Water Journal : Water Journal July 2012
my point of view of the country including Melbourne, Perth and the Victorian part of the Murray-Darling Basin. This persistent autumn dryness is associated with intensification of the atmospheric high-pressure cells over southern Australia. These cells, associated with clear skies and dry weather, have been intensifying since 1900, with strong intensification since 1979, correlated with increasing global mean temperatures since 1900, and consistent with a predicted expansion of the subtropical dry zone under global warming. Climate Change Signal The millennium drought partly reflects the natural variability of Australia's climate, and was more intense than the projections for reductions in mean annual rainfall under global warming -- but there does appear to be a persistent climate change signal beneath this variability. The floods and recent wet summers were also a feature of climate variability, but the atmosphere's water- holding capacity increases with global mean temperature so, arguably, there is also a signal of climate change in those floods. In the next few decades, changes in mean annual runoff will be of less concern than the possibility that the severity of droughts and floods could increase. Mean annual runoff is projected to change by up to 30%, but the variability of runoff from one year to another is as high as 300% around the mean, so it is still floods and droughts that will dominate how we experience and plan for the impacts of climate on water. It is pertinent then to plan for a range of circumstances in future, including circumstances outside of historical precedence. Some of these scenarios may have a low probability of occurrence, but their consequences could be high and require advanced planning. Formal risk management approaches provide a framework to deal with such circumstances. It would be of great benefit to better forecast future drought and flood risk. Lead indicators of seasonal weather combined with recent runoff are good predictors of the upcoming season's runoff, and such forecasts are now made routinely by the Bureau of Meteorology. In modern cities with diverse water supplies, seasonal forecasts can be used to choose the mix of supplies to manage risks such as floods or diminishing storages. Groundwater, for example, provides a much larger store of water than our dams, but is renewed more slowly. Groundwater can thus be used as an additional supply in dry times, especially if supplemented by managed aquifer recharge. Desalination plants can be brought online to manage both dwindling storages and a need to leave more air space in storages in times of high flood risk. Of greatest value to water planning would be to forecast water availability several years in advance, to help schedule new sources of water for growing cities. However, seasonal forecasts are based largely on El Niños and La Niñas, and these features tend to break up each autumn with uncertainty over how they will reform, so multi-year predictions are inherently difficult. Despite these challenges, there is much more that is known and predictable about the influence of climate on water than is at first apparent. This knowledge is growing all the time and can be usefully applied to water management.
Water Journal August 2012
Water Journal May 2012