provided evaporation data also for the storages listed above. Measured pan evaporations were adjusted by a pan factor.



Runoff to surface water in the Sydney region was estimated as the sum of:

  • runoff to storages managed by WaterNSW
  • runoff to other storages not managed by WaterNSW
  • runoff to rivers

 Locations of the stream gauging stations and their catchment areas are shown in Figure N2.


Figure N2 Runoff estimation for the modelled area

Figure N2 Runoff estimation for the modelled area


Data source: Bureau of Meteorology

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's Australian Landscape Water Balance webpage.

Data source: WaterNSW

Storages managed by WaterNSW contain sensors at the gauging sites which are linked to WaterNSW's radio telemetry system. Rating tables are used to convert water levels to a volume.

For all storages, mass balance calculation was used to estimate inflows to each storage. In the calculation, inflow, storage diversions, precipitation, evaporation and other known losses, and beginning and end storage volumes for each storage were balanced for the year. These volumes were either measured data or calculated data (interpolations, application of rating tables) from measured data.

The runoff to storages managed by WaterNSW was incorporated in the AWRA-L model to estimate the runoff for the whole of the region.



Data source: WaterNSW

The river outflow from the region represents the volume of water that flows out of the Sydney region. It comprises:

  • rule-based and variable environmental flows, other unspecified releases, and spills from most downstream storages and weirs to rivers
  • natural runoff from river catchments downstream from storages and weirs, and from streams leading to the sea (less downstream diversions)

For rule-based environmental flows, the water level in each river was monitored at these sites and converted to a flow volume using a rating table. The daily flows from these sites during reporting year were used to calculate the annual streamflow.

Natural runoff was estimated using available gauged flows. Runoff was estimated when gauges were not available.

The limitations associated with the calculations are:

  • The level of uncertainty of these gauging stations is estimated at +/– 20 % during low to medium flows and the uncertainty during high flows is ungraded. This is based on four manual physical flow gaugings performed per year. At these sites, the water surface level is measured constantly by on-site equipment. This water level is used to estimate a flow rate, based on a rating curve produced by physical flow gauging in as many flow conditions as possible.
  • There is some uncertainty in the flow rates. The river flows have not been gauged under all flow conditions and the river channel can change from time to time, due to deposition and movement of river sediments, which impacts the cross-sectional area of the channel and changes the velocity of the water.

Data source: Sydney Water

Treated water releases from wastewater treatment plants were metered.

Data source: Bureau of Meteorology

River outflow is estimated with the AWRA-R model using dummy nodes assigned at the end of of each river flowing out of the region. Observed flows (or simulated flows if observed flow is not available) at the most downstream gauges are routed to the dummy nodes. The residual catchment inflows not covered by the most downstream gauge are estimated using the AWRA-L model and are added to the routed flow to obtain the total outflow at the river mouth.

Environmental flow releases, unspecified releases, spills from most downstream storages and natural runoff as obtained from WaterNSW; and treated water releases from Sydney Water were incorporated in the AWRA–R model to estimate the outflow from the region.


Water storage product data, WaterNSW and Utilities


Storage volume at the start and end of the year was 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 uncertainty range for these volumes is +/–5%.

Storage volumes were collected form storage product of the Bureau, WaterNSW and other utilities (EnergyAustralia, Goulburn Mulwaree Council, Lithgow City Council, Shoalhaven City Council, and Wingecarribee Shire Council)

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.


Water resourcing licence database, water sharing plan and meter readings

Diversions: statutory rights

The estimated annual water abstraction under riparian right for stock and domestic purposes and for cultural purposes was extracted from the Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources and the Water Sharing Plan for the Kangaroo River Water Source.

Abstraction as part of cultural basic water rights is presumed to occur only in the Kangaroo River system.



The Water Sharing Plan for the Greater Metropolitan Unregulated River Water Sources and Department of Industry details the rules for surface water and groundwater allocations for access licences with shared components under several categories; individual holders for stock and domestic, urban holders (local water utility, major utility, stock and domestic (town water supply) and other lumped holders (unregulated).


Allocation remaining

The remaining surface water and groundwater allocation corresponds to the volume of water allocation that can be carried over between water years. The water allocation remaining at 30 June 2018 is calculated as shown in Table N4.


Table N4 Calculation of water allocation remaining
 Opening balance at 1 July 2017
lessAllocated abstraction
lessAdjustment and forfeiture
equalsClosing balance at 30 June 2018


Adjustment and forfeitures

While carryover of unused surface water allocation is allowed under the Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources, for licences categorised under 'local water utility', the carryover volume could not be quantified. Therefore, the closing balance of the remaining allocation is given as zero.


Allocated diversion: individual users

Allocated diversions for individual users were estimated by the Department of Industry based on the ratio of metered usage to entitlement volume within the Hawkesbury Nepean, as no meter readings were available outside of the Hawkesbury Nepean (excluding local water utilities and major water utilities). This usage ratio of 10% was assumed to be representative of the entire Greater Metropolitan Sydney Area and then applied to estimate the total volume of allocated diversions for individual users.

