Rewiring the Domestic U.S. Rice Trade for Reducing Irrigation ImpactsImplications for the Food–Energy–Water Nexus

Food trade connects distant places of food production to places of consumption. Through traded food, associated environmental impacts are also displaced as the consumer benefits from the product without incurring the externalities of production. Taking U.S. rice as an example, we discuss the sustainability implications of rewiring U.S. rice production and trade for reducing the impacts of irrigation (water and energy) and transportation greenhouse gas (GHG) emissions. We model a series of robust optimization scenarios that re-arrange the origin of trade and therefore the production to target virtual water use and GHG emission reductions. For the baseline case, virtual water trade amounts to 35 billion m3, and embodied irrigation and transportation GHG emissions amount to 6 billion kg CO2-equivalent and 0.7 billion kg CO2-equivalent, respectively. Rewiring consistently achieves better results compared to the baseline even in the presence of uncertainty. However, our findings reveal strikingly sobering national-level savings in optimizing the water use (2%) and GHG emissions (14%) with tradeoffs in other impacts. To achieve these results, all rice-producing states undergo changes, with the state of Mississippi completely stopping production. California’s unique ability to produce medium-grain rice at a large scale makes it indispensable for current rice production and hence a major constraint for rewiring rice production. The findings of this work reveal the inflexibility of our food system in balancing the food–energy–water nexus tradeoffs through restructuring trade.

粮食贸易将遥远的粮食生产地与消费地连接起来。与之伴生的环境影响也随之发生跨区域转移——消费者在享用产品的同时，并未承担生产环节产生的外部性成本。本文以美国大米贸易为案例，探讨重构美国大米生产与贸易格局，对降低灌溉（水资源与能源）消耗以及交通运输温室气体（Greenhouse Gas, GHG）排放所带来的可持续性影响。我们构建了一系列鲁棒优化（robust optimization）情景，通过调整贸易来源以重构生产布局，目标是减少虚拟水（virtual water）使用量与温室气体排放。基准情景下，虚拟水贸易总量达350亿立方米，隐含的灌溉与交通运输温室气体排放分别为60亿千克二氧化碳当量与7亿千克二氧化碳当量。即便存在不确定性，重构贸易网络始终能取得优于基准情景的效果。然而，研究结果却揭示出令人警醒的国家级层面的节水与减排空间：水资源使用量仅可减少2%，温室气体排放量可减少14%，但这会以其他环境影响为代价。为达成上述优化效果，所有大米主产州均需调整生产布局，其中密西西比州将完全停止大米生产。加利福尼亚州凭借大规模种植中粒稻米的独特优势，成为当前大米生产不可或缺的核心产区，因此也成为重构大米生产格局的主要制约因素。本研究结果表明，当前粮食系统在通过重构贸易以平衡粮食-能源-水纽带（food-energy-water nexus）的权衡关系方面，存在显著的刚性约束。