A study conducted by Professor Ashish Sharma at Australia’s University of New South Wales (UNSW) in Sydney explains that drying soils are at the root of a discrepancy between increased rainfall and shrinking water supplies.

The study, If Precipitation Extremes Are Increasing, Why Aren’t Floods?, relied on data from 43,000 rainfall stations and 5,300 river monitoring sites in 160 countries and found that for every 100 raindrops that fall on land, only thirty-six drops are rainfall that enters lakes, rivers and aquifers, and therefore, all the water extracted for human needs.

“This is something that has been missed,” said Sharma, an ARC Future Fellow at UNSW’s School of Civil and Environmental Engineering. “We expected rainfall to increase, since warmer air stores more moisture—and that is what climate models predicted too. What we did not expect is that, despite all the extra rain everywhere in the world, is that the large rivers are drying out.”

The remaining two thirds of rainfall is mostly retained as soil moisture, or green water, and used by the landscape and surrounding ecosystem.

“We believe the cause is the drying of soils in our catchments. Where once these were moist before a storm event—allowing excess rainfall to run-off into rivers—they are now drier and soak up more of the rain, so less water makes it as flow,” said Sharma.

As warming temperatures cause more water to evaporate from soils, those dry soils are absorbing more of the rainfall when it does occur, leaving less blue water for human use.

“It’s a double whammy,” said Sharma. “Less water is ending up where we can store it for later use. At the same time, more rain is overwhelming drainage infrastructure in towns and cities, leading to more urban flooding.”

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 on the planet, 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.”

“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.

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 U.S. research that shows that, in extreme rainfall events, if surrounding soils are wet before a storm, 62 per cent of the rain leads to flooding that is captured by catchments. But when soils are dry, only 13 per cent of the rain results in flooding.

“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.”

The findings were published November 5th in the journal, Water Resources Research.


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