Peru’s drying Rimac River. Photo by Jordan Kraft.
While a polar vortex provides much of the northern hemisphere with a global warming paradox to mull over, another climate paradox plays out across the globe: despite intensifying rainfall, which is attributed to global warming, freshwater supplies are shrinking. Professor Ashish Sharma and colleagues at Australia’s University of New South Wales (UNSW) now think that they have the answer.
Higher temperatures cause more water to evaporate, which dries the soil out and and makes the surrounding air more humid. When the water then falls back to the ground as rain, the dry earth soaks it up and prevents it from flowing into rivers as floods. These floods are needed to sustain many sources of fresh water, including reservoirs.
Dr. Sharma and his colleagues estimate that for every 100 raindrops that fall on land, only 36 drops are ‘blue water’, or rainfall that enters lakes, rivers and aquifers and can be used for human needs. The remaining two thirds of rainfall is mostly retained as soil moisture – known as ‘green water’ – and used by the landscape and the ecosystem. This figure is an average, though. Dr. Sharma explains that the amount of water retained as blue water is much higher in wetter regions as compared to drier parts of the world. What is concerning though, Sharma says, is that if one takes 100 extreme storm events, 62% of them will translate into floods if they fall on wet soil, while only 13% will become floods if the soil is dry. The fact that rising temperatures imply soils are drying, means fewer flood events will occur in the future, severely limiting the impact it has on reservoir inflows and the environment.
The findings are counterintuitive to past climate change research. “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.”
To Sharma, the answer to this phenomenon goes beyond just more dams. Given the scale and geographical complexity of this problem, each community, be it a city, region or country, will have to tailor its solutions to its own needs and environment. Because heavy rainfall gets trapped in dry soil, but is not absorbed by pavement, the various solutions will especially differ by urban versus rural setting.
Possible rural measures include improved irrigation infrastructure and incentives to change land use. Among the technological tools at city planners’ disposal is the “sponge city” concept, which includes measures to help cities adapt to extreme rainfall events. All settings will likely benefit from subsidies or other measures to reduce water consumption.
“There are no silver bullets,” says Sharma. “Any large-scale re-engineering project will require significant investment, but the cost of inaction could be monstrous.”
Fortunately, large-scale engineering projects of the kind needed to grapple with this problem are not without precedent. Tokyo, for instance, used to flood frequently until the city built a massive underground tank beneath the city that stores floodwater and releases it later. This has made Tokyo’s devastating floods a thing of the past.
Beyond cities, China’s North-South Water Transfer Project aims to transport water from the Yangtze River in southern China to the Yellow River Basin in the arid north.
To be sure, these are not simple solutions to enact. But as Sharma puts it, “Any large-scale re-engineering project will require significant investment, but the cost of inaction could be monstrous.”