National Water Account 2015

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Accountability statement

Sydney: Quantification approaches

• Summary of quantification approaches
• Detail of quantification approaches

Summary of quantification approaches

Table N13 outlines the quantification approaches used to derive the item volumes for the Sydney region. For a more detailed description of the quantification approach, click on the relevant item name in the table.

Table N13  Quantification approaches used to derive item volumes

Detail of quantification approaches

Water storage product data

Storages

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 estimate for these volumes 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, which in turn affect the accuracy of the storage–volume curves.

Unregulated river

The volume of water is an estimate of static volume held in the river reaches. The river sections for which data was available were taken for calculation as given below.

• Capertee River: from Glen Davis to Upper Colo
• Coxs River: from Lithgow to Kelpie Point
• Shoalhaven River: from Kadoona to Fossickers Flat
• Wollondilly River: from Jooriland to Golden Valley.

The volume at each river section was estimated by multiplying the average daily flow at the upstream and downstream river gauges by estimated travel time between the two sections.

The limitations associated with this approach are:

• travel times are estimated to the nearest day
• daily flow change between gauging sites is assumed to be linear.

The volumes for the following unregulated river channels were not estimated due to unavailability of reliable data.

• Hawkesbury and Nepean rivers (except the river sections given above)
• Shoalhaven River, except the reach from Kadoona to Fossiker's Flat
• northern Sydney rivers
• southern Sydney rivers
• rivers of the Illawarra.

Service reservoir data

Supply system

The storage volume of the urban water supply system comprises two components: the distribution pipe network and service reservoirs. This data was available only for the Wingecarribee Shire Council service area.

Gridded climate data and AWRA-L model

Precipitation and evaporation

Data source: Bureau of Meteorology

Monthly precipitation grids for the region were produced using daily data from approximately 6,500 rain gauge stations across Australia and interpolated to a 0.05 degree (5 km) national grid (Jones et al. 2009).

Potential evapotranspiration across the region was estimated using the Australian Water Resources Assessment system Landscape model (AWRA-L) version 5.0 (Viney et al. 2015). The AWRA-L model uses the Penman method to produce the potential evapotranspiration. Daily AWRA-L potential evaporation grids for the region were produced based on daily gridded climate data (including precipitation, solar radiance and temperature) that are available on a 0.05 degree (approximately 5 km) national grid (Jones et al. 2009).

The precipitation and evapotranspiration at each waterbody were estimated from the proportionally weighted average of grid cells that intersected each water feature. The volume was then estimated by multiplying the surface area of each waterbody. The average monthly surface area of the major storages was calculated from daily storage levels and capacity tables. In the Sydney region, the surface area of the four storages was calculated dynamically and the surface area of the three urban lakes was a static value produced from the Australian Hydrological Geospatial Fabric (AHGF).

The limitations associated with this approach are:

• The precipitation and AWRA-L potential evapotranspiration estimates were subject to approximations associated with interpolating the observation point data to a national grid.
• The dynamic storage surface areas calculated from the levels and storage-rating tables represent a monthly average and therefore do not capture changes that occur on a shorter temporal scale.
• The total surface area of the surface water store within the Sydney region included only the storages (and not the rivers).

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.

The precipitation and evaporation volumes were based on measured data. Estimated uncertainty based on accuracy of rain gauges, limitations in the calculation method and organisational practice is +/– 10%.

Runoff

Runoff to surface water in the Sydney region was estimated separately for:

• storages managed by WaterNSW
• storages not managed by WaterNSW
• runoff at gauging stations
• runoff for ungauged areas.

Figure N13 Runoff area to storages, gauged area and modelled area within the Sydney region

Data source: Bureau of Meteorology

The runoff volume related to the storages not managed by the WaterNSW and runoff volume for ungauged areas (landscape) were estimated by the Bureau of Meteorology. Runoff to surface water was estimated based on the AWRA-L Version 5.0 model streamflow outputs (Viney et al. 2015). Using gridded climate data for the Sydney region (including precipitation, temperature and solar radiation data), AWRA-L was used to estimate the runoff depth at each grid cell within the region. Only runoff from the landscape was considered; therefore, the surface areas of the major storages were excluded from the analysis.

The runoff from the catchment contributing to major storages and the remaining runoff within the region were separately calculated.

The average runoff depth from the landscape into the connected surface water store was determined as the weighted mean of the relevant grid cells within the region boundary. Cells 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).

The Bureau also calculated the runoff at several gauging stations using the flow data received from Sydney Water Corporation and WaterNSW, by calculating the flow volume and deducting the releases and diversions to the river at upstream of the gauging stations.

The limitations associated are:

• The runoff estimates were subject to the assumptions of the AWRA-L version 5.0 model detailed by Viney et all (2015).
• Uncertainty estimates were not available for the modelling method.

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.

Estimated uncertainty based on meter accuracy, professional judgement on calculation methods and organisation practice is +/– 10%.

Outflow

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

• rule-based 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)
• treated water releases from wastewater treatment plants.

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.

Treated water releases from wastewater treatment plants were measured using meters.

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.

