The impact of trade on local green and blue water availability

International trade of agricultural goods exported from water abundant to water scarce regions can, in theory, alleviate water scarcity problems, which is a widely and controversely discussed issue in the virtual water literature. But how much can trade really help to solve water scarcity problems? And more importantly does trade of virtual water save the valuable blue water which can be rededicated to other purposes than agriculture or does it rather let green water go unused for which an application to other usages is often difficult? To address these questions we use two models: First, MAgPIE trade (Model of Agricultural Production and and its Impact on the Environment), to give us explicit trade and production patterns for food crops. And second, LPJmL a dynamic, spatially explicit, global vegetation and agricultural model with closed water and carbon budgets, which simulates the blue and green virtual water content of the produced food crops. To assess the effect of trade on agricultural production and water availability, we compare two scenarios for the year 2005: one in which trade is completely disabled and one with a business as usual scenario. The difference between the two scenarios tells us how production patterns change when no trade is possible. Coming from there and using the explicit virtual water contents of LPJmL, we can calculate how much virtual water is saved by allowing trade. We can also determine how much blue water has been saved on the regional and the global level by this trade. MAgPIE trade gives us additionally spatially explicit patterns of agricultural production. Using the difference in the production of the two scenarios, we can now determine how production patterns change when no trade is possible. LPJml provides than consumptive green and blue water use for each of the produced crops. Combining the outputs of both models, we can now determine where and how much of green and blue water use increased or decreased when trade is eliminated. This information is already interesting but does not tell us if the important blue water has been saved in water abundant or in water scarce regions. Using the MAgPIE trade output of cell-based water shadow prices (which is a measure of water scarcity), we develop an indicator of the significance of water saving. This significance indicator can answer the question if and where blue water is saved.