National Water Account 2017

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Perth: Methods

Summary of methods

Table N4 outlines the methods used to derive the item volumes for the Perth region. For a more detailed description of a 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 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 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.

AWRA-L model

Runoff

Runoff to surface water was based on streamflow estimates from the AWRA-L version 5.0 model outputs (see the Australian Landscape Water Balance model website).

Using gridded climate data (including precipitation, temperature, and solar radiation data), the AWRA-L model 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 rivers were excluded from the analysis.

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. Points were weighted based upon the area they represented within the 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 modelled region (excluding storages).

As the AWRA-L runoff includes groundwater discharge to rivers, to avoid double counting, the modelled runoff from AWRA-L was discounted by the volume of Discharge: surface water estimated with the PRAMS and PHRAMS models.

The approach was subject to the assumptions of the AWRA-L version 5.0 model detailed in Viney et al. (2015).

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)

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:

• 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, and
• residual catchment inflows estimated from the AWRA-L model for ungauged portions of catchments below the most downstream gauge.

As the AWRA-R model for Perth does not include the Collie River in the far south of the Perth region, observed flows from the most downstream gauge on the Collie river, 612043 Collie River @ Rose Road (Figure N1), were added to the AWRA-R estimates to derive total outflow.

Inter-region inflow

Inter-region inflow for the Perth region includes flow in the two main rivers that cross the region boundary, the Avon River where it crosses the northeastern boundary of the region and the Collie River where it flows into the Wellington Reservoir. Inflows were estimated from the nearest gauging stations on each river: 615026 Avon River @ Stirling Tce Toodyay; and 612002 Collie River @ Mungalup Tower. Missing values for the Avon River were filled with simulated values using the AWRA-R model.

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).

River losses

The AWRA-R river and groundwater interaction module was used to model river losses. 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).

The AWRA-R output has been assumed to represent river losses to landscape, rather than groundwater, for the following reasons:

• In the Perth Region, most river losses occur east of the Darling Escarpment outside of the coastal plain, where the aquifers are generally of low productivity or have non-potable water quality. In these areas, groundwater has not been recognised as an asset in the Perth Account.
• On the coastal plain, where groundwater is recognised in the Perth Account, groundwater fluxes are estimated using the PRAMS and PHRAMS models. These models assume the river-aquifer flux is in the direction of groundwater discharge to rivers only and that all groundwater recharge is received via Recharge: landscape.

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 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.

Operational Data Storage System

Allocated abstraction: urban system

The allocated abstraction of surface water and groundwater to the urban supply system is based on measured data collected at the outlet of the water source using a cumulative water meter.

The expected error associated with these abstractions is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

Delivery: desalinated water

The 'Delivery: desalinated water' volume is metered at the outflow from the Perth Seawater Desalination Plant, located in Kwinana, and the Southern Seawater Desalination Plant, located near Binningup.

It is assumed that all desalinated water is supplied directly to the urban water supply system. Some desalinated water is transferred to surface water storages prior to consumption by urban water users. This water is included in Supply system discharge: surface water.

The expected error associated with this supply is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

Supply system delivery: inter-region

The 'Supply system delivery: inter-region' volume consists of potable and non-potable water and is based on metered information at the distribution infrastructure.

Water transferred to the Perth region’s supply system comes from Mundaring Reservoir, which is not considered a surface water asset in the Perth region as it primarily supplies the Goldfields and Agricultural Region outside the Perth region boundary.

The expected error associated with this transfer is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

Wastewater collected

The 'Wastewater collected' volume is estimated using the aggregated metered inflow to wastewater treatment plants within the region:

• minus any recirculation such as treated wastewater volume, to avoid double counting; and
• plus any reported wastewater losses or egress from the system before the metering point measuring inflow to the treatment plants (e.g. through emergency relief structure).

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.

The error in the inflow volume into wastewater treatment plants is estimated to be in the range of +/– 5–10%.

Supply system delivery: urban users

The 'Supply system delivery: urban users' volume includes urban consumption of potable and non-potable 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 (i.e., supply to irrigation schemes and the environment) is not included in the reported volume.

The expected error associated with the delivery of water to users is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

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.

This volume includes transfers to the Goldfields and Agricultural region and Western Australia's southwest region, measured on an almost continuous basis by flow meters.

The expected error associated with this transfer is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

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 to landscape and 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 Perth region’s urban water supply system monitors total supply using master meters in the network, which is used to derive remaining non-revenue water.

The expected error associated with these discharges is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

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.

