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Perth: Supporting information

  • A relatively dry winter in 2020 contributed to low runoff and storage inflows across the region.
  • End-of-year storage was at 42% of capacity, similar to the previous year.
  • 75% of the allocated water was taken in 2020–21, about 50% for urban supply.

Schematic representation of the Perth region. In 2020–21, runoff was 37% less than average. Storage volumes at 30 June 2021 were 42% full, marginally higher than the previous year. 52% of the total surface water allocation (372 GL) was taken. 94% of the total groundwater allocation (465 GL) was taken.

For further information on the region's water accounting statements scroll down this page or click on the links below:

 

Water assets

Wellington Dam, Perth region (Water Corporation © Darryl Peroni)

 

Surface water assets

  • Total surface water asset volumes in the Perth region at 30 June 2021 were 365 GL, 99% of which is water held in storages. The remaining amount is water held in rivers.

 

Bar graph of annual storage volume in the Perth region at 30 June for the years 2010–2021. Accessible storage plus dead storage is plotted against the storage capacity to show percentage full at 30 June each year. Storages at 30 June 2021 were 42% full, slightly higher than the previous year. Since 2010, the highest end-of-year storage volume was in 2019 when storages were 47% full, the lowest was in 2011 when storages were 23% full.
Figure S1 Total storage volume in the region at 30 June 2021 compared with the previous 11 years

 

  • Storage volumes increased marginally from 41% full at 30 June 2020 to 42% full at 30 June 2021.
  • The higher storage volumes over the previous few years is not only attributed to wetter winters, but also lower diversions and increased 'banking' of desalinated water in the storages. The Water Corporation's reduced reliance on surface water in recent years is a result of their adoption of a range of demand management strategies and continued development of climate-independent water sources in response to the long-term decline in storage inflows.

 

Map of Perth region showing the status of each storage at 30 June 2021. The storages are colour coded into ten categories of volume as a proportion of capacity. Canning, Wungong, Waroona, and Drakes Brook storages were over 60% full. Wellington Dam and Logue Brook, in the region's south, were over 50% full. All remaining storages were less than 50% full, including South Dandalup, which was less than 20% full. An inset line graph shows the time series of percentage-full storage volume between 1 July 2013–30 June 2021. Perth storages at 30 June 2021 were 41% full, less than last year.
Figure S2 Percentage-full volume on 30 June 2021 for each storage and total daily storage volume from 2013–2021 (inset)

 

  • Storage levels peaked in mid-October 2020 at around 51% full following the winter rainfall.
  • The increase in storage levels following the 2020 winter months was similar to the previous year.
  • More detailed information on the individual storages within the region is available on the Bureau of Meteorology's Water Storage website.

 

Groundwater assets

  • Volumes of water stored in aquifers cannot be quantified due to a lack of available data; however, information on the total annual change in aquifer storage is available.

 

Bar graph of annual change in aquifer storage volume in the Perth region from 2010–11 to 2020–21. In 2020–21, the volume of water in the aquifers decreased. This followed annual decreases in aquifer volume last year. Since 2010, the largest annual decrease in aquifer storage was in 2010–11 of more than 430 GL. The largest annual increase was during 2013–14 of around 285 GL.
Figure S3 Total annual change in aquifer volume in the region at 30 June 2021 compared with the previous 9 years

 

  • The low rainfall during the 2020 winter resulted in a decrease in groundwater storage during 2020–21.
  • This was the second consecutive year of decreasing aquifer storage within the region.

 

Water liabilities

Perth desalination plant, Perth region (Water Corporation © 2009)

 

  • Water liabilities in the Perth region refer to the volume of allocation remaining on licence entitlements at the end of the 2020–21 year.

 

Table S1 Volume of surface water allocation remaining at 30 June 2021
  Description Individual users (ML) Urban system (ML) Irrigation scheme (ML)
  Opening balance at 1 July 2020 8,522 0 31,831
add Allocation 45,173 247,950 79,366
less Allocated diversion 45,173 98,358 49,701
less Adjustment and forfeiture 0 149,592 29,698
  Closing balance at 30 June 2021 8,522 0 31,798

 

Table S2 Volume of groundwater allocation remaining at 30 June 2021
  Description Individual users (ML) Urban system (ML)
  Opening balance at 1 July 2020 102,375 0
add Allocation 300,494 164,588
less Allocation extraction 300,495 137,064
less Adjustment and forfeiture 0 27,524
  Closing balance at 30 June 2021 102,374 0

 

  • Many of the water supply licences for the region do not have a water management year that ends on 30 June; therefore, the allocation remaining is the unused component of the annual allocation for these entitlements.
  • A more detailed description of water allocations in the Perth region is given in the Water use section below.

 

Water flows

Serpentine Reservoir, Perth region (Water Corporation © Darryl Peroni)

 

Figure S4 Water inflows and outflows for the region's water stores and systems during the 2020–21 year

 

  • Total surface water and groundwater flows into the region's storages was estimated to be around 2,273 GL. The key item contributing to surface water and groundwater inflows was runoff and recharge: landscape, which together made up 70% of the storage inflows.
  • Runoff was lower than average (based on modelled data from 1971–2021) due to the relatively poor 2020 winter rainfall across the region.
  • A key item contributing to surface water outflows from the region's storages was river outflow from the region, which made up around 61% of the total surface water outflows. River outflow was well below average due to the dry conditions and low runoff across the region.
  • Water taken from surface water and groundwater resources for consumptive use was 715 GL, around 33% of the total surface water and groundwater outflow (see Water use section below).

