Water Journal : Water Journal July 2012
6 JULY 2012 water regular features my point of view Ian Prosser helps lead CSIRO's Water for a Healthy Country Flagship Program, which has over 300 research staff working across Australia to improve management of water resources for urban, rural, industrial and environmental uses. Ian has 25 years' research experience in river basin hydrology and water quality, leading national programs on water research. He also recently authored and edited the book, Water: Science and Solutions for Australia (CSIRO Publishing). After the unprecedented water shortages of the millennium drought and the devastating floods of the following two years, one could be forgiven for thinking that climate is a completely chaotic and unpredictable beast. Beneath the apparent chaos, however, are some consistent patterns that can assist water management decisions across southern Australia. Water is one of the most vulnerable sectors to climate variability and change. For every 10% increase or decrease in annual rainfall there is typically in Australia a 30%--40% increase or decrease in runoff. Australia is fundamentally water limited, because in most places potential evaporation exceeds rainfall, resulting in a strong reliance on large stores of water to see us through dry times. Climate can also fundamentally change the hydrology of the landscape. Our good understanding of Australia's climate and hydrology comes from the decades of detailed measurements of the oceans, atmosphere and rivers, as much as from global climate models that are most useful for explaining the observed patterns and for projecting forward. The South East Australia Climate Initiative (SEACI -- a partnership of the Murray-Darling Basin Authority, the Australian and Victorian Governments, CSIRO and the Bureau of Meteorology), along with other programs, has investigated these records and their implications for water management. Causes of the Millennium Drought SEACI found that the millennium drought in south-eastern Australia was largely the result of a persistent reduction in autumn rainfall and high temperatures. This had a greater than expected influence on runoff because it is usually autumn rains that soak catchments. Without these soaking rains in 2010 and 2011, unusually low runoff persisted well into spring -- even though winter and spring rainfall was not as low as autumn rainfall. The millennium drought was broken by the past two years' wet summers. However, summer rainfall and autumn rainfall are driven by quite different processes, which provides some insights into future climates. The rains of the last two summers filled dams, soaked parched soils, refilled wetlands and resulted in devastating floods -- but we cannot expect that in most years. In particular, the rains of the 2010--2011 summer were the result of an unusual coincidence of (1) a very strong La Niña episode in the Pacific Ocean and (2) unusual conditions in the Southern and Indian Oceans, which -- together with overall warm ocean temperatures -- brought strong tropical rainfall into south- eastern Australia. As the rains were tropically driven, Brisbane felt the strongest effects, followed by Sydney, with only moderate drought relief in Melbourne and little relief in Perth. The drought has ended and the pressure on water supplies has passed for now. However, while south-eastern Australia had two of the wettest years on record in 2010 and 2011, autumn rainfall in those years was only slightly above average and the long-term deficit continues in the southernmost parts Water Management Under an Unpredictable Climate Dr Ian Prosser, Science Director -- Water for a Healthy Country Flagship Program, CSIRO PHOTO: A. HOLLINGWORTH ©SCA Heavy rain in the drinking water catchments for Sydney earlier this year caused Warragamba Dam to fill and spill for the first time in over a decade.
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