Potable water is also discharged from the Perth region’s supply system for environmental purposes (riparian releases) and is metered at each environmental release point.

The expected error associated with these discharges is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

Aquifer recharge

The volume of aquifer recharge is based on metered data measuring the water recharged to aquifers, including potable, non-potable, and recycled water. Treated wastewater is recharged to groundwater from two wastewater treatment plants—Beenyup and Kwinana.

Treated wastewater from the Kwinana treatment plant is discharged to infiltration ponds where it infiltrates into the groundwater store. The volume of water discharged to the ponds is metered. It is assumed that this entire volume infiltrates into the aquifer; however, it is likely that some water would be lost through evaporation.  

Treated wastewater from the Beenyup treatment plant is transferred to the Advanced Water Recycling Plant where it is further treated to drinking water standards. This treated wastewater is then recharged to the Leederville aquifer. The volume of aquifer recharge is based on metered data at the recycling plant that is collected on an almost continuous basis.

The expected error associated with this volume is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

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 total volume of water discharged into the sea is estimated by subtracting the volume of recycled wastewater from the total inflow of water into the treatment plant. Both inflow and recycled volume data are based on measured discharge data collected at the site.

The wastewater discharge volumes are estimated to be in the range of +/– 5–10%.

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 in 'Discharge: surface water' and 'Discharge: sea'.

The total volume of water discharged to landscape from wastewater treatment plants is estimated by subtracting the volume of recycled wastewater from the total inflow of water into the treatment plant. Both inflow and recycled volume data are based on measured discharge data collected at the site.

The uncertainty of the wastewater discharge volumes are estimated to be in the range of +/– 5–10%.

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. Real losses related to pipe bursts are 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 revenue water was first adjusted to take into account the difference between total water delivered measured via bulk meter readings (done daily) and the customer meter readings (done every two months for each customer). This is known as the meter lag adjustment.

The volume of apparent losses comprises two components: unauthorised consumption (e.g., water theft); and customer meter under-registration (e.g., meter inaccuracies). The unauthorised consumption is assumed to be 0.1% of the metered volume of water supplied. Customer meter under-registration is assumed to be 3.5% of revenue water.

The volume of unmetered authorised consumption is calculated as 0.5% of the metered volume of water supplied. Unbilled authorised consumption includes estimates for fire use and Water Corporation’s internal operational use for activities such as scouring of water pipes and cleaning of tanks and service reservoirs.

These percentage estimates are based on the infrastructure leakage index calculations for Perth and Mandurah.

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.

Recycled water delivery: urban users

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

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

PRAMS version 3.5 and PHRAMS groundwater models

The Department of Water and Environmental Regulation uses the Perth Regional Aquifer Modelling System (PRAMS) version 3.5 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate the natural water movement to and from the groundwater store within the Perth region (Davidson and Yu 2008; URS 2009).

As shown in the Figure N2, PRAMS estimates groundwater movement for the area north of Mandurah. PHRAMS estimates groundwater movement for the Peel–Harvey area south of Mandurah. The volumes reported in this account refer to the natural water movement that occurs within each model area within the Perth region boundary.

Figure N2 Groundwater model areas relative to the Perth region boundary

Both models were initially developed for the purpose of estimating and assessing the impacts of changes in climatic conditions and varying extraction rates on the aquifers, not for the purposes of preparing water accounts. These models have since been modified to also provide data for water accounting.

Inter-region inflow and outflow

The Department of Water and Environmental Regulation uses the Perth Regional Aquifer Modelling System (PRAMS) version 3.5 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate groundwater flows to and from outside the region. Figure N2 shows each of the groundwater model areas within the Perth region.

Both groundwater models derive the lateral water movement between aquifers within the region and outside the region by defining the appropriate water balance zones and calculating the volume of water flowing across each boundary on an annual basis.

For estimating groundwater movement between aquifers within the Perth region and the ocean, both models apply a constant head at the coastline to estimate the volumes flowing through each boundary over the year. For the Peel–Harvey area, PHRAMS assumes that the estuary inlets along the coastline form part of the boundary.

The assumptions made were as follows:

• All groundwater flow to and from outside the region is via the north and south boundaries and the coastline. There is no groundwater flow between the model areas and the Darling Range in the eastern part of the region due to the geology of that area.
• Groundwater flow between the ocean and the modelled areas is estimated using a constant head at the coastline of zero (given the level of 0.0 m Australian Height Datum is mean sea level).
• Flows from the estuaries within the Peel–Harvey modelling area were estimated and included in the reported amount; that is, the estuary systems in the Peel–Harvey area were considered part of the 'coastline' boundary in PHRAMS. These estuary systems are relatively minor in the Perth modelling area and therefore were not considered in PRAMS.

