Spreading water shortages threaten to reduce global food supply by more than 10 per cent, according to the latest report from the Worldwatch Institute. If not addressed, it is likely to lead to hunger, civil unrest and even conflict over water in the new century, it predicts.
Irrigation accounts for two thirds of global water use, but less than half that water reaches the roots of plants. "Without increasing water productivity in irrigation, major food-producing regions will not have enough water to sustain crop production," said Ms Sandra Postel, author of Pillar of Sand: Can the Irrigation Miracle Last? which was published in the US at the weekend. "Some 40 per cent of the world's food comes from irrigated cropland and we're betting on that share to increase to feed a growing population," she explained.
However, the "productivity" of irrigation is in jeopardy from the over-pumping of groundwater, the growing diversion of irrigation water to cities and the build-up of salts in the soil. Water tables are dropping steadily in several major food-producing regions as groundwater is pumped faster than nature replenishes it.
Farmers rack up an annual water deficit of some 160 billion cubic metres; the amount used to produce nearly 10 per cent of the world's grain. With the population growing rapidly in many of the water-short regions, water problems are bound to get worse, she said. The number of people living in "water-stressed countries" is projected to climb from 470 million to 3 billion by 2025, the study concludes.
The heavy demands of irrigation are also damaging the aquatic environment - shrinking wetlands, reducing fish populations, and pushing species toward extinction.
"Most farmers today irrigate the way their predecessors did hundreds of years ago," she added. "Just as raising land productivity helped meet food needs during the last half of this century, boosting water productivity will be the agricultural frontier during the next century."
Water-short countries, nonetheless, are increasingly turning to the world grain market to relieve the stress. Jordan imports 91 per cent of its grain; Israel 87 per cent, Saudi Arabia 50 per cent and Egypt 40 per cent. To meet these challenges, Ms Postel proposes a "blue revolution" to double water productivity in the next 30 years.
She describes a diverse mix of strategies pointing the direction forward:
Farmers in India, Israel, Jordan, Spain and the US have shown that drip irrigation systems that deliver water directly to crop roots can cut water use by 30 to 70 percent and raise crop yields by 20 to 90 per cent.
In the Texas High Plains, farmers using highly efficient sprinklers raised their water efficiency to more than 90 per cent while simultaneously increasing corn yields by 10 per cent and cotton yields by 15 per cent.
Rice farmers in an area of Malaysia saw a 45 per cent increase in water productivity by better scheduling their irrigations, shoring up canals and sowing seeds directly in the field rather than transplanting seedlings.
Farmers in California's Imperial Valley are lining canals, recycling farm run-off and selling the saved water to southern California cities.
Israel is now re-using 65 per cent of its domestic waste water for crop production, freeing-up additional freshwater for households and industries.
For the blue revolution to succeed, Ms Postel said, it is up to governments and water authorities to adopt new rules on irrigation. Subsidies from governments worth $33 billion a year make it cheaper to waste water than to conserve it. Legal barriers often make it difficult for farmers to sell any water they save through conservation practices.
Ultimately, a failure to regulate over-pumping groundwater would leave the world vulnerable to sudden cutbacks in food production as water tables drop to greater and greater depths.