National Water Account 2016

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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.

Stream monitoring data

Outflow

There are four rivers that flow out to sea from the Perth region:

• Swan
• Canning
• Murray
• Harvey.

The total river outflow was estimated using flow data collected at the most downstream gauging stations nearest to the outlet to the sea (see Table N5). These data were used to determine the total annual discharge (in ML) at each station during the year.

The most downstream gauging stations (Figure N1) along the main channels of the Swan and Murray rivers are 616011 and 614065, respectively; however, the following figure shows that two tributaries flow into the Swan and Murray main channels at a confluence downstream of these two gauging stations. Therefore, the total outflow from the Swan and Murray rivers was the sum of the total annual discharge at the most downstream station plus the total annual discharge measured along the two tributaries.

Figure N1 Gauging stations used to calculate total outflow to sea

It is assumed that the river outflow to the sea is equal to the volume of discharge measured at the most downstream station along a river, that is, there is no adjustment made for the contributing area below the gauging station used to calculate the outflow.

This contributing area below the gauging stations is 9,173 km², approximately 43% of the total area of the Perth region. Based on a drainage–area ratio equation, estimated outflow is approximately 310,000 ML, which is 1.75 times that reported in the Statement of Water Flows (177,596 ML). Given, however, that the ungauged component of the Perth region mainly lies on the Swan Coastal Plain, which is an area of relatively high rainfall-recharge, it is unlikely that this area will generate such a large amount of runoff. Instead, it is considered that the reported outflow to sea may be underestimated by up to 10–20%.

Quality codes are assigned to flow data in accordance with Table N6, as given in the Bureau of Meteorology's Water Data Online.

The total volume of water that outflows into the sea from the Perth region has a quality code of E, which indicates the lowest quality of data recorded for all the station data.

Gridded climate data

Precipitation and evaporation

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). The precipitation at each waterbody (i.e., storages) was estimated from the proportionally weighted average of grid-cells that intersected each water feature. The volume was then estimated by multiplying by the surface area of each waterbody by the weighted average precipitation. The average monthly surface area of the storages was calculated from daily storage levels and capacity tables.

Evaporation from water bodies was calculated on a daily basis using the Morton's shallow lake formulation (Morton 1983a, 1983b, 1986). For annual evaporation estimate, there is no difference between shallow and deep lake evaporation (Sacks et al. 1994). The climate data required for the Morton's method are maximum temperature, minimum temperature, vapour pressure and solar radiation. The climate data for each waterbody was estimated from the proportionally weighted average of grid cells that intersected each water feature and input to the Morton's program to obtain the evaporation values. The volume was then estimated by multiplying the surface area of each waterbody by the evaporation values.

The limitations associated with this approach are:

• The dynamic storage surface areas calculated from the levels and capacity tables represent a monthly average and therefore will not capture changes that occur on a shorter temporal scale.
• The total surface area of the surface water store within the Perth region included only the storages (and not the rivers).

Streamflow data and AWRA-L model

Runoff

Runoff in the Perth region was based on a combination of streamflow data and runoff estimates from the AWRA-L version 5.0 model outputs. Runoff is only estimated from areas not modelled by the Department of Water's groundwater models (Figure N2), which already incorporate landscape water movement. Therefore, runoff was only estimated on the Darling Range in the eastern part of the region.

Figure N2 Runoff model area within the Perth region

Runoff in catchment areas upstream of the urban supply storages (Figure N2) was based on streamflow data. Streamflow is the volume of water entering the reservoirs, which was measured by the changing water storage levels. For more information, including historical streamflows into the storages, refer to the Water Corporation website.

The runoff in the remaining area was based on AWRA-L model outputs. 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 'ungauged' portion of 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).

The approach was subject to the assumptions of the AWRA-L version 5.0 model detailed in Viney et al. (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 2015–16 year is the unused component of the annual allocation for the licence. The allocation remaining at 30 June 2016 is calculated as shown in Table N7.

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 2015–16 year, the volume of abstraction is estimated to be the full licensed allocation.
• For licences that expired during the 2015–16 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 2015–16 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 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 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, entitlements, allocations and restrictions note.

Operational Data Storage System and Water Corporation Annual Report 2016

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 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 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 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 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 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 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 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 undergoes further treatment to produce water that meets Australian guidelines for drinking water. 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 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 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.

As shown in the Figure N3, 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 N3 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 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 N3 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 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 N3 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.

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 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 N3 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 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 N3 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 does not have sufficient metering information relating to actual extraction from domestic non-licensed bores to determine the total volume of water extracted. Consequently, the extraction was estimated based on Australian Bureau of Statistics survey results (2009) for the Perth metropolitan area. These usage figures in the Perth metropolitan area were then extrapolated to also include the southwest area (around Mandurah). The estimated annual extraction per bore is 0.405 ML per year, which is based on the initial results of the Department of Water's domestic bore metering project for several dozen domestic bores across Perth.

The estimate for domestic non-licensed extraction is based on a number of assumptions, including that:

• the ABS domestic bore survey is relevant for the southwest area, for which there is no alternative, independent research or evidence available; and
• the usage based on the Department of Water's research for several dozen bores was used to estimate the usage of all non-licensed bores in the region.

Supervisory Control and Data Acquisition system

Delivery: inter-region

The total volume of water transferred into Harvey Reservoir from the Collie River Irrigation District is based on measured data observed at the pump station located at the border of the Collie River and Harvey irrigation districts.

It is assumed that the volume of water measured at the flow meter at the border of the Collie River and Harvey irrigation districts is equal to the inflow at Harvey Reservoir (i.e., no leaks or transmission losses). This is a reasonable assumption given the transfer occurs via a closed pipe.

The expected error associated with this transfer is +/– 5%. The Department of Water 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).

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 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%.

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 2015–16 year was equivalent to the average annual leakage between 2010 and 2015.