How Dams Can Bring About Rainfalls and Drought

Wednesday, 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.