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Abstract:
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"This study quantifies the green, blue and grey water footprint of global crop production in a spatially-explicit way for the period 1996-2005. The assessment is global and improves upon earlier research by taking a high-resolution approach, estimating the water footprint of 126 crops at a 5 by 5 arc min grid. We have used a grid-based dynamic water balance model to calculate crop water use over time, with a time step of one day. The model takes into account the daily soil water balance and climatic conditions for each grid cell. In addition, the water pollution associated with the use of nitrogen fertilizer in crop production is estimated for each grid cell. The crop evapo-transpiration of additional 20 minor crops is calculated with the CROPWAT model. In addition, we have calculated the water footprint of more than two hundred derived crop products, including various flours, beverages, fibres and biofuels. We have used the water footprint assessment framework as in the guideline of the water footprint network. Considering the water footprints of primary crops, we see that global average wa- ter footprint per ton of crop increases from sugar crops (roughly 200 m3 ton-1), vegetables (300 m ton), roots and tubers (400 m ton), fruits (1000 m ton ), cereals (1600 m ton), oil crops (2400 m ton) to pulses (4000 m ton). The water footprint varies, however, across different crops per crop category and per production region as well. Besides, if one considers the water footprint per kcal, the picture changes as well. When considered per ton of product, commodities with relatively large water footprints are: coffee, tea, cocoa, tobacco, spices, nuts, rubber and fibres. The analysis of water footprints of different biofuels shows that bio-ethanol has a lower water footprint (in m GJ ) than biodiesel, which supports earlier analyses. The crop used matters significantly as well: the global average water footprint of bio-ethanol based on sugar beet amounts to 51 m3 GJ-1 , while this is 121 m3 GJ-1 for maize. Global freshwater withdrawal has increased nearly seven-fold in the past century. With a growing population, coupled with changing diet preferences, water withdrawals are expected to continue to increase in the coming decades. With increasing withdrawals, also consumptive water use is likely to increase. Consumptive water use in a certain period in a certain river basin refers to water that after use is no longer available for other purposes, because it evaporated."
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