Hydroponic greenhouse production is a proven approach to enhancing resource efficiency and overcoming limitations related to land availability, soil degradation, and climate variability. However, assessing the benefits and challenges of transitioning to greenhouses remains underexplored. This study provides a global assessment of the water-nutrient-salinity-energy nexus in lettuce production, spanning from soil-based open-field systems to closed-loop hydroponic greenhouse technologies. The study examines soil-water interactions in conventional open-field agriculture and highlights critical challenges, including soil salinization and nutrient leaching inherent to open-field farming. While transitioning to closed-loop hydroponics in greenhouses offers a potential solution to these issues, it comes with a considerable trade-off in the form of elevated energy demands for greenhouse operations. This study quantifies both the water demand for hydroponics and the energy required for heating and cooling, while also evaluating the contribution of harvested rooftop rainfall. Adopting closed-loop hydroponic systems in greenhouses can reduce water consumption for lettuce production by 6% to over 90%, while eliminating risks of soil salinization and groundwater contamination. Harvested rooftop rainfall can meet 100% of the hydroponics water demand in some locations, such as Bangladesh, during peak monsoon months (June–August). However, globally, the total energy demands for heating and cooling range from 39 to 123 MJ/kg, depending on climate. Furthermore, the minimum annual energy requirement for greenhouse operation was found to be 1854 MJ/m ² in Cwc climates (temperate, dry winters, and cold summers), with energy distribution favoring heating (62%) over cooling (38%). In India, specific water use (evapotranspiration and water for evaporative cooling) decreased by 93%, from 1146 L/kg in open-field furrow systems to 85 L/kg in closed-loop hydroponics, with an energy demand of 66 MJ/kg. These findings offer comprehensive, global-scale insights across 30 Köppen–Geiger climate zones and the 15 leading lettuce-producing countries, providing a foundation for policymakers to plan future agricultural development.

Soil salinization is a global phenomenon that affects large tracts of arid farmland worldwide. It contributes to the loss of soil fertility, declining yields, and – in the most severe cases – land unsuitability for cultivation. Irrigation water applications are both the main cause of and the solution to, anthropogenic (or ‘secondary’) salinization because salt typically enters the soil column as dissolved in irrigation water and leaves it through excess water applications (e.g., leaching). Excess leaching, which places additional water costs in areas affected by water scarcity, can be achieved with different irrigation techniques and practices. Here, by complementing a process-based crop water model with a salt balance of the shallow soil, we investigate the tradeoff between root zone salinization and water conservation to limit withdrawals from the water source. We evaluate how such a tradeoff is achieved under different irrigation technology and excess leaching practices. Considering as a case study the cultivation of tomatoes in Egypt, we find that drip and furrow irrigation allows for better control of salt accumulation, thus preventing crop exposure to salt stress. Drip irrigation achieves this goal with minimal water applications because it maintains the soil wetter. Thus, the (rare) rainfall events find more suitable conditions to drain the excess moisture. Conversely, by using more irrigation water (and ‘less efficiently’), furrow irrigation allows for higher rates of soil drainage and salt leaching. The irrigation schedule typically adopted with sprinkler irrigation allows for soil drying, thus limiting the ability of rainfall events to drain the soil and leach its salts. Collectively, these results highlight the key role of irrigation technology and practices in the management of secondary salinity in dryland agriculture. Specifically, there is a tradeoff between minimizing water use and preventing salt accumulation in the root zone. Drip irrigation exhibits the co-benefit of achieving both goals, while furrow irrigation limits soil salinity at the cost of requiring greater volumes of applied irrigation water.