ABSTRACT
The term “virtual water” began appearing in the water resources literature in the mid 1990s. Professor Tony Allan chose the term to describe the water used to produce crops traded in international markets (Allan, 1996, 2002). The notion of “water footprints” appeared a bit later, put forth originally in conjunction with the discussion of the amounts of virtual water “flowing” between countries as they trade goods and services (Hoekstra and Hung, 2002; Chapagain et al., 2006a, 2006b). In recent years, many authors have calculated the volumes of virtual water and water footprints within countries (Ma et al., 2006; Guan and Hubacek, 2007), while others have calculated both “internal” and “external” water footprints, suggesting that these measures distinguish between how much virtual water is used within a country from the volume involved in international trade (Hoekstra and Chapagain, 2007a, 2007b). Green, blue, and grey water footprints also have been described (Van Oel et al., 2009; Fader et al., 2011). Green water footprints are intended to represent the soil moisture from rainfall that is used for crop production, while blue water footprints describe irrigation with surface water or groundwater (Mekonnen and Hoekstra, 2010). The grey water footprint is offered as an estimate of the volume of water required to dilute pollutants to meet the prevailing ambient water quality standards (Hoekstra, 2013: 11).
