## 4 Conclusions  
Quantitatively evaluating the spatiotemporal patterns of ecosystem WUE and the underlying mechanisms are of great significance for understanding the effect of hydrological change on the ecosystem carbon budget. In this study, we assessed the driving environmental controls on crop WUE at corn/soybean rotation fields. As a proxy of vegetation responses to environmental conditions, time-series MODIS EVI data showed a strong correlation with the variability in crop WUE. Regional and global WUE have been estimated by integrating MOD17 GPP and MOD16 ET products. However, we observed large discrepancies between WUE\({}_{\text{RS}}\) and WUE\({}_{\text{EC}}\) for a few 8-day periods. The overall _RMSE_ for corn and soybean were 2.24 g C kg\({}^{-1}\) H\({}_{2}\)O and 1.58 g C kg\({}^{-1}\) H\({}_{2}\)O, which was mainly attributed to the great  
Figure 8: A linear comparison between ecosystem WUE from flux tower measurements (WUE\({}_{\text{EC}}\)) with predictions from MODIS products (WUEs) and estimates based on MODIS EVI data (WUE\({}_{\text{GR}}\)), respectively at the flux tower site US-Ro1 during the growing season of 2005 (corn).  
Figure 9: A linear comparison between ecosystem WUE from tower measurements (WUE\({}_{\text{EC}}\)) with predictions from MODIS products (WUEs\({}_{\text{RS}}\)) and estimates based on MODIS EVI data (WUE\({}_{\text{GR}}\)), respectively at the flux tower site US-Ro1 during the growing season of 2006 (soybean).  
uncertainties in the MODIS GPP product. Fortunately, an alternative approach that is solely based time-series EVI data performed reasonably well in capturing the seasonal variations in crop WUE from EC flux sites. _RMSE_ during the growing season of corn and soybean improved to 0.79 g C kg\({}^{-1}\) H\(\mathrm{O}\) and 0.66 g C kg\({}^{-1}\) H\(\mathrm{O}\). Independent validation also showed the model's potential for broad application. During the corn-planting year and soybean-planting year, the _RMSE_ between WUEEC with WUEER and WUEER were 2.21 g C kg\({}^{-1}\) H\(\mathrm{O}\)_vs_ 1.15 g C kg\({}^{-1}\) H\(\mathrm{O}\), and 1.61 g C kg\({}^{-1}\) H\(\mathrm{O}\)_vs_ 0.72 g C kg\({}^{-1}\) H\(\mathrm{O}\), respectively. This study also revealed that MODIS WUE estimates from the perspective of C\({}_{3}\) and C\({}_{4}\) may be a reasonable method to predict the seasonal patterns in WUE of farmland systems. Such analyses will be helpful for agricultural policy makers and managers to plan and implement strategic goals of water resource management and food supply.  
This study was jointly supported by the National Natural Science Foundation of China (41401221, 41271500, 41201496), the Natural Science Foundation of Jiangsu Province, China (BK20141058), the Key Research Program of the Chinese Academy of Sciences (KZZD-EW-10-04), and the Opening Funds of State Key Laboratory of Remote Sensing Science (OFSLRSS201502) and Key Laboratory of Poyang Lake Wetland and Watershed Research (Jiangxi Normal University), Ministry of Education (PK2014002). This work used eddy covariance data acquired by the AmeriFlux. A large number of technicians, graduate and doctoral students are acknowledged for help in site management, data collection and elaboration. We thank the principal investigators and 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. We also thank the group of [PERSON] at the University of Montana to provide satellite-derived global record of land surface evapotranspiration product.  
[PERSON] and [PERSON] contributed equally to design this study and write this manuscript including the figures and tables. [PERSON], [PERSON], [PERSON] and [PERSON] contributed significantly to the discussion of results and manuscript refinement.  
The authors declare no conflict of interest.