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Transference of the embedded water, so-called virtual water, in the trade of crops among regions within a country is often neglected, leading to no information about the impacts on the water resources of exporting regions, especially if those regions are water-stressed or, worse, water-scarce. Virtual water trade, if not considered through the lens of sustainability, could lead to adverse effects on the water resources of an exporting region. Previous related studies have quantified virtual water trade among the states in the United States providing valuable insights; however, information for specific crop trade among counties, its water footprint (WF) at the county scale, the resultant virtual water flow among counties, and the sustainability assessment of those virtual water flows are lacking. In this study, we calculate the green and blue WF of cereal and milled grain products at the county level and then, using trade data, calculate the virtual water flows among the counties. Then, we assess the sustainability of the import by introducing unsustainable import fraction (UIF), which is the ratio of virtual water imported from water-scarce counties to that of total virtual water imported in the form of cereal and milled grains. Finally, we quantify the change in UIF from the 2007–2017 period. A few of the significant insights discovered through this analysis include: (i) most of the cereal and milled grains trade is occurring among neighboring counties; ii) one-third of US counties import 75% or more virtual water from water scarce regions; (iii) in 2017, Texas and Missouri were the largest importer and exporter, respectively; and (iv) the number of counties importing cereals and milled grains from water-scarce counties increased from 2007 to 2017. Recommendations on alleviating the negative effects of the unsustainable import of cereal and milled grain are provided toward the end of the discussion.

Cotton, the world's most widely cultivated fiber crop, is associated with a multitude of environmental impacts, particularly the intensive use of water resources. While previous studies have assessed cotton's water usage through the water footprint concept, they largely focused on spatial variability, leaving temporal dynamics and the issue of unsustainable water use underexplored. This global study addressed these gaps by examining both the spatial and temporal variability of cotton's water footprint and assessing the unsustainable water footprint over time. Additionally, it explored the potential of flax as a water-efficient alternative for fiber production. Using a global gridded crop model, the study simulated yield and water use of flax and cotton from 1972 to 2018 at 30 arcmin resolution. The results showed that the global average blue, green, and total water footprint of cotton decreased by 37%, 48%, and 43%, respectively, due to yield improvements driven by improved crop cultivars, fertilizers, and better water management practices, with climate playing a minor role in these reductions. Despite this, the total water footprint of cotton production increased by 5%, with the blue water footprint rising by 17%. Additionally, unsustainable blue water usage in cotton cultivation increased from 59.3 km³/y to 70.9 km³/y between 1972-1976 and 2014–2018, with 71.1% of the unsustainable water traded virtually. USA, Pakistan, and India accounted for over 60% of this unsustainable virtual water trade. Flax, with its substantially lower water footprint, emerged as a promising alternative to reduce water consumption in fiber production. These findings offer critical insights for formulating strategies to mitigate unsustainable water use in global fiber production.