Perth: Water stores
These notes provide a water balance for each of the region's water stores for the 2015–16 year. Below-average rainfall conditions contributed to low runoff and aquifer recharge across the region and a drop in storage volumes. Despite aquifer recharge decreasing, groundwater extraction increased during the year due to the limited surface water availability.
Surface water store
The volume of water in the Perth region's surface water store decreased during the 2015–16 year from 202,362 ML at 1 July 2015 to 167,165 ML at 30 June 2016 (Table S5).
| 2016 ML | 2015 ML | |
| Opening surface water store | 202,362 | 204,898 |
| Inflows | 713,112 | 939,147 |
| Outflows | (377,204) | (618,395) |
| Balancing item | (371,105) | (322,288) |
| Closing surface water store | 167,165 | 202,362 |
A schematic diagram representing all surface water inflows and outflows during the 2015–16 year is provided in Figure S4.
Figure S4 Water inflows and outflows for the surface water store during the 2015–16 year
Surface water inflows
The largest natural water inflow in the region was runoff, and this was approximately 35% less than the previous year, reflecting the reduced rainfall experienced across the region during the year (see Climate and water). The remaining natural surface water inflows, precipitation and discharge: groundwater, also decreased by more than 20% and 10% respectively compared with the previous year.
The largest water transfer in the region (discharge: urban supply) comprised two components—internal water transfers (81,104 ML) and riparian releases (2,136 ML). Internal water transfers from the urban water system increased by approximately 12% from the previous year, likely due to operational requirements to mitigate the effects of the drier climatic conditions experienced during the year. The remaining two transfers of water to the surface water store were delivery: inter-region and point return: irrigation scheme. The point return flow was not measured and could not be quantified accurately.
Surface water outflows
The largest natural water outflow in the region was river outflow. Outflow to sea was more than 50% less than the previous year, which reflects the very low streamflows observed in these river systems during the year (see Climate and water).
Evaporation decreased by almost 40% from the previous year, which may be attributed to the decreased storage volumes and hence surface area of the storages during the year (see Statement details). The remaining two natural surface water outflows were leakage: landscape and river and floodplain losses. Leakage to landscape is a very small outflow and varies little annually; river and floodplain losses could not be quantified accurately due to a lack of available data.
Total surface water diverted in the region (170,546 ML) decreased by more than 10% from the previous year, which reflects the decreased availability of surface water in the region, particularly for urban supply (see Urban water system note below). Allocated diversion: urban system (101,204 ML) accounted for approximately 60% of the total allocated diversion in the Perth region. The volume of water diverted from each storage to the urban system is shown in Figure S5.
Figure S5 Surface water diversions to the urban system during the 2015–16 year; percentage of allocation diverted is also shown
Allocated diversion: individual users and allocated diversion: irrigation scheme accounted for approximately 20% of the total allocated diversion each. For a more detailed description of the water usage in the region, and the associated entitlements, see the Surface water rights note.
Surface water balancing item
The surface water balance (Table S5) yielded a balance of –371,105 ML, indicating that inflows are greater than outflows. This is larger than the total surface water store volume at the end of the 2015–16 year and more than half of the total surface water inflows during the year. The negative balancing item indicates that either the inflows are too high or the outflows are too low.
The balancing item is primarily attributed to the fact that river and floodplain losses cannot be quantified in the account. It is likely that these losses are a relatively large source of surface water decrease throughout the region. It is also likely that a portion of the balancing item is attributed to uncertainties associated with the runoff estimation (a large source of surface water increase). Runoff is estimated from a rainfall–runoff model (see Methods), and it is reasonable to expect a relatively high uncertainty around this volume.
Groundwater store
The volume of water in the Perth region's groundwater store is assumed to have remained unchanged throughout the 2015–16 year (Table S6).
| 2016 ML | 2015 ML | |
| Opening groundwater store | – | – |
| Inflows | 1,236,022 | 1,372,693 |
| Outflows | (1,429,120) | (1,518,521) |
| Balancing item | 193,098 | 145,828 |
| Closing groundwater store | – | – |
A schematic diagram representing all groundwater inflows and outflows during the 2015–16 year is provided in Figure S6.
