**5. Conclusions**

This study validated and optimized the performance of MODIS-derived GPP compared with EC-observed GPP at seasonal and annual time scales for the main arid ecosystems relying on flux networks constructed in arid and semi-arid ecosystems in China. Our study revealed that the current MODIS GPP products were significantly underestimated, as compared with the tower-observed GPP for most types of ecosystems in the arid region of China, especially the irrigated cropland and forest ecosystems, due to uncertainty of meteorological data and model parameters. Using ground-based meteorological data and updated land use data can improve GPP estimation. In addition to the light-use efficiency parameters, the temperature-limited stress factors and the VPD-limited factors also need to be recalibrated for ecosystems in arid regions. After using the proper model parameters, great improvements to the GPP model can be performed through a Bayesian approach. However, it is difficult to estimate GPP accurately in desert ecosystems because of the uncertainty of remote sensing vegetation products in arid regions. Hence, improvements in modelling GPP in desert ecosystems are needed in future studies. Moreover, this study implies that the current MODIS-derived GPP product requires further improvements to provide accurate monitoring of terrestrial ecosystem productivity in arid regions worldwide.

**Author Contributions:** H.W. and X.L. designed the research, analyzed data and prepared the manuscript. M.M. and L.G. provided suggestions to data analyses. All of the authors thoroughly reviewed and edited this paper.

**Funding:** This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA20100104), the National Natural Science Foundation of China (grant numbers: 91425303, 41871250, 41601482, 41471448), and the China Postdoctoral Science Foundation funded project (Grant No. 2016T90960).

**Acknowledgments:** We thank the two reviewers' insightful and helpful comments and suggestions to our manuscript. We are grateful to Kashif Jamal for the help of English editing of our revised manuscript and appreciate the availability of the MODIS data from the Oak Ridge National Laboratory Distributed Active Center (ORNL DAAC) website.

**Conflicts of Interest:** The authors declare no conflict of interest.
