Study finds that drying of soil due to rise in temperatures is not letting much water reach rivers and reservoirs
A global analysis of rainfall and rivers has found that despite intense rain due to climate change, water supplies across the world are decreasing. The study, conducted by a team from University of New South Wales (UNSW), led by Professor Ashish Sharma, used data from 43,000 rainfall stations and 5,300 river monitoring sites across 160 countries.
“This is something that has been missed. 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, the large rivers are drying out,” says Sharma in a press note released by UNSW.
“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. Less water into our rivers means less water for cities and farms. And drier soils means farmers need more water to grow the same crops. Worse, this pattern is repeated all over the world, assuming serious proportions in places that were already dry,” he adds.
The note explains that of every 100 raindrops that fall on land, only 36 drops are ‘blue water’—water that enters lakes, rivers and aquifers, used by humans. The remaining two third of rainfall gets retained as soil moisture, known as ‘green water’.
With rising temperatures arising out of climate change, more water is evaporating from soils, in turn making the soil absorb more rainfall. “It’s a double whammy. 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,” says Sharma.
The researchers found that despite rising precipitation extremes, there’s no evidence of an increase in flooding. This, they say, can be explained by the decline in the amount of moisture in the soil, coupled with the contraction in the geographical spread of each storm event.
“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. Small floods are very important for water supply, because they refill dams and form the basis of our water supply,” says 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.”
Further, Sharma says “extreme floods due to the larger storms are often too large to be stored for water supply. It is the less extreme floods our reservoirs depend on.”
The research is based on four papers, published across Nature Geoscience, Geophysical Research Letters, Scientific Reports and Water Resources Research.
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