Figure S6 Water inflows and outflows for the groundwater store during the 2015–16 year
Groundwater inflows
The largest natural water inflow in the region was recharge: landscape, which made up almost 95% of the total groundwater inflows. Recharge decreased by more than 10% from the previous year, reflecting the drier conditions across the region during the year (see Climate and water).
Inter-region inflow is less climate-dependent and increased marginally compared with the previous year. Consistent with previous years, inter-region coastal inflow and recharge: surface water was 0 ML.
Total aquifer recharge from the urban water system was less than previous years, which is attributed to the temporary cessation of managed aquifer recharge to facilitate the construction of Perth's first full-scale Groundwater Replenishment Scheme (see the Major water reforms section of 'Climate and water' for more information).
Leakage: urban system during the 2015–16 year was relatively similar to the previous year.
Groundwater outflows
The largest natural water outflow in the region was discharge: landscape, which made up more than 35% of the natural flows from the region's aquifers. Inter-region coastal outflow and discharge: surface water each made up approximately 30% of the natural outflows. Each of these flows decreased by more than 12% from the previous year, likely reflecting the reduced rainfall experienced across the region during the year (see Climate and water). The remaining natural outflow, inter-region outflow, also decreased from the previous year.
Total groundwater extracted in the region (517,477 ML) increased by approximately 7% from the previous year, which reflects the decreased availability of surface water (see Surface water store note above). Allocated extraction: individual users (299,754 ML) accounted for almost 60% of the total extraction in the Perth region. Allocated extraction: urban system (136,878 ML) accounted for approximately 25% of the total extraction; the remaining groundwater extracted refers to non-allocated extractions: individual users (80,845 ML).
For a more detailed description of the water usage in the region, and the associated entitlements, see the Groundwater rights note.
Groundwater balancing item
The groundwater balance (Table S6) yielded a balance of 193,098 ML, approximately 15% of the total groundwater inflows during the 2015–16 year. This volume actually represents the net change in groundwater store throughout the year.
During the 2015–16 year, total groundwater inflow was less than the previous year; in particular, recharge from the landscape was approximately 140,000 ML less than that reported for the previous year. This decrease in recharge reflects the decreased rainfall conditions observed throughout the region during the 2015–16 year (see Climate and water).
Therefore, the net change in groundwater storage during the 2015–16 year (represented by the balancing item volume) is mainly due to a relatively low recharge from the landscape compared with the combined total natural outflows and groundwater extractions that occurred during the year.
Urban water system
The volume of water in the Perth region's urban water system is assumed to have remained unchanged throughout the year (Table S7).
| 2016 ML | 2015 ML | |
| Opening urban water store | – | – |
| Inflows | 517,538 | 503,927 |
| Outflows | (517,992) | (503,488) |
| Balancing item | 454 | (439) |
| Closing urban water store | – | – |
A schematic diagram representing all urban inflows and outflows during the 2015–16 year is provided in Figure S7.
Figure S7 Water inflows and outflows for the urban water system during the 2015–16 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
Total net inflow to the water supply system was 297,576 ML, a slight increase from the previous year (Figure S8).
Figure S8 Water sources used in Perth's urban supply system during the 2015–16 year compared with the previous 5 years
Delivery: desalinated water and allocated extraction: groundwater made up almost the same proportion of urban water supply during the 2015–16 year, 47% and 46% of inflows respectively. Groundwater extraction for urban supply has decreased over the last 5 years with increasing desalinated water production; however, groundwater supply increased in the 2015–16 year due to the limited availability of surface water. Surface water diversion (net) made up only 7% of urban water supply, less than half that of the previous 5 years.
