Agricultural water sustainability under threat from the water use efficiency paradox

Improving crop water use efficiency may not necessarily alleviate water scarcity—a phenomenon that represents a specific manifestation of the Jevons Paradox in agricultural water resources. However, this paradox has not been quantified yet, which has limited our ability to understand the trade-off between efficiency gain and water loss and support informed irrigation decision-making in the face of climate change. Here, we introduce a novel index to quantify the crop water use efficiency paradox and explore its patterns and trends across China's 15 major crop types from 1991 to 2019. Our findings reveal clear evidence of the persistence and uneven threat of this paradox at both national and regional levels over the past three decades in China. Although the crop water efficiency in China has increased significantly by 48.78 kcal m^-3 yr^-‍1 since the 1990s, both the total blue water (+1.36 km³ yr^-1) and water scarcity index (+0.003 yr^-‍1) have also increased during the same period. The intensity of the paradox has, however, diminished since 2010, but climate change threatens to more than double its intensity in the mid-21st century if no proactive measures are implemented. Region-specific agricultural strategies are derived by quantifying water-use paradox intensity, scarcity, and efficiency trade-offs. These insights provide valuable scientific guidance for designing regional water-saving strategies in China and around the world, while also addressing the potential threats posed by the water use efficiency paradox to agricultural sustainability.