Climate change and waterways
The Western Australian government recognises that climate change is happening now. While our climate is naturally variable, the warming trend and climate changes observed due to enhanced greenhouse effect are predicted to continue into the future and will affect Western Australia's communities, industries and ecosystems as well as present challenges to managing our state's resources. The Indian Ocean Climate Initiative (IOCI), a long-running research partnership between the WA government, the CSIRO and the Bureau of Meteorology, has improved our understanding of climate changes in Western Australia.
Changes to our climate, including rainfall, temperature, evaporation and extreme weather events, influence the water cycle, which is the primary driver of the hydrology of waterways. The condition and stability of waterways depends not only on their hydrology, but also on a complex and dynamic network of interactions between bacteria, algae, plants and animals with sediments, rocks, surface water flow, groundwater and chemicals. Climate change will affect these components, and the processes and interactions that occur between them, in a range of different ways.
We monitor how changes in the climate affect the hydrology of our waterways and other water resources. Our challenge is to predict how climate change will affect our waterways and their ecosystems in the future, and to take this into account in land planning and in our approach to managing and restoring waterways, in order to maximise their resilience.
Changes in rainfall
Recent decades have a trend of higher-than normal rainfall in northern and central Australia and decreasing rainfall across south western Australia.
Since 1970, rainfall in south-west Western Australia has decreased significantly, with decreases in average rainfall of up to 200 mm/a in some places. Reduced rainfall and fewer very wet years have resulted in lower and less frequent flows in waterways. For more information on changes in flow, see climate and streamflow and surface water hydrology.
In some areas (such as around Perth), groundwater recharge and water tables have also fallen, affecting the waterways and other ecosystems that depend on groundwater inflow.
As this drying trend continues, south-west waterways are likely to experience altered patterns of flow with perennial streams becoming seasonal and seasonal streams having longer periods without flow. Some wetlands may disappear. 
Aquatic organisms will be affected by habitat changes. Many cannot disperse or migrate when river pools become disconnected.
For species that breed in autumn, the delay of winter rainfall and cooler temperatures that are cues for breeding, may not allow them sufficient time to complete their lifecycles.
Since 1960, rainfall in areas outside the south west of Western Australia has increased, with the highest increases recorded in the north-east parts of the state.
Many waterways in arid and semi-arid areas experience irregular large floods, with long dry periods between, where they exist as a series of disconnected pools or dry out completely. The aquatic and riparian fauna and flora that persist there are adapted to this extreme variability, so they may be resilient to future changes.
Increasing temperatures
Average temperatures have increased by 0.2-0.8°C across most of the state during the last 40 years; except in some parts of the central Kimberley, where temperature has declined by 0.2°C. Temperatures are predicted to continue to rise with more hot days, and extremely hot days (above 40°C) and less cold nights.
Temperature affects the rate of many of biochemical processes in aquatic ecosystems. Water temperature directly affects the distribution of aquatic organisms and extreme temperatures may exceed the tolerances of those adapted for the previous cooler climates, causing local extinction of vulnerable species. This situation is exacerbated in areas with extensive land clearing where little riparian vegetation exists to shade riverine pools. Increased temperatures may also disrupt lifecycles and reproduction, for example affecting species that have a sex ratio dependent on temperature (e.g. crocodiles and freshwater turtles).
Higher temperatures will increase evaporation and transpiration rates, reducing surface water availability, drying river pools and seasonal wetlands more quickly and affecting the survival and reproduction of aquatic organisms. For instance, tadpoles may not have time to develop into frogs before habitats dry out again.
Increased bushfires pose a direct threat to riparian and wetland vegetation and habitats, indirectly to water quality, due to ash and erosion after the removal of fringing vegetation.
Sea level rise
Sea level has increased around the coast, but not by a uniform amount. Sea levels recorded at Fremantle indicate a long-term average rise of 1.5 millimetres each year over 1897 to 2004. More recent shorter-term data from the Hillarys monitoring station indicates a greater increase of about nine millimetres each year between 1991 and 2011.Modelling of future climate scenarios predicts a 100mm increase from 1990 to 2030 for the median case.
As sea levels rise, low-lying coastal freshwater floodplain and wetland ecosystems are at risk as the incidence of inundation events increases and vegetation has less time to recover after more and more regular flooding by seawater.
Estuaries will be affected by increasing marine influence and changing habitat distribution and quality. Increase in saline habitats, altered sand bar dynamics, shoreline erosion and sediment redistribution due to increased tidal influence and wave height, and changes to stratification within the water column. South-west estuaries are more likely to experience reduced flushing of sediments, nutrients and pollutants as they are affected by both sea level rise and reduced river flows.
Planning for climate change and waterways
Waterways, their habitats and fauna in the south-west are the most vulnerable to climate change, due to the steady and long term decrease in rainfall and streamflow and added population pressures, including increasing demand for water and land development.
The Department of Water takes climate change and its influence on trends into account in the way it manages water resources, including:
- Assessing water resources and modelling water availability
- Making decisions about the amount of water that can be taken from surface and groundwater resources
- Advising about water and land planning and inundation risk
- Advising about management and restoration of waterways, their floodplains and estuaries.
In 2010, the Department partnered with the National Climate Change and Adaptation Research Facility (NCCARF), Murdoch University and then Department of Environment and Conservation to coordinate a symposium called 'Climate change and Western Australian wetlands and waterways'.
One hundred participants from a range of stakeholders met to review our understanding of the impacts of climate change on our aquatic ecosystems, and then identify adaptation responses, knowledge gaps and barriers.
An executive summary of the project is available in the form of a report card available at http://www.nccarf.edu.au/publications/aquatic-report-card
For further information:
The Western Australian government climate change strategy: Adapting to our changing climate (Department of Parks and Wildlife 2012).