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Spring 1999

Floods on the Yangtze: Another way of forecasting

An NCAR scientist is traveling to China this spring to forecast floods that may affect the gargantuan Three Gorges Dam, using techniques that were originally applied to such questions as future regional climate change.

Robert Wilby.

Hydrologist Robert Wilby is a project scientist in A Consortium for the Application of Climate Impact Assessments (ACACIA, housed in the Climate and Global Dynamics Division) working on downscaling. This is a relatively new technique in which data from comparatively coarse-resolution global climate models are adapted for use in models of climate impacts, which usually have a much finer scale. In this capacity, he's a convener for a session at the spring meeting of the American Geophysical Union that will cover, among other things, linking the downscaling technique directly to hydrological models. Wilby explains, "There's a number of people just beginning to experiment with these ideas."

Wilby himself is using these methods along with the U.S. Geological Survey's precipitation runoff modeling system (PMRS), a statistical model, to simulate past, present, and future river flow in basins such as the Cle Elum River in Washington state, the East Fork of the Carson in California, and the Animas in Colorado. He calibrates the model on high-quality observational data and then tests the model using river flows for earlier periods. "The preliminary results have been very encouraging," he says. "We're eager to do side-by-side comparisons with dynamical models to see how we compare with the more sophisticated models." Because the PRMS is less computer-intensive than dynamical models, it also allows Wilby to produce an ensemble of simulations. These offer clues about the range of natural variability, "which is just what the hydrologists want."

Wilby's work in China is the result of some earlier research, although it will be partially supported by ACACIA. In early 1998, Christian Dawson (University of Derby, England) and Wilby published a paper in Hydrological Sciences Journal on using an artificial neural network to forecast streamflow in U.K. river basins. Like the human brain, an artificial neural network is excellent at recognizing patterns. "It doesn't assume a priori how A and B are related, it just looks for the best way of relating them."

Chen Yangbo, director of the Institute of Water Resources Technology at the University of Hydraulic and Electrical Engineering in Yichang, China, read the paper and contacted Wilby. "He asked us to have a go at doing the same thing on the Yangtze," Wilby says. Other groups will also be developing flood models using different techniques. Managers of the Three Gorges Dam will use the results to plan how best to regulate the reservoir capacity that should be allotted to floodwater.

During their stay, Wilby and Dawson will deliver a course of lectures at the university on their forecasting technique. There they'll collect all the necessary data (precipitation, temperature, river flow) to make the Yangtze simulations. Then "we get to fly out to the Three Gorges site to meet the managers of the project." He calls it "the Holy Grail of hydrology to go and see this site."

The Yangtze, 6,300 kilometers (3,900 miles) long, drains an area the size of Mexico (1.8 million km2). The population of its basin is about the same as that of the United States, 300 million. Its runoff is 1012 cubic meters. The dam that will control this volume of water dwarfs all other dams that have ever been built. It will have a span of 2.3 km, or almost a mile and a half (the Hoover Dam's span is only 1,244 feet, or about 380 m) and a height of 185 m (600 ft). The construction has excavated a good-sized mountain of 102.6 million m3 of earth, and the finished dam will contain enough concrete to build a small city, 27 million m3. It's scheduled to be fully operational in 2009.

"It's an extremely controversial scheme, and Chris and I did a lot of soul searching before we agreed to go there," says Wilby. He points out that the construction of this dam "is as much a philosophical choice as it is a technical issue. The whole business depends on how you see man and nature." The Chinese think of the Three Gorges Dam as a way to generate huge amounts of electricity and to control nature's more damaging caprices--to "tame" a river. "In North America and Europe, the story's come full circle. Hydrologists are looking for a way to work with nature." He mentions as an example the use of zoning regulations in floodplains to maintain the integrity of river ecosystems by allowing flooding without adversely affecting people.

Besides its ecological impacts, the dam has a human cost. More than 1 million people are being resettled, most of them farmers who can't simply begin again in another location. China suffers from such a scarcity of high-quality farmland that the law allows some offenses against land-use codes to be punishable by death. "In my mind," says Wilby, "you've got to weigh up this huge cost."

What convinced Wilby and Dawson to participate is the Yangtze's huge death toll from flooding. In one season alone (1931), 145,000 people were killed. Last year, floods in the basin took as many lives as did Hurricane Mitch. And additional human suffering results from losses of farmland and food production after the flooding. "There's clearly a massive humanitarian issue here," Wilby says.

Wilby expects to have some Yangtze results by September, when Chen will visit him and Dawson. He plans to apply some of his downscaling experience to the project. "The methods have all emerged through the work I've been doing at ACACIA."


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