Wednesday, June 8, 2011

How Dams Can Bring About Rainfalls and Drought

[Note: The following text summarizes how dams - including the Three
Gorges Project - can influence rainfalls and droughts. Links to all
background documents are provided from the webpage. For a more
comprehensive update on dams and climate change see our post on today's
Huffington Post at]

How Dams Can Bring About Rainfalls and Drought
By Peter Bosshard, International Rivers
June 8, 2011

It is undisputed that dams can influence local rainfalls. Humidity
evaporates from reservoirs and irrigated fields and gets recycled as
rainfall. Evaporation from reservoirs can also cause more frequent
storms. On the other hand, dams and levees can reduce evaporation and
rainfalls when they drain wetlands and open up woodlands for deforestation.

The Niger Delta in West Africa illustrates how dams can influence
rainfalls. In September, the delta's wetlands extend to an area of
30,000 square kilometers – roughly the size of Belgium – and feed
rainfalls over a much larger region. Yet upstream dams on the Niger have
reduced the flows into the delta by 10-15%, and a major proposed
hydropower project upstream on the river would reduce inflows by a
further 33%. "Such a change would significantly reduce the window in the
seasonal cycle when the wetland can influence rainfall," warns
Christopher Taylor of the Centre for Ecology and Hydrology in Great Britain.

What does this mean for the Three Gorges Dam? A group of researchers in
the US and in China analyzed regional rainfall data before and after the
completion of the dam on the Yangtze. They found that precipitation
decreased somewhat south of the reservoir, and increased significantly
about 100 kilometers north of the reservoir.

Yet the rainfalls around the reservoir are only half the story. The dam
has impacts on wetlands throughout the lower Yangtze basin. During the
flood season, the Yangtze used to greatly expand the area of the
Dongting and Poyang lakes, two large flood basins in the Yangtze Valley.
Their combined surface used to expand from about 4,000 to about 24,000
square kilometers every year. Land reclamation for agriculture reduced
the size of the lakes, and by storing flood water for electricity
generation, the Three Gorges Dam is now greatly diminishing the seasonal
expansion of the two flood basins. During this year's drought, the
majestic Dongting Lake – home of the famous Chinese dragon boat races –
turned into a sad mudflat with isolated pools of water.

In late May, the hydropower operators increased the release of water
from the Three Gorges Dam, and claimed that the project was thus
contributing to the drought relief effort. This is disingenuous: without
the dam, much more water would have replenished the flood basins naturally.

The Three Gorges reservoir has inundated an area of 630 square
kilometers, and has thus influenced the rainfall patterns upstream of
the dam. The dam's operation reduces the Yangtze's flood basins by a
much larger area. How this has affected the persistent droughts in the
region has not been measured. Research on how wetlands influence
precipitation in other parts of the world suggests that the impact must
be significant.

No matter how dams are affecting rainfall and droughts, they are
themselves strongly affected by the vagaries of a changing climate. At
the height of this year's drought, more than 1000 reservoirs in Central
China did not have sufficient water to sustain the generation of
hydropower. Even in the Danjiangkou reservoir, the water levels fell
below the minimal height that will be needed to send water to Northern
China. Global warming could thus turn China's giant $62 billion
South-North Water Transfer Project into a white elephant. Diversified
sources of energy and decentralized water storage will make countries'
economies more resilient to climate change than big, lumpy dams.

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