Jim Giles, New Scientist 3 Jun 09;
Dams are supposed to help farmers manage water supply and boost crop yields, but in Africa they may actually have cut agricultural production, researchers warn.
Africa has over 1000 large dams, the majority of which are used for irrigation, say Eric Strobl of the École Polytechnique in Paris and his brother, Robert Strobl of the European Commission in Brussels.
The pair used satellite images to compare crops around the dams with those in nearby areas. Between 1981 and 2000, the dams caused an average 1 per cent drop in annual yields, conclude the Strobls.
Downstream from dams, farmers can benefit from a steady year-round water flow. But those around the dam itself are often not so lucky. In dry years, the reservoir behind a dam can only be maintained if local farmers are prevented from extracting too much water from the rivers that flow into it. That leads to less irrigation and lower yields around the dams, which more than cancels out the downstream benefits.
'No gains'
A 1 per cent drop in not necessarily a disaster, says Eric Strobl. "But if a country is already in poverty it could mean a lot."
Rohini Pande of Harvard University produced similar results when she examined dams in India. "The main way to look at this is that there is no [agricultural] gain from dams," she says. "And building them costs a lot of money."
Despite his findings, Strobl says that large dams can have a positive impact if they are more effectively located. He looked at the effect of building dams on around 1400 sites in Africa where as many farmers as possible benefit from the downstream irrigation. His preliminary calculations suggest that these dams could boost yields by almost 20 per cent even in dry years.
Rich before poor
But big dams often attract opposition from advocacy groups, and Strobl's optimistic predictions will not change that. Terri Hathaway, a Cameroon-based campaigner with International Rivers, says that major dam projects often benefit large farmers and industry at the expense of local people.
The 7-kilometre long Merowe Dam in Sudan, which began producing electricity earlier this year, is one example. The dam will ultimately pump out 1250 megawatts, more than doubling the country's electricity capacity.
The reservoir behind the dam contains 10 million cubic metres of water, but tens of thousands of people were displaced when it was created. Some were moved to distant desert areas and have since protested about water shortages.
The researchers' work can be viewed here (pdf format)
