Climate change impacts on the hydrology of south-western Australia

Climate change has profoundly affected the hydrology of south-western Australia since at least 1975. It took over a decade before the signal could be detected from annual variability. The impacts of rainfall reductions were exacerbated by higher temperatures and a decrease in wet periods when most recharge and runoff occurred. As a rule-of-thumb, runoff and recharge reduced by 3 percent for each percent reduction in rainfall.

Reductions in runoff were driven by falling groundwater levels. Stream- and dryland-salinity required levels be monitored, otherwise this driver would have gone unnoticed.

Runoff into reservoirs has almost ceased as processes irreversibly changed. Using historical records to estimate future runoff had limited application because of non-stationary processes.

While water resources have diminished, the threats posed by dryland salinity, stream salinity, flooding and waterlogging have decreased. While winter flood risks have dramatically reduced, summer flood risks appear to have increased.   

Almost all GCMs project an even drier and warmer future. Perth (population 2m) has avoided a ‘Day Zero’ by the rapid expansion of shallow- and deep-groundwater extraction, and seawater desalination. Highly treated wastewater has started to be added to augment drinking water aquifers.

Recharge under tree canopies have been most reduced. This is due to greater interception losses because showers have largely replaced heavy rain, and trees using a higher proportion of rainfall. Rainfall intensities, at least for long durations, have decreased despite the fear that higher sea surface temperatures (SST) and a warmer atmosphere will result in more intense rainfall. While SSTs have started to rise, there are complications related to El Niño– Southern Oscillation, the Indian Ocean Dipole and the warm Leeuwin Current that flows down the coast of Western Australia. This current results in much higher rainfall than would be expected and may weaken if El Niño becomes stronger and/or more frequent.  

As well as impacting water resources and rates of land degradation, climate change has affected ecosystems and industries. Abnormally hot and dry years have resulted in the deaths of trees able to withstand harsh Mediterranean summers. Wetlands have dried and groundwater-dependent ecosystems have been lost. Cereal crops are now grown in regions that used to be severely affected by soil waterlogging.  Tree plantations have become unviable due to slow wood growth and deaths.

Water restriction may have exacerbated urban heat islands as outdoor areas are irrigated less often, losing evaporative cooling. Fortunately, there are opportunities for diverting stormwater and treated wastewater to urban aquifers that provide a non-potable source of water for self-supply.

Government regulations and planning that have been set during the pre-1975 climate are struggling to keep pace with changes in understanding and future predictions. Restrictions tackling old problems are not being replaced with those needed for new issues. It is difficult to allocate water on a fixed volumetric basis when runoff and recharge are highly impacted. Society is also having to accept water reuse more quickly than is ideal.   

Lessons learned in SW Australia may be applicable to other Mediterranean climate zones.