The water footprint and virtual water exports of Spanish tomatoes

D. CHICO et al. 15standards. The grey water footprint is calculated by dividingthe pollutant load (L, in mass/time) by the differencebetween the ambient water quality standard for that pollutant(the maximum acceptable concentration c max, inmass/volume) and its natural concentration in the receivingwater body (c nat, in mass/volume).WFblue ijklET= ×Y10 blue ijklijklE [5]As it is generally the case, the production of tomato concernsmore than one form of pollution. In our case though,the grey water footprint was estimated only for Nitrogen. Thetotal volume of water required to assimilate a ton of Nitrogenwas calculated considering the surplus Nitrogen, which endsup leaching. The natural concentration of Nitrogen in the receivingwater body was assumed negligible whereas the maximumallowable concentration in the ambient water systemconsidered was 50 mgNO 3-/l, as the concentration stated inthe EU Nitrates Directive (91/676/EEC). The pollutant loadconsidered was the excess Nitrogen based on data from theMinistry of the Environment and Rural and Marine Affairsof Spain (MARM, 2008) (Annex IV). This excess Nitrogenavailable for leaching or run-off (kg/ha) was then multipliedby the corresponding area in order to obtain the total load ofNitrogen (kg) reaching the surface or groundwater systems.This was divided by the ambient water quality standard andthe corresponding crop yield (ton/ha) to obtain the grey watercontent in terms of m 3 /ton. Thus, a grey water footprint wasobtained for each year period and type of production system.WFgrey ijklExcessNk=LimitN×YijklE [6]