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solonavi 24 June 2007 Campaign, General, Habitat 143 views No Comment Print This Post  Email This Post

Discovery Channel

Two years before its completion, the world’s largest dam is already changing the local weather, say scientists studying the Three Gorges Dam on China’s Yangtze River. Both modeling and actual meteorological data suggest that the reservoir is cooling its valley, which is causing changes in rainfall.

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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.

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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.

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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.

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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.

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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.

Changed Landscape
At left is a 1987 bird’s eye view of the Three Gorges Dam region as seen with Landsat-5 satellite. At right is a 2006 bird’s eye view of the Three Gorges Dam region as progress on the dam is well underway.