Low prices of water and drop in productivity

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: With water shortages emerging as a constraint on food production growth, the world needs an effort to raise water productivity similar to the one that nearly tripled land productivity during the second half of the twentieth century. Worldwide, average irrigation water productivity is now roughly one kg of grain per tonne of water used. Since it takes 1,000 tonne of water to produce one tonne of grain, it is not surprising that 70% of world water use is devoted to irrigation. Thus, raising irrigation efficiency is central to raising water productivity overall.

In surface water projects—that is, dams that deliver water to farmers through a network of canals—crop usage of irrigation water never reaches 100% simply because some irrigation water evaporates, some percolates downward, and some runs off. Water policy analysts Sandra Postel and Amy Vickers found that “surface water irrigation efficiency ranges between 25% and 40% in India, Mexico, Pakistan, the Philippines and Thailand; between 40% and 45% in Malaysia and Morocco; and between 50% and 60% in Israel, Japan, and Taiwan.” Irrigation water efficiency is affected not only by the type and condition of irrigation systems but also by soil type, temperature and humidity. In hot arid regions, the evaporation of irrigation water is far higher than in cooler humid regions.

Raising irrigation water efficiency typically means shifting from the less efficient flood or furrow system to overhead sprinklers or drip irrigation, the gold standard of irrigation efficiency. Switching from flood or furrow to low-pressure sprinkler systems reduces water use by an estimated 30%, while switching to drip irrigation typically cuts water use in half. A drip system also raises yields because it provides a steady supply of water with minimal losses to evaporation. Since drip systems are both labour-intensive and water-efficient, they are well suited to countries with surplus labour and warer shortage.

A few small countries—Cyprus, Israel, and Jordan—rely heavily on drip irrigation. Among the big three agricultural producers, this more-efficient technology is used on 1-3% of irrigated land in India and China and on roughly 4% in the US.