Can astronauts really see the Great Wall of China from space? It's not really sure. But they'll be able to see the gigantic Three Gorges Dam reservoir along China's Yangtze River when it's completed in 2009, as NASA satellites already show. NASA has watched the dam since the beginning of its construction in 1994. And even if this dam is supposed to protect the 15 million people who lived in the lower Yangtze flood plains, it's still highly controversial, and for two major reasons: one million people will have to be relocated; and the huge dam will affect the climate area because of rain increase which has already be spotted by NASA.
Let's start with some images. You can see above the Three Gorges Dam area in 1987 showing the Yangtze River prior to the construction of the dam (left) and the same area in 2006 revealing the massive rise in water level (Credit: NASA). For more images and animations, you should visit this NASA gallery.
Here are some numbers about "the next Great Wall of China."
- The construction cost will be at least $625 billion;
- The dam will be about 2.3 kilometers long and almost 190 meters tall;
- It will produce more than 18,000 megawatts of electricity;
- It will increase commercial shipping access to China's cities;
- But almost 400 square kilometers of land will be submerged -- including the three gorges that give the dam its name;
- And the construction may seriously impact area rainfall.
- [Note: for additional numbers and information, please read the the Wikipedia entry about the Three Gorges Dam.]
Now, let's look at what NASA's satellites have found since the beginning of the dam's construction. "The satellite data and computer modeling clearly indicate that the land use change associated with the dam's construction has increased precipitation in the region between the Daba and Qinling mountains," said Liguang Wu of NASA Goddard Space Flight Center, who studied the dam, and who found that its construction also impacted land temperature.
Land surface temperature changes were also found to occur in the area where more rain fell. In the daytime, temperatures between the Daba and the Qinling mountains decreased by an average of 1.2 degrees Fahrenheit (0.67 degrees Celsius). Where there was more rainfall, there were more clouds, which reduced the amount of sunlight and heat that reached the land surface, creating cooler daytime temperatures.
The study suggests that the cause of these temperature changes was the expansion of the width of the Yangtze River and the formation of the dam's reservoir. After construction, a 401-square-mile reservoir formed in the mountainous area. Before the dam, the Yangtze River was only one-third of a mile in width. The larger mass of water created a "lake effect," causing cooler temperatures and increased rainfall between the Daba and Qinling mountains, but less rainfall in the immediate vicinity of the reservoir.
This research work about how the dam's construction has impacted rainfall in the area has been published last year in the American Geophysical Union's Geophysical Research Letters under the name "Three Gorges Dam affects regional precipitation" (Volume 33, Number 13, Article L13806, July 7, 2006). Here are two links to the abstract and to the full paper (PDF format, 4 pages).
We now have to wait until 2009, when the dam becomes fully operational and the reservoir reaches its peak size, to discover if precipitation changes increase even more.
Sources: NASA Goddard Space Flight Center, via EurekAlert!, June 12, 2007; and various websites
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