Posted by Karen Lykins - Thursday, February 10 2011
Researchers investigating how large dams can affect local climates say
dams have the clear potential to drastically alter local rainfall in
A study by researchers at Tennessee Tech University, Purdue University,
the University of Colorado and the University of Georgia, Pacific
Northwest National Laboratory and Hellenic Center for Marine Research
concluded that artificial reservoirs can modify precipitation patterns.
The study—published in Geophysical Research Letters— marks the first
time researchers have documented large dams having a clear, strong
influence on the climate around artificial reservoirs, an influence
markedly different from the climate around natural lakes and wetlands.
The results should spur consideration of more robust management of dams
and set the stage for further research on the regions and climates to
focus on, says Faisal Hossain, Tennessee Tech University civil
"This research shows you the smoking gun," said Hossain. "Logically and
physically we knew it was possible that a having a large body of water
and spreading it around would change the local climate. Now, our results
give us a better idea of which dams are most likely to gradually change
local climate and what that means for managing those reservoirs as time
With Hossain and TTU doctoral student Ahmed Mohamed Degu leading the
study, the research team looked at 30 years of climate data based on a
technique commonly known as reanalysis in the scientific community.
Reanalysis aims to recreate the gold standard record of weather
conditions everywhere in a domain by using as much information in
hindsight as possible. The data used spanned from 1979-2009 and was
collected 24/7 over North America.
Roger Pielke Sr. of the University of Colorado's Cooperative Institute
for Research in Environmental Sciences says the work was a breakthrough
study in scope and mission.
"it is a critically important, much needed study with multiple authors
and institutions using diverse datasets in order to obtain information
on how dams and their surroundings affect the region's climate rather
than a local snapshot that may not be representative for larger areas,"
The study reports that large dams influence local climate most in the
Mediterranean and semi-arid climates such as ones in California and in
the Southwestern United States.
So how does a large dam and its reservoir alter the climate? If the
dam's reservoir is large enough or if the water is spread around by uses
such as extensive irrigation or recreational activities, then the
expanded distribution of water creates an altered climate because it
allows the water to evaporate more easily.
"Think of your typical backyard swimming pool," said Hossain.
"If you pumped all the water out of your swimming pool and spread it
onto your lawn, it wouldn't take long for all that water to evaporate."
A change in water available for evaporation can change humidity,
temperature and other aspects of the climate system around a reservoir.
Under the right circumstances, all of these play an important role in
"We now know we need to do better building and managing dams and
reservoirs in those arid and Mediterranean regions where water is really
scarce," said Hossain.
Hossain says the report reflects a changing mindset in this area of
"We know a lot about how climate change affects reservoirs, but what we
didn't know a lot about was what a reservoir could do to the local
climate," he said. "We just reversed our thinking by saying that a
reservoir and the activities it supports are just as important a player
for climate as the larger climate is for the reservoir. Basically, it's
a two-way street."
Pielke says this framework, known as a vulnerability framework, is more
inclusive and promotes more effective decisions.
"The change in mindset is to identify the vulnerabilities from a
bottom-up, resource-based perspective," said Pielke.
Hossain agrees that this perspective changes the way civil engineers
think in the classroom and on the job.
"Our profession generally has never looked at climate and what we do to
it once we build large structures like dams, even cities, parks, ports,
etc.," said Hossain. "That work is missing at the interface of our
"We now need to adapt, be more climate cognizant and broaden our
horizons. Many of our dams in the U.S. are 50 years old and we need
answers for the future," he said.
"Now we have a better idea about how the local climate and rainfall may
change than we did 50 years ago, although more work is needed to
pinpoint exact causes at each dam location," said Hossain. Nevertheless,
we now can consider different scenarios and do a life cycle assessment
before even building a dam.
"This is like saying we can now forecast what a dam may do to itself as
it ages before even building it; then we build it according to a
specification that the profession is prepared for," he concluded.
The work was mainly supported by TTU's Office of Research and the Center
for the Management, Utilization and Protection of Water Resources.
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