June 14, 2007
NASA
SATELLITES WATCH AS CHINA
CONSTRUCTS GIANT DAM
Some call it the eighth wonder of world. Others say it's the
next Great Wall of China. Upon completion in 2009, the Three Gorges Dam
along China's Yangtze River will be the world's largest hydroelectric
power generator. NASA's Landsat satellites have provided detailed,
vivid views of the dam since construction began in 1994.
The
Yangtze River is the third
largest river in the world, stretching more than 3,900 miles across China before reaching
its mouth near Shanghai.
Historically,
the river has been prone to massive flooding, overflowing its banks
about once
every ten years. The
dam is designed
to greatly improve flood control on the river and protect the 15
million people
and 3.7 million acres of farmland in the lower Yangtze flood plains.
Observations from the NASA-built
Landsat satellites provide
an overview of the dam's construction. The first image shows the region
prior
to start of the project. By 2000, construction along each riverbank was
underway, but sediment-filled water still flowed through a narrow
channel near
the river’s south bank. The 2004 images show limited
development of the main
wall and the partial filling
of the reservoir,
including numerous side canyons. By mid-2006, construction
of the main
wall was completed and a reservoir more than 2 miles (3 kilometers)
across had
filled just upstream of the dam.
Engineered to
store more than 5 trillion gallons of water,
the Three Gorges Dam is
designed to produce more than
18,000 megawatts of electricity when all 26 turbines become operational
in 2009—twenty
times the power of Hoover Dam. The reservoir will also
allow 10,000-ton
freighters to enter the nation's interior, opening a
region burgeoning with agricultural and manufactured products,
increasing
commercial shipping access to China's
cities.
While Landsat is a premier
research tool for observing changes on the Earth's surface, other NASA
satellites are also helpful in determining how changing land cover and
use may
influence climate and the environment. Just as transforming forested
lands into
cities can change the local climate, scientists have found evidence
that Three
Gorges Dam and its enormous reservoir might have a similar effect.
In a recent
study, researchers used computer models and data
from NASA's Tropical Rainfall Measuring Mission satellite to estimate
how the
dam's construction impacted area rainfall. Information from NASA's
Terra and
Aqua satellites also revealed the dam's effect on land surface
temperatures.
"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 lead author Liguang Wu of NASA Goddard Space Flight
Center, Greenbelt, Md.,
and the University
of Maryland
- Baltimore
County.
The land changes also
reduced rainfall in the region immediately surrounding Three Gorges Dam
after
the dam's water level abruptly rose in June 2003.
The
researchers were surprised to see that the dam affected
rainfall over such a large area - a 62-square-mile region - rather than
just 6
miles projected in previous studies.
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.
When the dam
becomes fully operational in 2009 and the
reservoir reaches its peak size, scientists predict these regional
temperature
and precipitation changes may increase even more. The
2006 study was published in the American Geophysical Union's Geophysical Research Letters.
As
the world's landscape continues to evolve, NASA satellites will be
there to
monitor and better understand its effects. The next in the series of
Landsat
satellites -- the Landsat Data Continuity Mission -- is planned to
launch in
2011. Landsat satellites have orbited the Earth since 1972, collecting
valuable
information on Earth's surface to support global change research and
applications in agriculture, business, science and government. These data constitute the longest
record of Earth's
continental surfaces as seen from space.
More information and images:
http://www.nasa.gov/centers/goddard/news/topstory/2007/dam_construct.html
http://www.nasa.gov/vision/earth/lookingatearth/dam_construct.html
Landsat:
http://landsat.gsfc.nasa.gov/
Writer:
Mike Bettwy, NASA Goddard Space Flight Center
##
Contact:
Lynn Chandler
NASA Goddard
Space Flight Center
301-286-2806
This text is
derived from:
http://www.nasa.gov/centers/goddard/news/topstory/2007/dam_construct.html
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