Recharge: landscape

The Department of Water and Environmental Regulation uses the Perth Regional Aquifer Modelling System (PRAMS) version 3.5 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate groundwater recharge from the landscape within the Perth region. Figure N2 shows each of the groundwater model areas within the Perth region.

PRAMS calculates the recharge of water into the Perth region aquifers from the unsaturated zone above (i.e., the landscape store). The model delineates a series of horizontal cells called representative recharge units, and collates data on:

• land use, vegetation classifications, leaf area indexes, and soil classifications (spatial datasets)
• water table depths and plant root depths
• climatic data throughout the region.

The CSIRO WAVES model was then used to calculate flows from some of the specific land use areas, such as agriculture, pine plantations, and native bushland (simpler models are used for general land use areas including residential, industrial, and parkland areas). The Vertical Flux Model calculates recharge on a daily basis and MODFLOW was used to do the time step calculation (Xu et al., 2008).

PHRAMS calculates the inflow of water into the Peel–Harvey model area and assumes that most of the inflow is recharge from rainfall. The CSIRO WAVES model is used to estimate the maximum annual recharge based on the following equation:

Recharge = 0.8 x Rainfall – 280

The annual recharge and annual pan evaporation were distributed into monthly amounts using a monthly distribution table. The monthly accounts were then applied to the specific geological areas (which have their own recharge rates according to soil types and land cover) to calculate total recharge.

The two groundwater models use different techniques to calculate recharge. PRAMS is a more complicated and sophisticated model than PHRAMS. PRAMS uses daily climatic data to determine total recharge during the year, whereas PHRAMS is based on annual data inputs.

Recharge/Discharge: surface water

The Department of Water and Environmental Regulation uses the Perth Regional Aquifer Modelling System (PRAMS) version 3.5 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate groundwater recharge from the landscape within the Perth region. Figure N2 shows each of the groundwater model areas within the Perth region.

Both groundwater models calculate the flow of water between drains and rivers and the aquifers. The models assume that all groundwater flows between the aquifers and the surface water stores occurs from the aquifers (i.e., there is no recharge from surface water).

PRAMS removes water from drain and river cells when the water table rises above the specified invert level of the drain cell. The volume of water removed is the volume of groundwater discharged to surface water.

PHRAMS uses a simplified drain package developed within MODFLOW to calculate the discharge to these drains and rivers from the aquifer. Two classes of drains were assumed:

• major drains that are permanent and deep (including major rivers)—conductance is calculated at 10,000 m²/day and a depth of 2 m
• minor drains—conductance is calculated at 10,000 m²/day and a depth of 1.5 m.

Groundwater discharges to drains and rivers were calculated when the local groundwater level rises above the drain bed elevation.

Perennial lakes provide major groundwater sinks in the water table aquifer and are subject to rainfall and evaporation. These were included in the water table aquifer balance and modelled accordingly.

The assumptions made were as follows:

• Both models assume that all flows between the groundwater store and rivers and drains comes from the aquifers, hence recharge from surface water is zero. This assumption is consistent with measured and simulated discharges. Wetlands were considered to be part of the water table aquifer, hence any groundwater recharge from wetlands was not considered.
• It is assumed that there is no groundwater discharge to the major surface water storages within the region. Only groundwater discharge to rivers and drains is considered in this line item.
• The two models use different techniques to calculate discharge. PHRAMS uses a single conductance figure for all rivers and drains, and only two depth categories. It is assumed that once groundwater is above the drain bed elevation, there is a consistent flow of water out of the aquifer.

Discharge: landscape

The Department of Water and Environmental Regulation uses the Perth Regional Aquifer Modelling System (PRAMS) version 3.5 and Peel–Harvey Regional Aquifer Modelling System (PHRAMS) to estimate groundwater recharge from the landscape within the Perth region. Figure N2 shows each of the groundwater model areas within the Perth region.

Groundwater discharge to the landscape was estimated using PRAMS and PHRAMS. Both models calculate evapotranspiration from the saturated zone of the aquifers, which is considered to be equal to the groundwater discharge to landscape. Note that this should not be confused with evapotranspiration from the unsaturated zone, which is rainfall water that has entered the ground in the unsaturated zone but is removed before it reaches the water table at the top of the saturated zone.

