Global conversation needed
Professor Mark Hoffman, UNSW’s Dean of Engineering, welcomed Sharma’s research and called for a global conversation about how to deal with this unfolding scenario, especially in Australia, which is already the driest inhabited continent (apart from Antarctica).
“It’s clear there’s no simple fix, so we need to start preparing for this,” he said. “Climate change keeps delivering us unpleasant surprises. Nevertheless, as engineers, our role is to identify the problem and develop solutions. Knowing the problem is often half the battle, and this study has definitely identified some major ones.”
In a paper published in November in the American Geophysical Union’s Water Resources Research, Sharma and colleagues write that despite widespread global evidence of rising precipitation extremes, there’s no evidence of an increase in flooding, with evidence pointing more towards decreased flood peaks for the moderate flood events that form the key refill events in water supply reservoirs. “While extreme floods may increase due to the larger storms that are occurring, these floods are often too large to be stored for water supply. It is the less extreme floods our reservoirs depend on,” Sharma said.
“On the whole, flood magnitudes are decreasing,” write Sharma and his co-authors, Dr Conrad Wasko of the University of Melbourne, and Professor Dennis Lettenmaier of the University of California Los Angeles. (Wasko was Sharma’s PhD student at UNSW during most of the research).
Large declines in soil moisture
They suggest that large declines in the amount of moisture in the soil, coupled with the contraction in the geographical spread of each storm event, are the major reasons why increases in extreme rainfall are not resulting in corresponding increases in flooding.
They point to previous US research that shows that, in extreme rainfall events, if surrounding soils are wet before a storm, 62% of the rain leads to flooding that is captured by catchments. But when soils are dry, only 13% of the rain results in flooding.
Lake Hume, on the Upper Murray, New South Wales.
“This is kind of contradicting the increasing flood argument in past IPCC [Intergovernmental Panel on Climate Change] reports, but pointing to possibly a far worse scenario,” said Sharma. “Small floods are very important for water supply, because they refill dams and form the basis of our water supply,” said Sharma.
“But they’re happening less often, because the soils are sucking up the extra rain. Even when a major storm dumps a lot of rain, the soils are so dry they absorb more water than before, and less reaches the rivers and reservoirs.”
Reduced flows into reservoirs
Past research has so far missed this. “Everybody has been obsessed by the flood side of the equation but have ignored the more critical component, which is the embattled water supply that comes from reduced flows into our reservoirs,” he added.
So what is the solution? “One option is to wait for international agreements to take effect, so greenhouse gas concentrations can be reined in – but this will take a long time. The other option is to be proactive, and re-engineer our water systems so we can better adapt and cope.”
To adapt to this new reality, new policies and infrastructure is needed. In areas where water supply is shrinking, water-intensive farming will need to be curtailed or moved elsewhere, while reservoir storage capacities may need to be expanded. In urban areas, where flooding is becoming more common, incentives to create ‘green cities’ and to store or divert flood water will need to be explored.
Re-engineering on a massive scale
“We need to adapt to this emerging reality,” said Sharma. “We’re going to need re-engineering on a massive scale in some places if we are to continue living in them. But it’s possible: places like Arizona and California receive barely 400mm of rain each year, but have engineered their water supply systems to make previously uninhabitable places liveable.
“Or take the Snowy Mountain Scheme: it’s not just about hydroelectricity, it’s also a complex water supply scheme with 225km of tunnels, pipelines and aqueducts.”
Sharma said the answer was not just more dams. “Re-engineering solutions are not simple, they have to be analysed on a region-by-region basis, looking at the costs and the benefits, looking at the change expected into the future, while also studying past projects so mistakes are not repeated. There are no silver bullets. Any large-scale re-engineering project will require significant investment, but the cost of inaction could be monstrous.”
In urban areas, the reverse will be needed: flooding is becoming more common and more intense. Global economic losses from flooding have risen from an average of $500 million a year in the 1980s to around $20 billion annually by 2010; by 2013, this rose to more than $US50 billion. The Intergovernmental Panel on Climate Change expects this to more than double in the next 20 years as extreme storms and rainfall intensify and growing numbers of people move into urban centres.
Adapting to this is possible, but will require large-scale re-engineering of many cities, says Sharma. “Tokyo used to get clobbered by floods every year, but they built a massive underground tank beneath the city that stores the floodwater, and releases it later. You never see floods there now.”
IMAGES, VIDEO AND BACKGROUND AVAILABLE
- PHOTOS: Prof Ashish Sharma and Dr Conrad Wasko at UNSW, and inside the labs at UNSW’s Water Research Centre. Also, aerial views of the Snowy Mountain Scheme and the California State Water Project, plus recent droughts and floods.
- AUDIO: A full recording of the news conference held at the UNSW's City Centre campus in Sydney at 9am on Thursday 13 December 2018. Speakers are Prof Ashish Sharma and Prof Mark Hoffman.
- VIDEO: Prof Ashish Sharma and UNSW Dean Engineering, Prof Mark Hoffman, summarising the research and calling for an international discussion on its implications.
- SCIENTIFIC PAPERS: For the full text of the four papers on which this research is based; in Nature Geoscience, Geophysical Research Letters, Scientific Reports and Water Resources Research.