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Remember the water cycle from junior high? It doesn’t work that way anymore

A new analysis of global water use finds that precipitation is failing to replenish water lost from many of the world’s largest river basins.

Lake Powell on the Colorado River near Page, Arizona
REUTERS/Rick Wilking

A new analysis of global water use, focused on large-scale diversions for agriculture and electric power, finds that precipitation is failing to replenish water lost from many of the world’s largest river basins.

If these deficits are fairly counted as human use, two Swedish scientists argue in a paper published last week in the journal Science, then current estimates underestimate human freshwater consumption by a factor of five – and actual use may already exceed a widely accepted upper limit of sustainability.

Their research focuses on the balance, in 100 of the world’s largest river basins, between water subtracted via evapotranspiration (the combination of evaporation directly to the atmosphere and indirectly through green plants’ transpiration) and returned via precipitation.

And it sort of shatters the simplistic notion of “the water cycle” that many of us have been carrying around from junior high earth science, which holds that these two factors will always be more or less in balance because consumption will always be balanced by a prompt return flow.

Not just drink and tinkle

Much human water use does indeed follow that scheme: you drink a glass of water from the kitchen tap, and a few hours later return it via the bathroom plumbing. Apartment buildings, industrial plants, even whole cities do essentially the same thing on increasingly large scales and in easily measurable amounts.

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Of course, if you drink a glass of water drawn from one river basin and then fly to another before returning it, you’ve shifted the balance very slightly – perhaps just enough to recall Tom Robbins’ amusing observation, in “Another Roadside Attraction,” that “human beings were invented by water as a device for transporting itself from one place to another.”

But what we’ve done with dams, reservoirs and other elements of what the Swedish scientists call “human-controlled flow regulation and irrigation,” or FRI, is to change the patterns of evapotranspiration so significantly that large volumes of water are withdrawn and then transported across distance, time or both so that they never return to their source basin in this “normal” way.

The 100 basins comprising the research sample were selected because good data on precipitation, human consumption and evapotranspiration were available back to the beginning of the last century.  

A century of data

The researchers looked at changes between 1901 and 2008, divided into two 54-year periods, and found that FRI infrastructure changes “have either moderately or strongly affected 59 percent of the world’s largest river systems.”

They include around 45,000 large dams and many other smaller ones, spread over 140 countries around the world and constructed mostly over the past century to store water for irrigation, flood control, urban water supply, hydropower, or a combination of such purposes.

These developments are linked with approximately 12 to 16% of the current global food production and 19% of the world’s electricity supply. …

In general, they write, assessment of environmental impact has focused mostly on “river fragmentation and diversion and water storage,”  although some recent work has considered a shift in the evapotranspiration/precipitation balance at local or regional scales.

At a global scale, however, assessment of balance shifts has been limited to modeling in the absence of “observation-based evidence of the global importance of FRI as a driver of freshwater change.”

Having filled the information gap for the 100-basin sample, they “further quantified the magnitude of the FRI-driven hydro-climatic changes hydroclimatic changes in each basin and assessed their implications for global human consumption of fresh water.”

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In general, the results were an increase in water loss through evapotranspiration relative to water return through precipitation, strongly correlated to the degree of FRI infrastructure present in a particular basin.

U.S. basins strongly affected

They mapped the impacts worldwide, with strongly affected basins coded in red, moderately affected basins in yellow and non-affected basins in blue. Virtually all of the U.S. basins show up in red, with a few pockets of yellow in the eastern seaboard region and one blue spot Mississippi.

This map of 100 of the world’s largest river basins shows strong dam/reservoir impacts on the normal water cycle in 53 of them, representing 50% of the basins’ total area, and moderate impacts in another 30, covering 46 percent of total area. Only 17 basins, comprising 4 percent of total area, showed no effects.

Significantly, I thought, the study concluded that the shift toward greater evapotranspiration losses was not correlated with measures of atmospheric climate change, among other possible explanations.

The researchers also concluded that if the increased losses are counted as consumption – which is only fair, since they amount to human-caused withdrawals that aren’t returned more or less promptly to their source – then most of the figures commonly used for per capita water use are way off base.

A prevailing estimate for per capita use worldwide puts annual consumption at 807 cubic kilometers per year, they write, but adding in the accelerated evapotranspiration losses brings that to 4,370 cubic kilometers, and well above “a proposed freshwater planetary boundary” of 4,000 cubic kilometers for sustainable use. (A cubic kilometer is equivalent to 264.2 billion gallons.)

According to a fine piece in Smithsonian, where I first encountered the study, observations of highly local impacts associated with Swedish dams piqued the interest of Fernando Jaramillo, a physical geologist at the University of Gothenburg, and Georgia Destouni, a hydrologist at the University of Stockholm.

But the work was made more complicated by a dearth of accessible, accurate data. The team collected public data for nearly 3,000 water basins, but found complete data sets on only 100. Still, using that sample they were able to analyze each basin over two periods covering the years 1901 to 2008.

Though the team suspected a strong link between water infrastructure and evapotranspiration, they first had to rule out other possible factors. “You have to differentiate the direct effects of humans,” Jaramillo says, adding that he was skeptical that his team could find that particular footprint among the deafening noise.

“Okay, we have deforestation, we have non-irrigated agriculture, urbanization, melting glaciers, permafrost thawing, climate change,” laughs Jaramillo. But when the team corrected for things like temperature and climate change and looked at evapotranspiration rates over time, they always ended up with the same result.

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Both their results and their research design are gathering praise in earth science circles for rigor and originality. Shannon Sterling, an earth scientist at Dalhousie University who was not involved with the paper, told the Washington Post:

What is really novel and exciting about what Dr. Jaramillo and Destouni did was they took observational data, so measured flow data, on major watersheds, and they were able to detect a signal of a specific human impact. And that’s remarkable.

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The full paper as published in Science can be found here, but if you’re not already a subscriber you’ll have to pay for access.