The water footprint and virtual water exports of Spanish tomatoes

14 THE WATER FOOTPRINT OF TOMATO PRODUCTIONThe green water footprint of the crop (m 3 /ton) has beenestimated as the ratio of the green water use (m 3 /ha) to thecrop yield (ton/ha). The blue water footprint of the crop isassumed equal to the ratio of the volume of irrigation waterconsumed to the crop yield.WFWFgreenijklblue ijklET= ×10green ijklY10×ET=Y ijklijklblue ijklE [4]In which:i = year season (early, middle or late season).j = production system (rainfed, open-air irrigated or covered).k = province of the country.l = year of the study period (1997-2008).ET green ijkl= Green water evapotraspiration of the provincek, under the production system j in the yearseason i in the year l (mm).Y ijkl= Yield of the province k, under the production systemj in the year season i in the year l (t/ha).ET blue ijkl= Blue water evapotraspiration of the province k,under the production system j in the year seasoni in the year l (mm).Finally, the grey water footprint of a primary crop is anindicator of freshwater pollution associated with the productionof the crop (Hoekstra et al., 2009). It is defined asthe volume of freshwater that is required to assimilate theload of pollutants based on existing ambient water quality