the internal water footprint is the amount of water used from domestic water resources; the external water footprint is the amount of water used in other countries to produce goods and services imported and consumed by the inhabitants of the country. when assessing the water footprint of a nation, it is crucial to take into account the international flows of virtual water (also called "embodied water", i.e. the water used or polluted in connection to all agricultural and industrial commodities) leaving and entering the country. when taking the use of domestic water resources as a starting point for calculating a nation's water footprint, one should subtract the virtual water flows that leave the country and add the virtual water flows that enter the country.