IT is generally believed that there has been a decline in rainfall across Australia and that as a consequence cities like Melbourne must suffer severe water restrictions. Indeed if you live in Melbourne you must get prior written approval to fill a swimming pool, there are strict rules explaining how and when you can water your garden, and it is illegal to wash to your car with a garden hose.
In Melbourne reducing water demand and ensuring the efficient use of water is now government policy and the public is continually reminded of this imperative.
Melbourne’s broadsheet, The Age, recently published an opinion piece entitled ‘Our hot, dry future’ by David Jones, head of climate analysis at the Bureau of Meteorology. The piece reinforced the popular belief that there has been a long term decline in rainfall as a consequence of climate change. Dr Jones wrote:
“We also know that over the past 11 years Melbourne’s rainfall has been about 20% below the long-term average, and that south-east Australia as a whole has now missed out on more than a year’s worth of its normal rainfall over the duration of the event. The run-off into Melbourne’s dams has been 40% below average over this drought period compared with the longer term, while regional areas have fared even worse. And the drought hasn’t ended.”
Total rainfall for the major water-harvesting catchments feeding Melbourne is archived on a weekly basis at the Melbourne water website as well as total dam storage levels back to September and August 1998, respectively. My assistant at the Institute of Public Affairs, Nichole Hoskin, asked the Water Commission if we could have this information in an excel format for ease of manipulation, but a Mark Kartasumitra, explained we would have to make-do with what was at the website. So Nichole extracted the individual weekly values for rainfall and water storage from their archives and entered these values into a spread sheet and then plotted a chart for rainfall, shown below, and also a chart for water storage.
There has been a steady decline in the amount of water in Melbourne’s dams since 1998, but the chart of total catchment rainfall shows no such decline. Indeed rainfall over the last decade appears to have been fairly steady.
When Dr Jones writes that rainfall has been 20% below the long-term average I wonder what time frame he uses by way of comparison? When Dr Jones writes that runoff has been 40% below average it is interesting to again ponder time frames and also what changes in land management in the catchment may have contributed to the reduction. Indeed the available data suggests that dam levels have fallen significantly even though there has been reasonable rain.
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Part 1 of ‘How Melbourne’s Climate Has Changed: A reply to Dr David Jones’ was published on October 14th, 2008, and can be read here.
Jennifer Marohasy BSc PhD has worked in industry and government. She is currently researching a novel technique for long-range weather forecasting funded by the B. Macfie Family Foundation.