Spreading water shortages are threatening to reduce the global food supply by more than 10 per cent. Left unaddressed, these shortages could lead to hunger, civil unrest, and even wars over water. 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," says Sandra Postel, the author. Some 40 per cent...