Western Australia summary from water balances

Water balances were prepared for six areas within Western Australia – Perth, Gnangara Mound, South West Yarragadee, Harvey River, Collie River and Carnarvon. The water balance for the Ord River crosses both Western Australia and Northern Territory.

Key points

  • The water balances for Perth, Gnangara Mound and South West Yarragadee were all based on the boundaries of a number of combined groundwater management units, while the balance for Carnarvon was based on the single groundwater management unit boundary.
  • Although rainfall conditions across each of the water management areas were above average for the 2004-05, in general the far south west region of Western Australia has experienced a long term downturn in rainfall observed since the 1970’s. This, in conjunction with the absence of very wet years, has resulted in greatly reduced runoff and recharge within the catchments. Streamflows across the south west of Western Australia have more than halved since 1975. This is reflected in this report, with streamflow for 2004-05 below the long term average for the Perth, Gnangara Mound and Harvey River water management areas.
  • As a result of the below average rainfall since the mid 1970’s, water supplies have increased their reliance on groundwater. In 1980 the amount of licensed use was relatively evenly distributed between surface water and groundwater, however by 1997 groundwater use was determined to be approximately 75% larger than surface water use. For 2004-05 each of the water management areas was found to have a high reliance of groundwater for both domestic use and irrigation with groundwater use ranging between 68% for Gnangara Mound to 99% for South West Yarragadee.
  • Approximately 97% of domestic water is supplied by the Water Corporation of Western Australia, with the Integrated Water Supply Scheme (IWSS) linking Perth’s wellfields and surface water sources with demand centres. Currently groundwater supplies 60% of the IWSS public water supply, with 60% of this sourced from confined aquifers. In addition, 55% of irrigation water within Western Australia is self-supplied, with 90% of this from groundwater.
  • A seawater desalination plant in the Perth suburb of Kwinana was commissioned in November 2006. When fully operational the plant, owned by Water Corporation, will produce approximately 130 million litres per day, supplying 17 per cent of the city’s needs.
  • The understanding of groundwater within Western Australia is advanced with the majority of groundwater data for the Perth, Gnangara Mound, South West Yarragadee and the Collie River water balances derived from detailed groundwater models with a high level of reliability. With regard to surface water resources a project currently being developed involves the mapping of farm dams within the south west region. This project has allowed for an estimate of the volume of water in farm dams to be obtained for the South West Yarragadee, Collie River and Harvey River water management areas. For each of these areas the volume held in farm dams was less than 10% of the total surface water runoff figure, indicating that farm dams are not having a significant impact on runoff.
  • A common element of the water balances prepared in Western Australia is that there was no opening or closing balance of water in store. This is primarily due to there being no major surface water storages within the area and no defined management trigger levels from which renewable or non-renewable groundwater stores are managed. In addition, for many of the water management areas two recharge estimates were available for the water balance. In general the recharge figure supplied by the Department of Water groundwater model was much larger than that obtained from the BRS Water 2010 project. This could be due to the Department of Water groundwater model giving an evapotranspiration loss from groundwater which is not included in the Water 2010 estimates, or alternatively, could be due to different climatic representations. Two runoff estimates were also available for many of the water management areas. The estimate used within many of the water balances was taken from the Department of Water REG75 model. This estimate was often much greater than the estimate given by the BRS Water 2010 project. Both estimates are based on approximate analyses and interpolation and extrapolations, which relates to data reliability Category B.
  • In general, data could not be obtained for the following items in the water balance:
    • Groundwater storage volumes (renewable and non-renewable)
    • Soil - unsaturated zone storage volumes
    • River channels storage volumes
    • Irrigation drainage returns and irrigation channel outfall returns
    • Seepage from surface water features
    • Losses from minor catchment dams (farm dams)
  • For more information on the water resources of Western Australia go to the Context report on south-west water resources prepared by CSIRO.

Further discussion of the water balances for Western Australia is provided in the Water Availability National Perspective report, and the Regional Water Balances report, which are both available on the Publications page.

For individual results of water balance assessments go to Regional Water Resources Assessments

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Last Updated 12/06/2007