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In addressing the role that irrigation might play in the global water challenge, Peter Rogers explained that precipitation falling on the earth’s surface is the ultimate source of water. He described its eventual separation into “green” and “blue” water, a concept first introduced by the Stockholm International Water Institute and further illustrated in Water for Food, Water, For Life: Comprehensive Assessment of Water Management in Agriculture published by the International Water Management Institute in 2007. According to IWMI, blue water is water in rivers, groundwater aquifers, reservoirs and lakes and is the main water source for irrigated agriculture. Green water refers to the soil moisture generated from rainfall that infiltrates the soil and is available for uptake by plants. It constitutes the main water resource in rainfed agriculture. On average, about 56 percent of the water falling on the surface evaporates or transpires from forests, grazing lands and other natural habitats. About 4.5 percent evaporates or transpires from rainfed agriculture and another 2 percent from irrigated agriculture. The percentage of rainfall consumed by cities and industry is only 0.1 percent of the total rainfall.
How scarce is water? Given that irrigated agriculture uses such a low percentage of precipitation, how can the Earth run out of water? The total available blue water, which is available for use from streams and groundwater, is about 12,500 cubic kilometers. The rest of the blue water is unavailable because it is either in the wrong place, such as remote arctic streams, or comes at the wrong time, such as during a flood. Based on these estimates, humans use 50 percent of the available blue water supply, which is close to the edge of sustainability. If blue and green water are considered, humans use only 23 percent of the available water supply. Most of that is used by rainfed food, fiber and forestry crops. “You can heave a sigh of relief and say, well, gee, 23 percent is a lot better than 50-something percent,” Rogers said. “So part of my argument again is, we have a global water problem coming up and … how close to the edge are we?”