## 5 Conclusions  
The spatio-temporal variation and its driving force of WUE across the FPENC during the two decades with the support of a GEE platform was examined. In addition, the ecosystem resilience over the FPENC was also explored. This study reveals a non-significant declining trend in WUE (0.52-2.60 gC kgH\({}_{2}\)O\({}^{-1}\)) over the FPENC in the period of 2003-2022. Notably, WUE decreased markedly from 2003 to 2012, followed by a slight increase from 2013 to 2022. Among vegetation types, the evergreen needle forest exhibited the highest WUE, while grassland maintained relatively low values. In terms of climatic controls on WUE, Temp, VPD, and VSWC played the dominant role in controlling the WUE from 2003 to 2022. During the drought years, 40.18% of the whole FPENC showed ecosystem resilience, mainly in the vegetation of DBF, DNF, CRO, CSH, ENF, MF, SAV, and WSA, and grassland and open shrubland were shown to be moderately non-resilient. This study can enhance our understanding of WUE characteristics at both ecosystem and regional scales within the farming-pastoral ecotone of Northern China and shed light on ecosystem resilience amid global warming.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/agriculture15111164/s1](https://www.mdpi.com/article/10.3390/agriculture15111164/s1)]([https://www.mdpi.com/article/10.3390/agriculture15111164/s1](https://www.mdpi.com/article/10.3390/agriculture15111164/s1)), Figure S1: The slope of inter-annual water use efficiency variations during 2003 to 2022 in the farming-pastoral ecotone of Northern China; Figure S2: Annual averages of water use efficiency and its trends during 2003 to 2010, 2011-2022 and 2003 to 2022 in the farming-pastoral ecotone of Northern China; Figure S3: The annual WUE during the study period from 2003 to 2022 in the farming-pastoral ecotone of Northern China; Figure S4: Monthly average water use efficiency during 2003 to 2022 in the farming-pastoral ecotone of Northern China; Figure S5: The spatial distribution of characteristics of annual averages of water use efficiency (WUE) in each season: (a) Spring, (b) Summer, (c) Autumn and (d) Winter during the study period from 2003 to 2022; Figure S6: Comparison of the auunal water use efficiency (WUE) observed from EC sites and the calculation from the MODIS GPP and ET; Figure S7: Comparison of precipitation obeseved from meteorological stations and the CHIRPS data.  
Conceptualization, X.G. and Z.-L.L.; methodology, X.G. and M.W.; software, X.G. and Z.S.; validation, G.S., Q.M. and H.L.; writing--original draft preparation, X.G. and M.W.; Z.-L.L. and L.H. provided conceptual and editorial advice and rewrote significant parts of the manuscript; funding acquisition, M.W. and Z.-L.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Natural Science Foundation of Hebei Province (No. C2023403003, D2022403013), the Postdoctoral Research Foundation of China (No. 2020M670543), the National Key Research & Development Program of China (2022 YFB3903505), Shijiazhuang Science and Technology Program (247790759A). We also would like to thank the editors and anonymous reviewers for providing valuable comments and suggestions on the manuscript.  
Not applicable.  
The data are available on request from the authors, the raw data supporting the conclusions of our manuscript will be made available by the authors on request.  
The authors declare no conflict of interest.