Water is heavy and cheap, which makes shipping it across oceans impractical. Food is comparatively light relative to the water it took to produce. So while very little water moves internationally, enormous volumes of water are traded in the form of crops.
How the accounting works
Producing a ton of wheat requires a certain volume of water. When an importing country buys that wheat, it avoids using its own water to grow it. Researchers call the avoided volume virtual water, and mapping these flows reveals a global network of water dependence.
Several water-scarce nations import a large share of their calories precisely because domestic production would consume water they do not have.
Why this is a strategy and a risk
Importing food to conserve domestic water can be sound policy. It also transfers exposure. A drought in an exporting region becomes a price shock or supply disruption thousands of miles away, and countries that concentrate imports in few sources concentrate that risk.
The productivity question
Virtual water trade is most beneficial when crops are grown where water is used efficiently. Often the pattern is the opposite, with export production concentrated in regions already under water stress. Improving irrigation efficiency in major exporting regions therefore has effects well beyond their borders.