Comment on "Explaining virtual water trade: A spatial-temporal analysis of the comparative advantage of land, labor and water in China," published by Zhao et al. [Water Research (2019) 153: 304-314].

The virtual water hypothesis offers the reasonable proposition that if water-abundant regions export water-intensive products to water-scarce regions, the latter could devote their scarce resources instead to uses yielding higher economic returns. Zhao et al. show that trade flows in China do not adhere to this hypothesis, use the economic theory of comparative advantage to explore why, and seek a solution where both the hypothesis and the theory are apparently satisfied. However, they have not fully utilized the power of the theory: this is made evident by the fact that their calculations quantify what they call "the comparative advantage of land, labor and water," as stated in the title of the article. This Comment describes the significance of comparative advantage for this inquiry and our comparable investigation for the regions of Mexico. It concludes with the case for a collaborative effort to situate such single-country studies in the context of a model of global comparative advantages.