Africa Water, Sanitation & Hygiene January - February 2016 vol.11 no.1 | Page 20

Water Resources

Humans Are Draining Even More of Earth’s Freshwater
Than We Thought
Ironically, building dams and irrigation systems may end
up driving food and water shortages
By Erin Blakemore
Humans have
been trying to
wrangle Earth’s
freshwater since
the dawn of
civilization. Case
in point: the
3,000-year-old
Sadd Al-Kafra
Water gushes out of Aswan Dam in Egypt. (Jack embankment
Fields/CORBIS) dam in Egypt.
Things like
dams and irrigation obviously affect local waterways, but
it’s much harder to figure out how those local changes
influence freshwater supplies worldwide.
Now, an analysis of water basins shows that the global
impact of humans’ water consumption is much larger than
initially thought. That’s because local attempts to divert
and control water actually increase global consumption by
ramping up evapotranspiration, or the process by which
water cycles from the land into the atmosphere.
Physical geologist Fernando Jaramillo of the University
of Gothenburg and hydrologist Georgia Destouni of
the University of Stockholm embarked on the study
after linking local dams in Sweden to surprising spikes in
regional evapotranspiration. To take the research global,
they decided to abandon complex modeling in favor of a
formula inspired by the water basins themselves.
“We knew we could use the data in a simpler way,”
Destouni says. At the most basic level, a hydrological basin
is a closed system, she explains—precipitation comes in,
and runoff and water withdraws go out. Any difference
between the input and the output must leave the basin via
evapotranspiration.
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
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Africa Water, Sanitation & Hygiene • January - February 2016

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.
“These were landscape drivers—things changing the
landscape itself,” says Jaramillo. “The signal was just so
clean and clear.”
Based on their analysis, published recently in Science,
they calculate that reservoirs, dams and irrigation are
responsible for increasing evapotranspiration so much that
humans’ overall water consumption is 18 percent higher
than the most recent estimate.
In effect, we are using an average of 4,370 cubic
kilometers of water every year, if you’re counting—and
perhaps you should be. In 2011, environmental scientist
Johan Rockström defined 4,000 cubic kilometers of annual
global freshwater use as a critical planetary boundary that,
if crossed, could spell irreversible environmental changes.
Experts suggest that when the world tips into a full-scale
freshwater crisis, it will spur food shortages and civil
unrest. Stud Y\