Sourcing Water for Use
The amount of water available for the IWSS water grid is known as the water yield. The yield of existing water supplies is somewhat uncertain due to:
- reduced inflows to dams;
- the potential requirement to increase flows from surface and groundwater resources to the environment; and
- increasing and competing land use practices within water supply catchments rendering them unsuitable for use or requiring higher levels of treatment.
Streamflows over the last 30 years averaged 163 gigalitres a year. In this period, there were some wet years early on, however over the past six years, when rainfall was the lowest on record, streamflows into our dams were only half this amount.
Groundwater availability is also being reduced because of declining rainfall.
The IWSS water yield is estimated to average 315 gigalitres a year to 2020, based on the low rainfall period from April 2001 to May 2007.
This figure includes water from both the Perth and Southern Seawater Desalination Plants and also includes the water available from trading with the Harvey Water irrigation cooperative. This water yield is based upon a probability of a total outside sprinkler ban occurring once in fifty years.
Based on work undertaken by CSIRO and the Indian Ocean Climate Initiative, climate change is expected to result in reduced rainfall in the Perth-Mandurah area. This will adversely impact yields from both surface water and groundwater sources.
In particular, surface water yields (including water from trading with irrigators) will be most severely impacted. By 2060, the most extreme projection is for a 60% decrease in rainfall, which would virtually eliminate water from dams as a reliable water source in most years, due to evaporation.
CSIRO projected under a median emissions scenario, that by 2030, rainfall could decrease by 20% from 1980 to 1999 averages. Given that we have already experienced a 12% reduction in rainfall since this time, a further 8% loss in rainfall is estimated to reduce streamflows by a further 20 gigalitres by 2030. This may also further reduce water availability from groundwater resources.
This scenario also projects a further 20% reduction in rainfall by 2060, resulting in greater loss of surface and groundwater supply.
Climate science is evolving and these projections are based on probabilities. However, it is important to understand the potential impact of reductions in rainfall on public water supply during this period of rapid change.
Decisions to accelerate water conservation initiatives or invest in new supply capacity will be made incrementally. There is an opportunity for us to progressively adapt as our knowledge of climate and experience of rainfall unfolds.
Planning supports sustainable decisions by making the impact of different scenarios transparent. Engaging with the community allows us to evaluate a range of responses to these possible futures.
The Department of Water determines the amount of water required to sustain the ecological, social and economic values of water resources. Retaining water for these reasons is important, but can reduce the amount of water available for other uses, such as public water supply.
Environmental water provisions are currently being reviewed for a number of surface and groundwater sources in the metropolitan area. No allowance has been made in the above figures for reduced surface water availability as a result of regulatory requirements for increased environmental water provision.