Water Week

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Rainfall figures going back 100 years in Sydney’s catchment area show an alarming long-term pattern; extreme events determine water levels

Posted by waterweek on 27 September 2007

 Rainfall figures going back 100 years in Sydney’s catchment area show an alarming long-term pattern, reported The Sydney Morning Herald (22/9/2007, p. 25). Before 1992, in a key part of the Warragamba catchment, Moss Vale, there was one of these events on average every 15 months. From 1992 till June this year, there was just one such event in 15 years – the flood of 1998. Apart from that year, Trewin says, there has been almost a complete lack of the those extreme events. June was the first time in seven years when a downpour in Moss Vale cracked the 200-millimetre mark.

Long-term average rainfall is irrelevant for Sydney water storages: While dam levels shot up to 60 per cent after the big June rains this year, Bureau of Meteorology experts warn against being lulled into a false sense of security. “Long-term average rainfall is almost irrel­evant for Sydney water storages [the dams],” explains bureau climatologist Blair Trewin.

“Extreme events” determine catchment levels: “What really matters is that they depend on a relatively small number of ‘extreme’ events” – in other words, the big downpours of more than 200 millimetres like those that fell in June.

The Sydney Morning Herald, 22/9/2007, p. 25

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