The net surface water diversion (20,100 ML) as shown in Figure S8 is different to the allocated diversion: surface water (101,204 ML) as shown in Figure S7. This is because the total diversion volume is partially made up of groundwater as well as desalinated water produced in the winter and then transferred to storages to enable summer peak supply capacity. Therefore, surface water diversion is the difference between the total diversion (101,204 ML) and the volume of internal transfers (81,104 ML).
Supply system delivery: inter-region made up less than 1% of urban water supply, which is similar to the previous few years.
More details on water allocations and abstractions associated with the urban water system are provided in the Water rights note.
Wastewater and recycled water inflows
Wastewater collected was 138,858 ML during the 2015–16 year.
Water supply outflows
Total outflows from the water supply system (378,021 ML) comprised the following (Figure S9):
Figure S9 Outflows from the water supply system
The majority of water supply outflow was supply system delivery: urban users. During the 2015–16 year, the delivery was marginally higher than the previous year, which aligned with the increased inflows to the water supply system during the year (see Water supply inflows above). Supply system discharge: surface water increased by approximately 12% from last year, likely due to operational requirements to mitigate the effects of the drier climatic conditions experienced during the year.
Approximately 8% of the total outflow from the urban supply system was related to losses (Figure S9). The losses, which comprised leakage: groundwater and other supply system decreases, were similar to the previous year. Supply system transfer: inter-region was also similar to the previous year.
Wastewater and recycled water system outflows
Total outflows from the wastewater and recycled water system (139,971 ML) comprised the following (Figure S10):
Figure S10 Outflows from the wastewater and recycled water systems
Approximately 8% of the treated wastewater was recycled, which comprised recycled water delivery: urban users and aquifer recharge (Figure S10). Aquifer recharge was less than previous years, which is attributed to the temporary cessation of managed aquifer recharge to facilitate the construction of Perth's first full-scale Groundwater Replenishment Scheme (see the Major water reforms section of 'Climate and water' for more information).
Most of the remaining treated wastewater comprises discharge: sea and, to a much lesser extent, discharge: landscape.
Recirculation of treated wastewater and recycled water that occurs in the system may be subjected to additional treatment and/or re-use on site. The reported urban system outflows exclude any such recirculated volumes in the system.
Urban water system balancing item
The urban water balance (Table S7) yielded a balance item of 454 ML. This is less than 0.1% of the total urban water system inflows during the year. The misbalance may be due to a number of factors, such as metering inaccuracies, unaccounted losses, and inaccuracies in the estimation of certain volumes.
Irrigation scheme
The Perth region's irrigation scheme represents the Harvey and Waroona irrigation districts within the Harvey Water Irrigation Area. The volume of water in the irrigation scheme is assumed to remain unchanged throughout the year (Table S8).
| 2016 ML | 2015 ML | |
| Opening irrigation scheme store | – | – |
| Inflows | 30,633 | 32,958 |
| Outflows | (32,197) | (31,426) |
| Balancing item | 1,564 | (1,532) |
| Closing irrigation scheme store | – | – |
A schematic diagram representing all irrigation scheme inflows and outflows during the 2015–16 year is provided in Figure S11.
Figure S11 Water inflows and outflows for the irrigation scheme during the 2015–16 year
The volume of allocated diversion: irrigation scheme (30,633 ML) during the 2015–16 year was marginally less than the previous year. More details on water allocations and diversions associated with the irrigation scheme are provided in the Surface water rights note.
The delivery of water to customers was 32,197 ML. There are more than 1,250 customers within the Harvey and Waroona irrigation districts; most of the water is delivered to customers for irrigation purposes (Figure S12).
Figure S12 Water delivered for each irrigation scheme customer category during the 2015–16 year
The remaining transfer of water is point return: irrigation scheme, which was not measured and could not be quantified accurately.
Irrigation scheme balancing item
The irrigation scheme water balance yielded a balancing item of 1,564 ML (Table S8), which is approximately 5% of the total irrigation scheme inflows for the year. The misbalance may be due to a number of factors, such as metering inaccuracies and unaccounted losses.