National Water Account 2017

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Perth: Water stores

Surface water store

• The volume of surface water in the region increased by 13% from 262,516 ML at 1 July 2016 to 297,520 ML at 30 June 2017.  
• The increase of 35,004 ML represents the rise in the volume of water stored in surface water storages (see Surface water assets).
• The balancing item is likely attributed to uncertainties associated with quantification of runoff, river losses and river outflow (see Methods).

Surface water flows

Figure S4 Water inflows and outflows for the surface water store during the 2016–17 year

• The largest surface water inflows in the region were runoff and inter-region inflow, which together comprised more than 75% of the total natural inflows. Both were much higher than the previous year due to higher rainfall in the first few months of the year and unseasonably high rainfall in late January–early February.
• The increase in inter-region inflow is mostly attributable to flow into the region in the Avon River in February–March following the high summer rainfall (see Climate and water).   

• About 7% of the natural surface water inflows in the region were captured in the surface water storages, mostly in the first few months of the year.
• The majority of the surface water inflows to the region (about 70%) discharged to the sea as river outflow, which was much higher than the previous year due to higher runoff, inter-region inflow and groundwater discharge.
• The largest water transfer in the region, discharge: urban supply, increased by 22% from the previous year due to increased 'banking' of desalinated water and groundwater in the urban surface water storages as the urban supply mix changed in response to low surface water availability (see Urban system).

Figure S5 Allocated surface water diversions during the 2016–17 year by allocation area

• The total allocated surface water diversions in the region was 189,453 ML, which was about 9% lower than the previous year.
• Both urban and irrigation surface water diversions declined due to low surface water availability and reduced demand during the unusually wet summer.
• Allocated diversion to the urban system accounted for about half of the diversions, and mainly came from the urban storages in the Canning River, Dandalup River System and Serpentine River Catchment. The urban diversions included groundwater and desalinated water 'banked' in the storages.
• Allocated diversions by individual users and allocated diversions by the irrigation scheme accounted for approximately 20% and 30% of the diversions respectively; most individual user diversions came from unregulated rivers in the Murray River and Tributaries allocation area, and all irrigation scheme diversions came from the irrigation storages in the Harvey and Collie allocation areas.
• For a more detailed description of the water usage in the region, and the associated entitlements, see the Surface water rights note.

Groundwater store

• The groundwater balance yielded a balancing item of –116,555 ML for the year.
• As groundwater storage has not been quantified, the balancing item is attributed mainly to the increase in groundwater storage (see Groundwater assets) but also reflects uncertainties in the estimation of the individual water balance components (see Methods).

Groundwater flows

Figure S6 Water inflows and outflows for the groundwater store during the 2016–17 year

• Almost 96% of the total groundwater inflows is attributed to recharge: landscape. Recharge increased by 35% from the previous year due to higher rainfall, including during the unseasonally wet summer (see Figure S24), and was the highest in three years.

• Aquifer recharge was low as the Perth region's Groundwater Replenishment Scheme had not yet started operation in 2016–17 (see Major water reforms).
• The largest natural groundwater outflows in the region were discharge: landscape and discharge: surface water, which each made up around 30–40% of the total natural outflow volume. 

Figure S7 Allocated groundwater extractions during the 2016–17 year by allocation area

• Total allocated groundwater extraction in the region was 430,972 ML, which was similar to the previous year. 
• 68% of the allocated extraction was for licensed individual users and 32% was for the urban system.
• The Perth groundwater allocation area had the highest groundwater extraction, mainly by individual users. The Other category in Figure S7 includes 12 groundwater allocation areas, mainly located on the coastal plain in the west of the region, which had average allocated extraction of 12 GL each.
• For a more detailed description of the water usage in the region, and the associated entitlements, see the Groundwater rights note.

Urban water system

• The volume of water in the region's urban system comprises water in the pipe network. As no major pipe network changes were made, this volume is assumed to remain unchanged throughout the year.
• The balancing item is likely attributed to metering inaccuracies, unaccounted losses, and uncertainties in the estimation of certain volumes (see Methods).

Urban water system flows

Figure S8 Water inflows and outflows for the urban water system during the 2016–17 year

• A more detailed breakdown of the inflows and outflows of the urban water system components is provided in the urban water balance diagram.

Water supply inflows

Figure S9 Water sources used in the Perth region's urban supply system during the 2016–17 year compared with the previous 6 years

• Following very low surface water storage inflows in the previous year, the Water Corporation's strategy in 2016–17 was to rely primarily on groundwater and increased desalination for urban supply.

• Desalinated water made up 53% of urban supply, an increase of 8% from the previous year and part of a steadily increasing trend since Perth's first desalination plant was brought online in 2006.
• The increased reliance on desalination required greater 'banking' of water production in the urban surface water storages during low demand periods to buffer peak period supply requirements and cater for forecast demand increases.
• Due to the unexpectedly wet summer conditions (see Climate and water), urban water delivery actually decreased by 5% from the previous year. As a result, supply system discharge to the surface water storages exceeded urban diversions from the storages by 8,532 ML, resulting in the negative net surface water supply shown in Figure S9.
• The net discharge to the surface water store by the urban system contributed to the increase in surface water storage during the year (see Surface water assets).

Wastewater and recycled water inflows

• Wastewater collected was 140,439 ML during the 2016–17 year.

Water supply outflows

Figure S10 Outflows from the water supply system

• The majority of water supply outflow was delivery to urban users.
• Supply system discharge to surface water has increased consistently over the past several years, and by more than 20% in the past year, due to the increasing reliance on desalinated water which is 'banked' in the storages during periods of low demand and delivered during peak demand times.
• 8% of the total outflow from the urban supply system was related to losses.

Wastewater and recycled water system outflows

Figure S11 Outflows from the wastewater and recycled water systems

• About 7% of the treated wastewater was recycled, either through delivery to urban users or via aquifer recharge.
• Most of the remaining treated wastewater was discharged to the sea.

Irrigation scheme

• The Perth region's irrigation scheme is the Harvey Water Irrigation Area which comprises three irrigation districts: Waroona, Harvey, and Collie River.

• The volume of water in the region's irrigation system comprises water in the pipe and channel network. As no major changes were made to this network, this volume is assumed to remain unchanged throughout the year.
• The balancing item may be due to a number of factors, such as metering inaccuracies and unaccounted losses (see Methods).

Irrigation scheme flows

Figure S12 Water inflows and outflows for the irrigation scheme during the 2016–17 year

• The volume of allocated diversion: irrigation scheme was about 15% less than the previous year, reflecting lower allocations for the Waroona and Harvey irrigation districts and above-average rainfall during the typically drier summer months.
• Leakage represents losses from the open channel network in the Collie River irrigation district.

Figure S13 Water delivered for each irrigation scheme customer category during the 2016–17 year

• There are about 1,621 customers in the Harvey Water irrigation scheme, most of whom receive water from the scheme for irrigation purposes.