Finally, global virtual water trade can physically save water if products are traded
from countries with high to countries with low water productivity. For example, Mexico
imports wheat, maize and sorghum from the US, which requires 7.1 BCM of water per
year in the US. If Mexico would produce the imported crops domestically, it would
require 15.6 BCM of water per year. Thus, from a global perspective, the trade in cereals
from the US to Mexico saves 8.5 BCM/yr. Although there are also examples where waterintensive
commodities flow in the other direction, from countries with low to countries
with high water productivity, Oki and Kanae (2004), De Fraiture et al. (2004), Chapagain
et al. (2006a) and Yang et al. (2006) have shown that the resultant of all international
trade flows works into the positive direction.