Allocated diversions by EnergyAustralia from Thomsons Creek Reservoir, Lake Lyell and Lake Wallace for the Mt Piper and Wallerawang power stations were quantified using metered data.


Allocated diversion: urban system

The allocated diversion of surface water to the urban system comprises WaterNSW diversion and supply to urban entitlement holders (Goulburn Mulwaree Council, Sydney Water Corporation, Shoalhaven City Council, and Wingecarribee Shire Council), as well as raw and unfiltered water supplied by WaterNSW to retail customers.

This volume is calculated based on metered raw water diversion from surface water and inflow to water treatment plants. Raw water supplied directly as untreated (nonpotable) water is the metered diverted volume from the water source. For sources managed through WaterNSW entitlements, the surface water diversion volumes are as reported by WaterNSW, and may have some slight differences to the inflow to water treatment plants reported by the respective water authorities.

The uncertainty in this volume is estimated to be +/– 10%.


Discharge: user

EnergyAustralia discharged treated wastewater from Wallerawang Power Station to the Coxs River. Ultrasonic flow meters were used to measure the discharge.

The uncertainty associated with this volume is estimated to be +/– 2%.


Delivery: inter-region agreement / inter-region claim on water

This volume is based on metered flow measurements provided by Department of Industry, Lands and Water for the inter-basin claim on the Fish River Water Supply Scheme by WaterNSW and EnergyAustralia. There are provisions to carryover allocations up to 20% into the following year, as shown in Table N5.


Table N5 Entitlement, diversion and carryover volumes for the Fish River Water Supply Scheme
AuthorityEntitled maximum allocation (ML)Diversion (ML)Carryover as at 30 June 2018 (ML)


In addition to the above, Lithgow City Council has entitlements from the Fish River Water Supply Scheme to supply water to its storages under urban inter-region claims on water. The diverted volume associated with this claim was measured using magnetic flow meters.


Metered and estimated data provided by water authorities

Wastewater collected

The 'Wastewater collected'  volume is estimated using the 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.

The assumptions made were as follows:

  • 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 as it assumes no losses during wastewater treatment.
  • This volume does not include wastewater collected for individual or community wastewater management systems.

The uncertainty of the estimated volume is estimated to be in the range of +/– 20%.


Allocated diversion: urban system

The 'Allocated diversion: urban system' volume is calculated from the volume of surface water diverted (metered at the source) and inflow to water treatment plants.

WaterNSW reports surface water diversions volumes for entitlements that they manage. These volumes may be slightly different to the reported inflow to water treatment plants by the respective water authorities.

The assumption made was as follows:

  • Where metered inflows to water treatment plants are not available, these volumes are assumed to equal the metered outflow volume (i.e. no water losses occur during the treatment process).


Supply system delivery: urban users

The 'Supply system delivery: urban users' volume includes urban consumption of potable and nonpotable 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 assumption made was as follows:

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

The uncertainty is estimated to be +/– 7%.


Leakage: groundwater

This volume now includes volumes that were historically reported under “leakage: landscape” which accounted for pipe bursts, losses in water treatment, and disposal to sewer from the water supply system. 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).

 The assumptions made were as follows:

  • 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.
  • Sydney Water estimated leakage within their wastewater system at wastewater treatment plants as well as from the water supply system.

The uncertainity is estimated to be +/– 25%.

Leakage in the wastewater system is reported under 'other wastewater system decreases'.


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).

The assumptions made was as follows:

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

The uncertainty is estimated to be +/– 10%.


Recycled water delivery: urban users

The 'Recycled water delivery: urban users' is derived from :

  • customer meters; and
  • billing meters onsite re-use water meters.

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

Urban consumption consists of residential, commercial, industrial, municipal, onsite (water and wastewater treatment plant) use, small scale agriculture/irrigation uses, and ‘other’ use which includes non-revenue recycled water volumes due to pipe-bursts, leakage, apparent losses  and other non-revenue recycled water.


Wastewater and recycled water discharge: surface water

The 'Wastewater and recycled water discharge: surface water' volume is metered and includes:

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

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


Other supply system decreases

The 'Other supply system decreases' volume was assumed to be the non-revenue water associated with apparent losses, the remaining non-revenue water from the urban water supply system (if not reported in 'Leakage: groundwater'), and ‘own use’ which is the volume of water used by the utilities on site for purposes such as wash down, toilet flushing and watering gardens.

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.

Remaining non-revenue water is estimated using:

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

Own use was not reported.

The uncertainty is estimated to be +/– 20%.


Other wastewater and recycled water system decreases

The 'Other wastewater and recycled water decreases' volume is the sum of the following two components:

  • losses from the wastewater treatment system
  • losses during management of treated wastewater.

Losses from the wastewater system are estimated based on the metered inflow and outflow/disposal/customer meters, or estimated based on observations.

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