A paradoxical fact has emerged from a global study—water supplies worldwide are shrinking at a time when climate change has made rains more intense. The reason, ascertained by the researchers, is the drying of soils due to global warming, thus pointing towards a situation where drought-like situations will become more prevalent, especially in regions that are already dry.
The study was done on an exhaustive global scale data, and was led by Ashish Sharma at the University of New South Wales, Sydney, Australia. It analysed actual data from 43,000 rainfall stations and 5300 river-monitoring sites in 160 countries.
The increased intensity of rain with global warming is quite an expected phenomenon because warmer air stores more moisture, and this was predicted by various climate models as well. What was unexpected is drying up of large rivers despite the extra rains.
As an explanation of this paradox, lead scientist Ashish Sharma said, “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.”
He added, “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. It is extremely concerning.”
Blue Water and Green Water
Blue water is the portion of the rainfall that flows into lakes, rivers and aquifers, and therefore is the portion that is extracted for human needs. The blue water is only about one third of the total rain water; out of every 100 raindrops falling on land only 36 drops are blue water. The remaining two thirds of rainfall is the green water, retained as soil moisture used by landscape and the ecosystem.
The warmer the globe becomes, more water evaporate from the soils, leaving it drier. Those dry soils now absorb more of the rainfall, and in the process, lesser blue water is left for human utility.
The situation is very worrisome. Less water is making their way to the places where it is stored for later use, and at the same time overwhelming rain is wrecking drainage infrastructure in cities and towns, causing more urban flash floods.
The study’s conclusion is based on research done in the past four years that have been published in Nature Geoscience, Geophysical Research Letters, and Scientific Reports.
Declines in Soil Moisture
The researchers suggest that large declines in the moisture level of the soil and the contraction of the geographical spread of each storm event are the main reasons why increase in rainfall is not resulting in increases of water supply.
Previous researches show that in extreme rainfall events, if the surrounding soil is wet, then most of the rainfall (almost 62 per cent) makes its way to catchments. But when soils are dry, only a meager amount (13 per cent) of the rain can flood the catchments.
Sharma and colleagues also wrote that despite the more intense rainfalls, evidence shows that the level of moderate floods is decreasing globally. “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 argued.