Perth: Supporting information
- A relatively dry winter in 2019 contributed to low runoff and storage inflows across the region.
- Surface water storage dropped to 41% full at 30 June 2020, the first annual drop in storage in four years.
- 76% of the allocated water was taken in 2019–20, about a third for urban supply.
For further information on the region's water accounting statements scroll down this page or click on the links below:
Water assets
Surface water assets
- Total surface water asset volumes in the Perth region at 30 June 2020 were 360 GL, 99% of which is water held in storages. The remaining amount is water held in rivers.
Figure S1 Total storage volume in the region at 30 June 2020
compared with the previous 10 years
- Storage volumes decreased from 47% full at 30 June 2019 to 41% full at 30 June 2020, the first annual drop in storage in four years.
- 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.
Figure S2 Percentage-full volume on
30 June 2020 for each storage and total daily storage volume from 2013–2020 (inset)
- Storage levels peaked in September 2019 at around 55% full following the winter rainfall.
- The increase in storage levels following the 2019 winter months was much lower than the previous three years.
- About 60% of the surface water stored in the region was for urban supply and 40% for irrigation.
- 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.
Figure S3 Total annual change in aquifer volume in the region at 30
June 2020 compared with the previous 9 years
- The low rainfall during the 2019 winter resulted in a decrease in groundwater storage during 2019–20, the first annual drop in aquifer storage in four years.
- The dry conditions during the latter half of 2019 were influenced by a a positive phase of the Indian Ocean Dipole, one of the strongest on record.
Water liabilities
- Water liabilities in the Perth region refer to the volume of allocation remaining on licence entitlements at the end of the 2019–20 year.
| Description | Individual users (ML) | Urban system (ML) | Irrigation scheme (ML) | |
| Opening balance at 1 July 2019 | 8,522 | 0 | 40,634 | |
| add | Allocation | 43,998 | 247,950 | 102,164 |
| less | Allocated diversion | 43,998 | 101,929 | 73,827 |
| less | Adjustment and forfeiture | 0 | 146,021 | 37,140 |
| Closing balance at 30 June 2020 | 8,522 | 0 | 31,831 |
| Description | Individual users (ML) | Urban system (ML) | |
| Opening balance at 1 July 2019 | 102,375 | 0 | |
| add | Allocation | 301,028 | 164,388 |
| less | Allocation extraction | 301,028 | 135,516 |
| less | Adjustment and forfeiture | 0 | 28,872 |
| Closing balance at 30 June 2020 | 102,375 | 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
Figure S4 Water inflows and outflows for the region's water stores and systems during the 2019–20 year
- Total surface water and groundwater flows into the region's storages was estimated to be around 2,557 GL. The key item contributing to surface water and groundwater inflows was runoff and recharge: landscape, which together made up more than 75% of the storage inflows.
- Runoff was lower than average (based on modelled data from 1971–2020) due to the relatively poor 2019 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 60% 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 740 GL, around 30% of the total surface water and groundwater outflow (see Water use section below).
Water use
Figure S5 Water taken during the 2019–20 year in the region's water management areas
- Total surface water diverted in the region was 220 GL.
- 46% of the diversions were for the urban system, which included groundwater and desalinated water 'banked' in the storages
- 34% of the diversions were to the irrigation scheme from storages in the Harvey and Collie river catchments; 20% of the diversions were for individual users.
- Total groundwater extracted in the region was 521 GL, around 40% of which was from the Gnangara plan area.
- 74% of the extractions were for individual users, including allocated and non-allocated use, 26% was for the urban system.
Figure S6 Surface water allocations and diversions
in the Perth region for the years ending 30 June, from 2012–2020
- In 2019–20, total surface water allocation was 394 GL, mostly for urban system and irrigation scheme supply. 56% of the allocated surface water was taken.
- Allocations for the irrigation scheme decreased from the previous year following a dry winter in 2019.
- Diversions to the urban system were lower 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).
Figure S7 Groundwater allocations and extractions
in the Perth region for the years ending 30 June, from 2012–2020
- 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 11% more than last year as 'banking' of groundwater in the surface water storages was increased due to the low storage inflows.
Urban system supply
Figure S8 Water sources used in the Perth region's
urban supply system during the 2019–20 year compared with the previous nine years
- Total inflows to the water supply system was 298 GL, around 4% more than the previous year.
- Low storage inflows following a dry winter in 2019 meant the Water Corporation had to rely mostly on desalinated water and groundwater for urban supply.
- The decreased surface water contribution in 2019–20 compared to the previous year 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
| Transaction type | Number of transactions | Volume (ML) |
| Surface water transactions | ||
| Entitlement trades | 21 | 1,239 |
| Allocation trades | 207 | 9,994 |
| Groundwater transactions | ||
| Entitlement trades | 171 | 17,781 |
| Allocation trades | 0 | 0 |
| Total | 399 | 29,014 |
- Most of the surface water transactions that occurred in the Perth region in 2019–20 were within the Harvey Water irrigation scheme.
- All groundwater transactions were entitlement trades, which represented about 4% 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.
Figure S9 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.
Figure S10 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
Figure S11 Daily flow in the Canning River compared with
the required flow regime during the 2019–20 year
- In 2019–20, 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 almost 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).
Figure S12 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
Figure S13 Average groundwater levels in the Gnangara Mound from 1997 to
2020
- 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 2020 were at their highest since 2008.
- Further information on the status of the Gnangara Mound is available from the Department of Water and Environmental Regulation's Gnangara groundwater microsite.