Virtual groundwater transfers from overexploited aquifers in the United States

Significance Irrigated agriculture is contributing to the depletion of the Central Valley, High Plains, and Mississippi Embayment aquifer systems. Agricultural production within these aquifer regions comprises a significant portion of the domestic and international cereal supply; thus, potential food security implications arise if production significantly decreases to bring groundwater withdrawals within sustainable limits. For the first time to our knowledge, this study tracks and quantifies the food and embodied groundwater resources from these aquifer systems to their final destination and determines the major US cities, US states, and countries that are currently most reliant upon them. Tracing virtual groundwater transfers highlights the role of distant demands on local groundwater sustainability and the fact that aquifer depletion must be considered within its global context. The High Plains, Mississippi Embayment, and Central Valley aquifer systems within the United States are currently being overexploited for irrigation water supplies. The unsustainable use of groundwater resources in all three aquifer systems intensified from 2000 to 2008, making it imperative that we understand the consumptive processes and forces of demand that are driving their depletion. To this end, we quantify and track agricultural virtual groundwater transfers from these overexploited aquifer systems to their final destination. Specifically, we determine which US metropolitan areas, US states, and international export destinations are currently the largest consumers of these critical aquifers. We draw upon US government data on agricultural production, irrigation, and domestic food flows, as well as modeled estimates of agricultural virtual water contents to quantify domestic transfers. Additionally, we use US port-level trade data to trace international exports from these aquifers. In 2007, virtual groundwater transfers from the High Plains, Mississippi Embayment, and Central Valley aquifer systems totaled 17.93 km3, 9.18 km3, and 6.81 km3, respectively, which is comparable to the capacity of Lake Mead (35.7 km3), the largest surface reservoir in the United States. The vast majority (91%) of virtual groundwater transfers remains within the United States. Importantly, the cereals produced by these overexploited aquifers are critical to US food security (contributing 18.5% to domestic cereal supply). Notably, Japan relies upon cereals produced by these overexploited aquifers for 9.2% of its domestic cereal supply. These results highlight the need to understand the teleconnections between distant food demands and local agricultural water use.