Extreme Weather Makeover: The Dam Effect

06/08/2011 03:56 pm ET | Updated Aug 08, 2011

It's been an unusually rainy spring here in the San Francisco Bay Area. The damp weather has a few California farm-belt politicians mad as wet hens, as some farmers in their districts will get less water than usual this year despite high rainfall. These politicians -- who call the situation a "man-made drought" (without a touch of irony) -- are now trying to dismantle groundbreaking agreements that would restore the beleaguered San Joaquin River with increased flows from huge dams that cut off the natural flow decades ago.

Man-made droughts are nothing new to those who live downstream of big dams. Some half a billion people living on the dry side of dams have suffered from the massive ecological and hydrological changes they cause. They experience both man-made droughts from dams holding back the river's flow (in some cases, to the point of the river disappearing completely from some of its course), and man-made floods caused by poor dam design and management.

The Three Gorges Dam has been much in the news in recent weeks, in part because it has been implicated in worsening the impacts of a historic drought now underway in China. According to the New York Times, water levels in two lakes downstream of the dam -- Dongting in Hunan Province and Poyang in Jiangxi Province -- have fallen dramatically in part because of the storage of water in the reservoir behind the dam.

The Miami Herald reports: "Chinese officials pushed for water levels to hit a height of 574 feet at the dam in 2008 and 2009 -- seeking maximum power generation capacity -- and finally succeeded last October. That campaign came at the cost of the downstream water supply."

In a measure of how desperate they are, China -- which has the world's biggest weather-modification program -- is running out of cloud-seeding shells after a massive effort to ease the drought in the Yangtze delta.

Three Gorges is only the latest dam to be associated with worsening drought. In Venezuela last year, the Guri Dam -- the world's third biggest dam -- experienced a severe drought, leading to blackouts across the country. The Guri Dam supplies half of the nation's electricity.

Dams don't just worsen droughts; it turns out that large reservoirs can also be rainmakers. A new scientific study concluded that artificial reservoirs can modify rainfall patterns in ways that natural lakes do not. Their findings show that dams can increase the intensity of extreme rainstorms in their immediate vicinity.

As Wired magazine notes, "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."

The researchers, which included scientists from a number of U.S. universities and research labs, note that the problem is worse in hot, dry climates. Basically, the dam's surface area allows water to evaporate more easily than it would from a river. The problem is worsened if the water is spread around by large-scale irrigation. Some areas show as much as a 20 percent increase in extreme precipitation events after dams were built.

Concerns about dam safety from rainmaking dams have not yet reached the engineers who design them. The new study notes: "Indeed, dam design protocol in civil engineering continues to assume unchanging [patterns of] extreme precipitation events." My organization has for years been warning of the same problem regarding climate change: large dams are being designed based on past hydrological patterns, with little thought to how a changing climate will affect dam viability or safety. Such practices create risks for future generations that we can't begin to imagine.

Big dams are, in effect, experimental technology. Those who design, build and operate them can't predict with any certainty the many serious impacts they will have on life-giving river systems. With a changing climate, that learning curve is about to get a whole lot steeper.