With water shortages constraining food production growth, the world needs an effort to raise water productivity similar to the one that nearly tripled land productivity over the last half-century.
Since it takes 1,000 tonnes of water to produce one tonne of grain, it is not surprising that 70 percent of world water use is devoted to irrigation. Thus, raising irrigation efficiency is central to raising water productivity overall. Data on the efficiency of surface water projects - that is, dams that deliver water to farmers through a network of canals - show that crop usage of irrigation water never reaches 100 percent simply because some irrigation water evaporates, some percolates downward, and some runs off. (...)

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http://www.ipsnews.net/news.asp?idnews=51912