## 5 Conclusions  
As an important linkage coupling the global carbon and water cycles, a better understanding of terrestrial ecosystem WUE will help us track the responses in water-use strategies of ecosystems to environmental stress and adopt appropriate ecosystem management, especially in the arid and semi-arid areas with limited water resources. Fortunately, the development of the EC technique and frequent MODIS observations make it feasible from site-level evaluations to large-scale modeling, as well as the underlying mechanisms with environmental controls. Ecosystem WUE of both grassland and maize cropland in the Heihe River Basin of Northwest China exhibited distinct seasonal dynamics, but with troughs during the peak growing season in summertime. A sharp decrease in GPP, but no apparent changes in ET, jointly led to the reduction of ecosystem WUE under a warm temperature. Pearson correlation analysis revealed that local temperature (T\({}_{\text{s}}\) and T\({}_{\text{a}}\)) and precipitation are the two most important environmental factors in determining the coupled WUE variability. Time-series MODIS EVI data that comprehensively reflected the complicated climate conditions was strongly relevant to ecosystem WUE with strong correlation coefficients reaching up to 0.960 and 0.941, respectively. Given the large uncertainties in the MODIS WUE estimates from GPP and ET products especially for the cropland, an alternative method by exclusive use of time-series MODIS EVI data (GR) performed rather well in capturing eight-day variations in WUE at both Arou grassland and Daman cropland stations. Therefore, our findings have important implications for understanding climate change effects on the coupling of carbon and water interactions in dry regions. In spite of these improvements, more site-year data are still required for further investigation and larger scale extrapolation. Meanwhile, the responses of ecosystem WUE and flux dynamics to extreme climate events such as drought are also crucial in the terrestrial feedback to changing climate.  
**Acknowledgments:** This study was jointly supported by the National Natural Science Foundation of China (41401221, 41371532), the Interdisciplinary Frontier Project of Nanjing Institute of Geography and Limnology, CAS (NIGLAS2016 QY02), the Fundamental Research Funds for the Central Universities in China (SWU116088) and China Postdoctoral Science Foundation (2017M610109). This work used the original flux data of Arou grassland site and Daman cropland site acquired from Heihe Watershed Allied Telemetry Experimental Research (Hi-WATER). All technicians are acknowledged in site management and data collection. We also thank the principal contributors of the MODIS products, the Distributed Active Archive Center of the Oak Ridge National Laboratory, and the Earth Observing System Data for making these MODIS data available.  
**Author Contributions:** [PERSON] designed this study, performed data analysis and wrote the manuscript. [PERSON], [PERSON] and [PERSON] contributed significantly to the discussion of results and manuscript refinement. [PERSON], [PERSON], [PERSON] and [PERSON] helped to process the flux measurements and remote sensing data.  
**Conflicts of Interest:** The authors declare no conflict of interest.