Increased Water Use Efficiency in China and Its Drivers During 2000–2016

Water use efficiency (WUE), the carbon uptake per unit of water consumption, is a critical indicator of the coupling between water and carbon cycles in terrestrial ecosystems. Understanding the spatiotemporal changes in WUE and its drivers is vital for predicting ecosystem responses to environmental changes. Herein, we used a remote sensing-driven analytical model to estimate spatiotemporal variations of WUE in China during 2000–2016 and quantified its key drivers. The simulated WUE matched well with the eddy covariance observations across different ecosystems in China, with coefficients of determination of 0.86 and 0.81. The annual WUE increased significantly at a rate of 0.021 g C mm⁻¹ H₂O y⁻¹ (P < 0.01), with the mean value of 1.62 g C mm⁻¹ H₂O during 2000–2016. Analyses based on a factorial experiment (FE) and the sensitivity method (SE) showed that the increased leaf area index was the largest contributor to the increase in WUE, with contributions of 50.1% and 42.8% determined via FE and SE, respectively. CO₂ enrichment was the second largest contributor (FE: 20.7% and SE: 31.4%), followed by the decrease in the vapor pressure deficit (FE: 15.4% and SE: 13.4%). The increased ratio of water loss by canopy interaction had a negative impact on WUE due to the increased non-productive water loss. Although these attributions had some inherent uncertainties, the results highlighted that the significant greening in China, which was mainly caused by human land-use management, and increased CO₂ concentrations significantly promoted WUE during the recent 17 years.