The water that is used in the production process of a commodity is called the 'virtual water' contained in the commodity. International trade of commodities brings along international flows of virtual water. The objective of this paper is to quantify the volumes of virtual water flows between nations in the period 1995-1999 insofar related to international crop trade and to analyse national virtual water balances in relation to national water needs and water availability. The basic approach is

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... o multiply international crop trade flows (ton/yr) by their associated virtual water content (m 3 ton À1 ). The calculations show that the global volume of crop-related international virtual water flows between nations was 695 Gm 3 yr À1 in average over the period 1995-1999. For comparison: the total water use by crops in the world has been estimated at 5400 Gm 3 yr À1 . This means that 13% of the water used for crop production in the world is not used for domestic consumption but for export (in virtual form). This is a conservative estimate because only a limited number of crops--although the most important ones--have been taken into account and because crop products (such as cotton clothes) have been excluded from the study. The countries with the largest net virtual water export are United States, Canada, Thailand, Argentina and India. The largest net import appears to be in Japan, the Netherlands, the Republic of Korea, China and Indonesia. r Crop evapotranspiration E c [mm day -1 ] Crop water requirement CWR [m 3 ha -1 ] Crop yield CY [ton ha -1 ] Specific water demand SWD [m 3 ton -1 ] Global crop trade CT [ton yr -1 ] Global virtual water flows VWT [m 3 yr -1 ] Ref. crop evapotranspiration E 0 [mm day -1 ] Crop coefficient