Both models use the MODFLOW evaporation module to calculate evapotranspiration from the saturated zone open water wetland surfaces and near surface water table areas.

Non-allocated extraction: individual users

The Department of Water and Environmental Regulation estimates domestic garden bore use through surveys and metering projects. Domestic garden bore users are exempt from licensing and are generally not metered, so both the number of bores and the volume of water used per bore is an estimate only.

Across the Perth metropolitan area there is good information on the rate of domestic garden bore instalment based on:

• on-the-ground surveys by the Water Corporation
• surveys by the Australian Bureau of Statistics in 2003, 2006 and 2009 and
• phone surveys conducted by the department in 2012 and 2016.

The estimated annual use per domestic garden bore is 0.430 ML based on the department’s project on Perth garden bore metering.

Supervisory Control and Data Acquisition system

Allocated diversion: irrigation

The entitled diversion of allocated water for the Harvey Water Irrigation Area during the licensed water year is equal to the total volume of water diverted from the storages. The volumes diverted are metered at the storage outlet continuously and reported on a monthly basis.

The expected error associated with this diversion is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c).

Delivery

Total water supply to irrigation scheme users within the Harvey Water Irrigation Area during the year is the total of consumers' meter readings. Meter readings are read on a monthly basis and recorded in Harvey Water's customer information and billing system.

This volume is estimated to be in the range of +/– 5–10%.

Leakage: landscape

Leakage: landscape for the irrigation scheme is estimated as the difference between the metered diversions from Wellingtom Dam and the metered deliveries to the Collie River irrigation district, which occur via an open channel network. This difference is assumed to be lost mainly to seepage into the ground, leaks in the channels and structures and end of system outflow.  

Estimated data

Leakage: landscape

The volume of leakage to the landscape from surface water storages is estimated based on six years of annual metered data collected between 2010 and 2015. Leakage data are collected at most of the urban supply storages within the Perth region.

During this 6-year period, total annual leakage at these storages changed little from year to year, so it was assumed that leakage to the landscape during the 2016–17 year was equivalent to the average annual leakage between 2010 and 2015.

Water resourcing licence database and annual reports/meter readings

Allocation remaining

The water management year commences on the date the licence is issued. In most cases, particularly for individual users, the licence anniversary falls outside the standard 1 July–30 June water year. As a result, the water allocation remaining at the end of the 2016–17 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June 2017 is calculated as shown in Table N5.

Adjustment and forfeiture

The portion of water allocation that has not been abstracted at the end of the licence water year is forfeited (i.e., there is no carry-over of entitlements). Therefore, forfeiture is calculated as the total annual allocation for each licence minus the allocation abstraction during the licence water year. Individual user entitlements that are terminated during the year are also considered to be forfeitures.

Allocated abstraction: individual users

The entitled abstraction of allocated water by individual users (both surface water and groundwater) during the licensed water year is derived from a combination of metered data and estimates. Where metered data are available, the abstraction is calculated as the actual abstraction during the year. Where metered data are not available, the following methods were used to estimate the volume of abstraction:

• For licences that expired and were renewed during the 2016–17 year, the volume of abstraction is estimated to be the full licensed allocation.
• For licences that expired during the 2016–17 year (and were not renewed), the volume of abstraction is estimated to be the allocation remaining at 1 July 2015 (i.e., the allocation remaining at the start of the year is assumed to be abstracted before the licence expired).
• For new licences that were created during the 2016–17 year, the volume of abstraction is estimated to be the full allocation, multiplied by the ratio of the number of days from the licence issue date until 30 June 2016 to the number of days in the year.

There is insufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences, particularly individual users. The pro-rata estimates of abstraction assume that the full annual entitlement is abstracted each year and that the rate of abstraction is uniform throughout the year. It is unlikely on both counts that this will be the case for all licences.

The expected error associated with measured abstraction data is +/– 5%. The Department of Water and Environmental Regulation requires that all water meters, when tested under in situ conditions, must be within 5% accuracy across the full flow rate range (Department of Water 2009c). For estimated data the uncertainty is unquantified.

Allocation

Individual user licences are generally issued for periods of between 1–10 years, with an annual abstraction amount specified and with annual compliance arrangements in place.

The maximum amount of abstraction for each year for urban water and irrigation scheme supply is announced by the Western Australian Minister for Water on an annual basis. The announced allocation is made after a review by the Department of Water and Environmental Regulation of storage and aquifer levels in the Perth region in October of the reporting year.

More information on these allocations and the associated water access entitlement is given in the Water rights and allocations note.