Source:
The Yomiuri ShimbunCountries exporting food to Japan use about 42.7 cubic kilometers of water annually to produce that food, according to an estimate by a group of researchers at Tokyo University. The figure, equivalent to 42.7 billion tons of water, works out at about 80 percent of the total agricultural water used annually for food production in Japan.
...
Almost 10 percent of the water used abroad in producing food for the Japanese market comes from underground sources. In recent years, concern has been growing that water supplies from underground sources in many food-exporting nations could dry up in the not too distant future, threatening Japan's efforts to secure food from overseas. "Underground water is such a scarce resource and yet it accounts for such a large percentage of water
. If it dries up, it could dramatically push up food prices," Oki said.
...
The volume of underground water was estimated at about 2.9 cubic kilometers, or about 6.8 percent of the total. The group found that the United States, the largest food exporter to Japan, was particularly dependent on underground sources of water for food shipped to Japan.
Read more: http://www.yomiuri.co.jp/dy/features/science/20080302TDY02308.htm