M.M. Aldaya, J.A. Allan and A.Y. Hoekstra in their paper Strategic importance of green water in international crop trade (Ecological Economics 2009) doi:10.1016/j.ecolecon.2009.11.001 map global flows of virtual water in the wheat trade.
In their paper they explain their figure:
The map presented in Fig. 6 shows the virtual-water ‘flows’ to the five major importing countries for wheat for the period 2000–2004.
By ‘importing’ virtual water embodied in agricultural commodities, a nation “saves” the amount of water it would have required to produce those commodities domestically.
Though from an importing country perspective it is not relevant whether products have been produced using green or blue water in the country of origin, from a global point of view it has important implications (Chapagain et al., 2006a). For instance, Egypt is the largest importer of wheat, with the USA providing about 45% of the country’s imports. Wheat from Egypt has an average virtual-water content of 930 m3/ton of which 100% is blue water (Chapagain et al., 2006a), while the USA has a virtual-water content for wheat of 1707 m3/ton of which 39.8% is blue water (Table 3).
By importing wheat, Egypt saves 930 m3 of water per ton of wheat. Globally, when imported from the USA, there is not a total water saving because wheat production in the USA requires more water than in Egypt. Exports to Egypt from this country result in a considerable net global water loss of 777 m3 per ton. However, if we just look at blue water only, importing wheat from the USA to Egypt saves 251 m3/ton (since USA production requires 679 m3/ton of blue water and wheat production in Egypt 930 m3/ton).
Along these lines, Egypt, as some other water-scarce importing countries, has formulated policies to import low value but high water consuming food like cereals (Van Hofwegen, 2005). Nevertheless, even if the potential of trade to “save” water at national level is substantial, most international food trade occurs for reasons not related to water resources (CAWMA, 2007).