This is a link to a good paper on the methodologies associated with water footprinting and the idea of virtual water - -
How much water does it take to make a cup of coffee? About 140 litres. The idea of virtual water, the water that goes into the hamburger (the cow) you ate last night, the shirt you are wearing (the cotton), and drink in your hand (the Coke), is an interesting intersection of our ignorance and our goal of sustainability. The water embedded in everything from coffee to a steak is invisible - - which produces a high level of public ignorance.
An increase in water sustainability will need a much stronger understanding and commitment to the metrics of virtual water and water footprinting.