The current issue of Scientific American has a bunch of great charts on which countries use the most fresh water. Not surprisingly, most of the world’s water goes toward agriculture — only a tiny fraction is for household use — and the biggest users are China, India, and the United States.

But the most unexpected aspect of water use is the massive global trade in what’s known as “virtual water.” Japan, for instance, imports a lot of food and other products from abroad. Other countries need to use their water to make those products. That makes Japan the world’s biggest net importer of “virtual water,” followed by Mexico, Italy, and Germany. By contrast, countries like the United States and India are net exporters of virtual water.

Here’s a fascinating map from a recent paper in the Proceedings of the National Academies of Sciences that charts the global water trade — green countries are virtual water exporters, while red and orange countries are virtual water importers:

The authors of the PNAS study, Arjen Hoekstra and Mesfin Mekonnen, estimate that one-fifth of the world’s “water footprint” goes toward export. And that could increase a lot in the coming years. Currently, about 90 percent of China’s water needs are supplied internally. But, the authors note, that’s likely to change as China grows rapidly and its own freshwater supplies in the north get depleted. The country will start importing more and more “virtual water” from elsewhere. (China has already been moving to buy up farmland in Africa, for instance.)

There’s not really a specific policy issue addressed by the PNAS paper, although the research does suggest that some countries are extremely dependent on foreign water supplies — possibly far more than they realize. Kuwait, for instance, imports 90 percent of its virtual water from abroad.

And if, as is expected, climate change and a growing world population put increasing stress on the world’s freshwater supplies, these trade flows could become more important. Perhaps we’ll one day reach the point where countries fret as much about “foreign water dependence” as they do foreign oil dependence.