The water footprint and virtual water exports of Spanish tomatoes

16 THE WATER FOOTPRINT OF TOMATO PRODUCTIONIn 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).WF grey ijkl= the grey water footprint of the province k, underthe production system j in the year season i inthe year l (l/kg).ExcessN k= Nitrogen excess of the Nitrogen balance in theprovince k (kg N/ha).-Limit N = limit concentration of NO 3in the receiving waterbody according to the EU Nitrates Directive(91/676/EEC) (kg NO 3-/l).Although this approach is based on some assumptions, itallowed us to have a preliminary estimate of the grey waterfootprint for each type of production. A more local approachwould be desirable if a more accurate quantification issearched.2.2. Water footprint calculation of tomatogreenhouse productionIn the case of tomato production under greenhouses, themethodology was similar to that followed for the open-air systemsbut differed in some points. In this case, since the plantingdates and crop growing period vary significantly, beingdecided by the producer based on climatic, agronomical andeconomical reasons (Reche, 2009), these two parameters wereprovided at the national level. The crop parameters were assumedto be the same for all the provinces and different fromthose of the calculus of open-air production (Table 4).