Chinas Three Gorges Dam changing the weather

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Chinas Three Gorges Dam changing the weather

Its going to do a lot more than that, from other books I have been reading, millions of people downstream will be affected as the water flow to the Mekong and other rivers decrease

Two years before its completion, the world’s largest dam is already changing the local weather. Both modeling and actual meteorological data suggest that the reservoir is cooling its valley, which is causing changes in rainfall.

"In China there are a lot of people who complain because of the construction of the dam" and specifically about changes in local weather, said climate modeler Liguang Wu of NASA’s Goddard Space Flight Center and the University of Maryland in College Park.

To find out if the dam was really to blame, Wu and his colleagues collaborated with Chinese scientists to study the changing climate around what will soon be a 401-square-mile reservoir of more than 5 trillion gallons of water and a hydroelectric power plant 20 times more powerful than the Hoover Dam.

The researchers combined satellite data and ground weather stations to create a computer climate simulation, which they then compared to what has already happened in recent years.

The construction of the dam and changes to the land and vegetation around it have been recorded for years by the NASA-US Geological Survey Landsat satellites. They show steady progress from 2000 to today, with the biggest changes in 2004, when the reservoir was partially filled and water backed up into many side canyons.

By last summer the main wall of the dam was done and the water in the reservoir was two miles across.

"Frequently people tend to use these (Landsat images) in a time series," said Jeff Masek a Landsat scientist at NASA. Because Landsat satellites have been operating since 1972, there are a lot of human changes to be seen, he said.

What’s more, since the data is available to the world, many countries, like China, can use them to get a different view of what’s happening on their own land.

More recently, other NASA satellites have been watching the weather changes, said Wu. The Tropical Rainfall Measuring Mission (TRMM) provided some data to estimate changes in rainfall, while the Terra and Aqua satellites kept track of surface temperatures.

Among the surprise weather changes has been the increase in rainfall between the Daba and Oinling mountains, said Wu.

The rains come from a "lake effect" intensification of precipitation, like that seen around the Great Lakes of North America. The lake effect happens when already moist air picks up more moisture as it crosses over a warm body of water, then rains or snows it out quickly upon reaching the shore.

"It’s not totally bad news," Wu said of the added rain. Some people want the added moisture. On the other hand, as the water was rising in 2003, some areas around the dam saw less rainfall, Wu said.

In all, a whopping 62 square miles of land are expected to see weather effects from the dam, he said. That’s more than ten times the area originally predicted, he said.

In a way, he said, Three Gorges is a great laboratory for studying how well local climate changes caused by very local land-use changes can be detected and distinguished from larger-scale global climate change.

"To me it’s a scientific issue," Wu told Discovery News. Wu and his colleagues published their work in a recent issue of Geophysical Research Letters.

At almost 4,000 miles, the Yangtze is the fourth longest river in the world, discharging into the sea about twice the water of the Mississippi. For as long as people have kept records, the Yangtze has been in the habit of periodically overflowing its banks and flooding vast areas. Controlling that ancient threat, along with producing electricity, are the main goals of the Three Gorges Dam.

NASA animation of the Three Gorges Dam
By netchicken: posted on 30-6-2007

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