26 Mar 2008

Virtual Water

Professor John Anthony Allan of the University of London won the 2008 Stockholm Water Prize for introducing the concept of "virtual water," which captures the amount of water is takes to produce a (food product). Examples:
  • Cup of coffee: 140 liters of water to grow, produce, package and ship the beans.
  • One kg wheat: 1000 liters
  • One kg beef: about 15,000 liters
Bottom Line: A kg of rice will use more water if it relies on irrigation, uses lots of other inputs (fertilizer, etc.) or is grown in a hot, dry climate (e.g., California versus Thailand). Since food exports are also water exports, we should worry about growing food for export in water-poor areas. We should worry a lot if that water is subsidized and/or the food crop is grown for nationalist purposes (e.g., wheat in Saudi Arabia).


  1. I don't understand 'virtual water' very well.

    If I burn coal and get CO2, the coal is gone, but if I irrigate a field to make rice, the water is not gone it just passes through the field and the rice. I would think that almost all of the water needed to grow rice either sinks into the ground or evaporates. In either case the water moves on to accomplish other things that need water.

    So, I would like a water accounting that includes the fact that, for the most part, water does not get used up but moves on to other places. Such an accounting, as in carbon accounting, would clarify a lot of topics.

  2. Eric -- remember that water is "used" when it evaporates. Virtual water refers to the "virtual" increase in supply that one gets from using food that was grown elsewhere...

    I have a post on "water use" going up soon...


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