Water Sharing Plan for the Greater Metropolitan Unregulated River Water Sources, Water resourcing licence database, annual reports 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 Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources and the Water Sharing Plan for the Kangaroo River Water Source.

No cultural basic rights are estimated to occur in areas other than the Kangaroo River system.

Allocation

The Water Sharing Plan for the Greater Metropolitan Unregulated River Water Source and DPI Water licence system detail the rules for surface water and groundwater allocations for access licences with shared components under several categories; individual holder for stock and domestic, urban holder (local water utility, major utility, domestic and stock (town water supply) and other lump 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 2015 is calculated as shown in Table N14.

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, for the purposes of the National Water Account, the closing balance of remaining allocation is zero.

Water Sharing Plan for the Greater Metropolitan Unregulated River Water Sources

Allocated diversion: individual users 

DPI Water estimated the usage (17,612 ML) to be 13.5% of entitlement based on metered usage within the Hawkesbury Nepean. The 13.5% usage ratio was assumed to be representative of the entire Greater Metropolitan Sydney Area, as no meter readings were available outside of the Hawkesbury Nepean (excluding local water utility and major water utility).

Energy Australia diverted 7,315 ML ML from Thomsons Creek Reservoir, Lake Lyell and Lake Wallace for the Mt Piper and Wallerawang power stations. It was based on meter reading data.

Diversion: statutory rights

The annual extraction for domestic and stock rights in the water accounts is assumed to be the volume stated in the water sharing plan.

Discharge: user

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

Uncertainty of measured volume is based on meter accuracy (+/– 2%).

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

The WaterNSW and Energy Australia have an inter-basin claim on the Fish River Water Supply Scheme for which there is an upper limit for annual allocation. This claim is subject to a number of operating rules and restrictions depending on the levels observed in the supply storages (Oberon Dam and Duckmaloi Weir).

The diverted volume is based on metered flow measurements provided by DPI Water. There are provisions to carryover allocations up to 20% into the following year.

Claims: inter-region

In addition to the above, Lithgow City Council has entitlements for which there is an upper limit for annual allocation from the Fish River Water Supply Scheme to supply water to its storages under urban inter-region claim on water. There is also the ability to carry over a maximum of 20% of the total allocation into the following year. The volume was measured using ABB magflow 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
• plus any reported wastewater losses or egress from the system before the metering point at which inflow is measured to the treatment plants (e.g. through emergency relief structure).
  

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; (c) unreported wastewater overflows to stormwater; and (d) exfiltration of wastewater to groundwater. 
• 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 to be in the range of +/– 2% to +/– 10%.

Delivery: inter-region agreement

The 'Delivery: inter-region agreement' volume is metered and includes raw water, potable water and nonpotable water.

Allocation diversion: surface water

The 'Allocated diversion: surface water' 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 assumptions made were 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.
• Shoalhaven City Council factors the total metered volume from billing records to estimate the water usage within the region.

 The uncertainty is estimated to be +/– 2% to +/– 7%.

Supply system transfer: inter-region

The 'Supply system transfer: inter-region' volume measures the transfer of potable and non-potable water outside of the region. The volumes are based on metered information at the distribution infrastructure. The uncertainty is estimated to be +/– 10%.

The assumption made was as follows:

•  Shoalhaven City Council estimates the amount of water transferred outside the region by considering total water supplied with the Shoalhaven region and total water used within the region. The balancing figure is recorded as water transferred out of the region.

Evaporation: urban system

The volume of evaporation from the urban water system is:

• calculated using the Bureau of Meteorology's climate data

• calculated using a water balance approach through available inflow and outflow metering data; and/or

• the metered treated wastewater volume disposed to evaporation lagoons.

The assumption made was as follows:

• Evaporation losses are only reported for the wastewater system.

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. Where there are available volumes associated with only pipe burst, this is reported in 'Leakage: landscape'.

Non-revenue water is estimated using:

• the difference based on a water balance between metered water produced and revenue water (metered/estimated customer consumption charges); and/or
• modelling software of network real losses (leakages and bursts) and apparent losses (unauthorised/authorised unbilled use); and
• time to repair leaks.

 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:

• 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.
• Goulburn Mulwaree Council and Sydney Water also estimated leakage within their wastewater system at wastewater treatment plants.
• All other utilities estimated leakage from potable water supply system only.

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

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 and small scale agriculture/irrigation uses.

The assumption made was as follows:

• Shoalhaven City Council factors the total metered volume from billing records to estimate the water usage within the region.

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
• known egress to stormwater from the wastewater collection system occurring before metered inflow to wastewater treatment plants.

Egress to stormwater is 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.

The assumption made was as follows:

• 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 is the remaining non-revenue water from the urban water supply system (if not reported in 'Leakage: landscape' and 'Leakage: groundwater' respectively).

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.

The assumption made was as follows:

• No nonpotable water losses were reported and likely underestimate the losses in the system.

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

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 (if not reported in 'Evaporation: urban system)
• known losses from the wastewater collection system
• known egress or exfiltration from the wastewater collection system occurring before metered inflow to wastewater treatment plants.

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.