Virtual water is the term that links water, products, and trade. Each product, in fact, is characterized by a specific virtual water content: when the product is sell (or buy), it virtually carries the amount of water that was necessary for the production. By using this concept, the international trade can be seen as a complex virtual water network in which each country involved is a node. Among nodes there are flows of virtual water whose magnitude depends on the quantity of products traded, and on the water footprint characteristic of the place where the product has been produced.
For example the picture below shows the share in the export of the maize produced in India in 2011.
Apparently countries import maize to eat, to satisfy their needs (biofuels, animal feeding), but effectively the importation means for them releasing the pressure on their domestic water resources: virtual water import is an instrument to face water scarsity and limit the local depletion. This concept is particularly true for that regions in which the anthropic demand exceeds the local supply.
The map in the figure below shows the relationship between the local supply and the demand. America, Northern Asia and Australia are independent from others countries because their local water supply is bigger than the water demand for food, so they can support themselves and export the exceed goods to other parts of the world. Europe is generally able to support itself thanks to the importation from other countries that allows the re-exportation of some of the food imported. Africa and South of Asia have a very low local supply (there are only few exceptions) that is not sufficient to meet the local demand.
The virtual water network makes an increasing inter-dependance among countries, that can reduce the societal resilience. In fact if a node (country) of the network becomes weaker because of water deficit, the node dependent from it becomes weaker as a consequence.