Unsustainable groundwater use for global food production and related international trade

Non-technical summary. Most of the water humans consume is for agriculture. Rapidly increasing water demand has led to overexploitation of water resources in many important food-producing regions. In particular, growing groundwater-based irrigation causes potentially damaging depletion. Food systems are increasingly globalized, leading to large export-oriented production. Much research has focused on quantifying the amount of water resources embedded in traded products, but less attention has been given to the role of groundwater use and the related sustainability of agriculture globally. We assess current knowledge of virtual water trade in light of groundwater use and sustainability and highlight remaining challenges in this field. Technical summary. Increasing pressure on water resources globally is mostly driven by irrigation. The use of non-renewable groundwater for irrigation is estimated to have tripled from 1960 to 2000, now making up approximately 20% of irrigation globally. This leads to fast aquifer depletion in many regions, which is a major concern as groundwater can provide an alternative source during droughts, which are expected to become more frequent under climate change. Importantly, water resources management is a local issue, but global trade strongly links producing regions to remote locations of consumption. Much research has analysed global patterns of virtual water trade - measuring the water consumed to produce traded food and highlighting the dependence of some nations' food supplies on others' water resources. Recently, concern regarding the sustainability of water use has emerged in this field, and many challenges remain, including groundwater sustainability estimates. We analyse how the consideration of different water sources and water sustainability has emerged in virtual water trade studies, describe the challenges and advances in evaluating the water sustainability of agriculture globally and recommend topics for future research: distinguish groundwater from other irrigation sources while considering all sources; exploit new modelling and data opportunities to improve estimates of groundwater storage and dynamics; and better integrate ecological needs for water.