As if America's aging dams were not in enough trouble already, new research suggests that their reservoirs could be increasing the intensity of extreme rainstorms in their immediate vicinities.

That's a problem because the dams were designed for the climate that existed in the area before they were built. If by virtue of their creation, they increase the chance that an extreme weather event will exceed the dams' capacity, they could be less safe than previously thought.

"What if the dam itself, its reservoir, could have accelerated or intensified the heavy rainfall patterns?" said Faisal Hossain, a hydrologist at Tennessee Tech University, who has co-authored a paper and editorial on the topic accepted for publication in Natural Hazards Review and Water Resources Research, respectively.

There is strong evidence that a standing body of water, like a lake, can alter precipitation patterns, Hossain said. Increasing the amount of liquid water in a region increases the amount of evaporation in a region, too. That water vapor will eventually condense and fall as precipitation. So, it's logical to think that a dam's reservoir could have the same impact. And dams allow irrigation, which can transform the land in the area, possibly leading to local climactic impacts.

Marshall Shepherd, a research meteorologist at the University of Georgia, called the findings "interesting and plausible" in an e-mail to Wired.com.

"The literature contains many examples of how extreme land use changes alter precipitation patterns," wrote Shepherd, whose own work focuses on climactic changes induced by cities.

Shepherd would like to see more detailed analysis of the mechanics behind how a dam could change local precipitation.

That kind of work could help explain why the changes Hossain and his collaborators have observed vary so much between sites. Some areas like southern Africa and Europe show as much as a 20 percent increase in extreme precipitation events after dams were built, but other areas, particularly in the United States, show just a percent or two increase.

Still, even a relatively small increase in the amount of precipitation could become a problem. As dams age, they lose some of their storage capacity as silt builds up along the reservoir bed, cutting the volume of water that physically fits in the reservoir. Hossain pointed to Lake Mead as an example, which he says has lost about 30 percent of its storage capacity.

Reservoirs with less room for precipitation are more susceptible to overflowing. One solution to the problem could be to decrease the amount of water in the reservoir to provide more space for rainwater.

"You can always lower the level of a reservoir to handle when that heavy cloudburst or flood happens," said Hossain.

But that could negatively impact dams' hydroelectric and irrigation capacity and may not be popular with the dam's users. Before such steps are taken, however, scientists will have to determine how much of problem the dam's alteration of local climate could be, Hossain admits.

Image: NASA/Lake Powell

See Also:

WiSci 2.0: Alexis Madrigal's Twitter, Google Reader feed, and green tech history research site; Wired Science on Twitter and Facebook.**