Temporal Characteristics and Spatial Homogeneity of Virtual Water Trade: A Complex Network Analysis

International trade of commodities causes an international water flow in virtual form. This study focuses on the virtual water flow related to international crop trade. We build complex networks of virtual water trade (Virtual Water Trade Networks, VWTNs hereafter) with annual data of trade and its virtual water content from 1986 to 2013 among all countries. By analyzing the temporal characteristics and spatial homogeneity of the VWTNs, we find that trade networks display a feature of scale-free. This suggests those core export and import countries play dominant roles in international virtual water flows. However, the structure of the VWTN becomes more heterogeneous with time, indicating that virtual water trade has been getting more globalized. By spatially recoding the trade volumes and using detrended fluctuation analysis, this study finds that geographical distance becomes less binding in international virtual water trades. Other important findings from our VWTNs analysis include that virtual water trade patterns remain relative stable within trading groups and that geopolitical, political, and economic environments can significantly influence world virtual water trade flows. This study provides an understanding of the international water trade and its relationship with the macro social and economical conditions at the global scale. Its method thus can be applied to analyze the temporal and spatial dynamics of other networks with directed trade flow.