## 5 Conclusions  
In the current study, the impact of seasonal drought on WUE was analyzed using ridge regression, which appeared in the temporal and spatial distribution characteristics of seasonal drought and WUE in the Loess Plateau from 2001 to 2019. The results show that from 2001 to 2019, there was significant spatial heterogeneity in seasonal drought and WUE, with generally mild drought levels and the highest WUE values in the summer. Seasonal drought had different effects on WUE, and the sensitivity coefficient also differed significantly as the SPEI changed. On the other hand, the sensitivity of seasonal WUE to drought magnitude was affected by regional dry and wet conditions. The impact of drought intensity on WUE in the four climate regions showed a clear seasonal divergence, and the sensitivity of WUE to drought magnitude in arid areas was generally higher than that in semi-arid, semi-humid, or humid areas. However, there are still many deficiencies in this study. Future research should pay more attention to the response processes of GPP and ET to drought and consider the impact of drought accumulation and lag effect on WUE. Our findings are crucial for understanding the impact of climate change on the Loess Plateau's ecosystem carbon and water cycles. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/f13050634/s1](https://www.mdpi.com/article/10.3390/f13050634/s1)]([https://www.mdpi.com/article/10.3390/f13050634/s1](https://www.mdpi.com/article/10.3390/f13050634/s1)), Figure S1. The area proportion of the sensitivity coefficient (WUE to seasonal SPEI) passing the significance test. **Author Contributions:** Conceptualization, T.P. and Q.H. ; methodology, T.P. and Q.H. ; software, Q.H. ; validation, Q.H. and Y.C. ; formal analysis, Y.C. ; investigation, Z.J. and H.W. ; resources, Q.H. and Z.J. ; data curation, Q.H. ; writing--original draft preparation, Q.H. ; writing--review and editing, T.P. and Y.C. ; visualization, Q.H. ; supervision, B.X., P.Q. and J.Z. ; project administration, Y.C. ; funding acquisition, T.P. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the University Innovative Fundation Project of Gansu Province (2020B-117), Youth Science and Technology Fund of Gansu Province (21 JR7 RA853), the Youth Scholar Supporting Foundation of Gansu Agricultural University (GAU-QDFC-2021-06). **Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon request. **Acknowledgments:** This paper would not have been possible without the help of all organizations or institutions that provided multi-source data, including the precipitation and temperature dataset from the National Meteorological Science Data Center, as well as and MODIS GPP (MOD17A2H) and MODIS ET (MOD16A2) data are from the United States Land Processes Distributed Active Archive Center. **Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that might affect the work reported in this article. * (13) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; et al. Response of vegetation to drought time-scales across global land biomes. _Proc. Natl. Acad. Sci. USA_**2013**, _110_, 52-57. [CrossRef] [PubMed]
* (14) [PERSON]; [PERSON]; [PERSON]; [PERSON] Ecosystem level water use efficiency: A review. _J. Acta Ecol. Sin._**2009**, _29_, 1498-1507. * (15) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Drought-induced dynamics of carbon and water use efficiency of global grasslands from 2000 to 2011. _Ecol. Indic._**2016**, _67_, 788-797. [CrossRef]
* (16) [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Research progress in water use efficiency of plants under global climate. _Change J. Chin. J. Appl.