 

Water availability

Bar graph of annual surface water availability in the Perth region for the years 2011–2021. Storage at 1 July plus estimated inflows during the year is plotted against the mean water availability (based on 1971–2021 data). Surface water available during 2020–21 was well below average for the second consecutive year. Water availability was near average during 2017–18 and 2018–19.
Figure S5 Annual surface water available in the Perth region, 2011–12 to 2020–21

 

  • Physical surface water availability is calculated as the sum of storage at the start of the year (see Water assets) and estimated inflows into storages and rivers during the year (see Figure S4).
  • Surface water availability in the Perth region in 2020–21 was well below average for the second consecutive year.
  • Water taken during the year is influenced by the physical water availability. Typically, water taken from surface water resources is lower than water availability. This is discussed in more detail in the Water use section below.

 

Water use

Desalination plant infrastructure, Perth region (Water Corporation © 2013)

 

Figure S6 Water taken during the 2020–21 year in the region's water management areas

 

  • Total surface water diverted in the region was 193 GL.
  • 51% of the diversions were for the urban system, which included groundwater and desalinated water 'banked' in the storages
  • 26% of the diversions were to the irrigation scheme from storages in the Harvey and Collie river catchments; 23% of the diversions were for individual users.
  • Total groundwater extracted in the region was 522 GL.
  • 74% of the extractions were for individual users, including allocated and non-allocated use, 26% was for the urban system.

 

Bar graph of annual surface water allocation and diversion for different users in the Perth region from 2011–12 to 2020–21. Most of the surface water allocations are associated with the urban system. These allocations change from year to year. In 2020–21, 40% of the allocation for the urban system was taken, similar to the previous year. Allocations for the irrigation scheme vary from year to year. In 2020–21, allocations were less than the previous year. 63% of the allocation was taken. 100% of the allocation for individual users was taken. Non-allocated surface water is also taken for the urban system.
Figure S7 Surface water allocations and diversions in the Perth region for the years ending 30 June, from 2012–2021

 

  • In 2020–21, total surface water allocation was 372 GL, mostly the urban system and the irrigation scheme supply. 52% of the allocated surface water was taken.
  • Allocations for the irrigation scheme decreased from the previous year following a dry winter in 2020.
  • Diversions to the urban system were 4% less than last year due to reduced storage inflows. A portion of these diversions is groundwater and desalinated water transferred to the reservoirs for storage (see Urban system supply section below).

 

Bar graph of annual groundwater allocation and extraction for different users in the Perth region from 2011–12 to 2020–21. Most of the groundwater allocations are associated with individual users. The allocation in 2020–21 was similar to the previous year. 100% of the allocation for individual users was taken. In 2020–21, allocation for the urban system was similar to the previous year. 83% of the allocation was taken. Non-allocated groundwater is also taken for individual users.
Figure S8 Groundwater allocations and extractions in the Perth region for the years ending 30 June, from 2012–2021

 

  • Total groundwater allocation was 465 GL, mainly for private and commercial use. 94% of the allocated groundwater was taken.
  • Groundwater extraction for the urban system was similar to last year as 'banking' of groundwater in the surface water storages was increased due to the low storage inflows.

 

Urban system supply

Stacked bar graph showing the annual water sourced for Perth region's urban supply system during the period from 2010–11 to 2020–21. Each bar in the stack progressively represents surface water, inter-region inflow, groundwater, and desalinated water. Since 2010–11, total water sourced for the urban system has been relatively consistent. Groundwater and desalinated water are the predominant water sources. Key aspects of the graph are described in the text below the figure.
Figure S9 Water sources used in the Perth region's urban supply system during the 2020–21 year compared with the previous ten years

 

  • Total inflows to the water supply system was 303 GL, around 2% more than the previous year.
  • Low storage inflows following a dry winter in 2020 meant the Water Corporation had to rely mostly on desalinated water and groundwater for urban supply.
  • The decreased surface water contribution in the last two years meant more desalinated water was required to meet urban demand; the region's two desalination plants were run at near capacity, similar to the 3-year period between 2015–2018.
  • The supply strategy requires 'banking' of groundwater and desalinated water in the urban surface water storages during low demand periods to buffer peak period supply requirements.

 

Water restrictions

 

  • A permanent daytime sprinkler ban and 2-day a week sprinkler roster have been in place for urban water users since 2001 and 3-day a week sprinkler roster for domestic garden bores since 2007. A permanent winter sprinkler ban was introduced in 2010.
  • For more information, refer to the Bureau of Meteorology's Water restrictions website.

 

Water market activity

 

Table S3 Water trade in the Perth region during the 2020–21 year
Transaction type Number of transactions Volume
(ML)
Surface water transactions    
Entitlement trades 60 4,283
Allocation trades 143 8,501
Groundwater transactions    
Entitlement trades 121 7,657
Allocation trades 0 0
Total 399 29,014

 

  • Most of the surface water transactions that occurred in the Perth region in 2020–21 were within the Harvey Water irrigation scheme.
  • All groundwater transactions were entitlement trades, which represented about 2% of the total groundwater allocation. Groundwater trading was most common in the Superficial Aquifer within the Gnangara plan area.
  • More information on water trading in Western Australia is available from the Bureau's Water Market Dashboard and the Department of Water and Environmental Regulation website.

 

Cultural and environmental water

 

Environmental and cultural surface water provisions

  • The main mechanisms for protecting ecological and cultural values of surface water systems in the region are regulated releases and annual allocation limits.
  • Release requirements are defined in operating strategies negotiated between the Water Corporation and the Department of Water and Environmental Regulation, and in water allocation plans for the Middle Canning and Lower Collie rivers.
  • The releases are made from water mains or the base of the dams, and are used to maintain baseflows, generate pulse flows important for ecological function, and meet cultural objectives.

 

Map of 6 environmental water release points along the Canning River. There are 4 release points in Canning River upstream of the gauging station at Seaforth, station number 616027, and 2 release points downstream of the station.
Figure S10 Middle Canning River environmental release points

 

  • The Middle Canning River allocation plan sets out requirements for releases from six locations along the Canning River to maintain downstream environmental and cultural river values.

 

Example line graph of daily flow in the Canning River at Seaforth, station number 616027. The flow is compared with the required environmental water provisions: a summer-flow release threshold (1.8 ML/day) and a summer-flow release trigger (9.3 ML/day). The release schedule commences when flow is less than 9.3 ML/day for 3 consecutive days in October–November and ends when cumulative rainfall at a designated rain gauge reaches 40 mm during April–May. There must be 3 pulses of flow per year of at least 5 days duration. The high-flow pulses occur when flow is less than 9.3 ML/day for 15 consecutive days during November–January.
Figure S11 Example water provisions for the Canning River; required baseflows and higher flow pulses are also shown

 

  • Environmental water releases to the Canning River are made during the drier months between November and May. The release schedule is based on rainfall and flow triggers.

 

Environmental and cultural surface water outcomes

Line graph of daily flow in 2020–21 in the Canning River at Seaforth, station number 616027. The flow is compared with the required environmental water provisions: a summer-flow release threshold (1.8 ML/day) and a summer-flow release trigger (9.3 ML/day). Key aspects of the daily flow graph are described in the text below the figure.
Figure S12 Daily flow in the Canning River compared with the required flow regime during the 2020–21 year

 

  • In 2020–21, releases to the Canning River began in mid-October based on the summer-flow release trigger of three consecutive days of flow below 9.3 ML/day.
  • Flow remained above the target summer-flow release threshold for the entire release period.
  • Following the higher-flow pulse event in November, no further pulses of flow of at least 5-days duration occurred during the summer months, which reflects the dry conditions throughout this period.
  • The releases finished in late-May based on a rainfall trigger of 40 mm of rainfall recorded at Gosnells weather station.

 

Environmental and cultural groundwater provisions

  • Annual extraction limits in water allocation plans are the main instrument used to protect the environmental and cultural values of groundwater in the region (see Water management note).

 

Map of groundwater monitoring sites in the northern part of the Perth region. There are more than 30 monitoring sites in both the Gnangara groundwater mound north of Perth and in the Jandakot groundwater mound just south of Perth.
Figure S13 Sites where Ministerial criteria have been set for groundwater levels

 

  • In the Jandakot and Gnangara groundwater mounds, water level criteria for key sites have been set as Ministerial conditions under the Environmental Protection Act 1986 (see Ministerial Statements 688 and 819).
  • Over the past several years a number of the sites have been non-compliant with the Ministerial conditions due to a combination of factors including drying climate, groundwater extraction, and tree plantations limiting recharge (Department of Water 2017).
  • In response the Department of Water and Environmental Regulation implements strategies to stabilise and restore groundwater levels. This includes reducing allocation limits, increasing recharge through land-use changes, improving water use efficiency, and investing in alternative water sources.
  • For more information see the department's compliance reports for the Gnangara and Jandakot mounds

 

Environmental and cultural groundwater outcomes

Line graph of groundwater level in the Gnangara Mound between January 1997 to June 2021. The level data are based on analysis of 43 superficial aquifer monitoring bores across the Gnangara Mound. There was a decreasing trend in groundwater levels between 1997 and 2016. Between 2016 and 2019 groundwater levels increased, although still relatively low compared to earlier years.
Figure S14 Average groundwater levels in the Gnangara Mound from 1997 to 2021

 

  • Although groundwater levels remain relatively low, the improved rainfall conditions during the 3-year period between July 2016–June 2019 have led to an increase in water levels since 2016 in the Gnangara Mound.
  • The end-of-year levels at 30 June 2021 were the lowest in five years following two consecutive less-than-average winter rainfalls.
  • Further information on the status of the Gnangara Mound is available from the Department of Water and Environmental Regulation's Gnangara groundwater microsite.