# Evaluation of Clumping Effects on the Estimation of Global Terrestrial Evapotranspiration
## 7 Conclusions  
The significance of leaf clumping to global terrestrial ET simulation is revealed from its influences on sunlit and shaded LAI stratification, on the calculation of true LAI from effective LAI, and estimations of leaf-level irradiance. This is the first time that foliage clumping effects on ET estimation are assessed globally. The following main conclusions are drawn from this study:  
1. Even though accurate global LAI data is used, neglecting leaf clumping will overestimate global terrestrial ET by about 4.7%. The reason is that leaf clumping reduces sunlit LAI while increases shaded LAI, resulting in an overall reduction in ET.
2. If \(\mathrm{L_{e}}\) is used, neglecting leaf clumping will underestimate global terrestrial ET by 13.0%. The reason is that \(\mathrm{L_{e}}\) is lower than true LAI for a clumped canopy. If \(\mathrm{L_{e}}\) instead of true LAI is used in ET simulation ignoring leaf clumping impacts, shaded LAI would be substantially negatively biased while sunlit LAI is invariant, causing negative bias in ET.
3. Although the accuracy of global terrestrial ET simulation still needs improvements, leaf clumping impacts quantified by the RDs between the model simulation cases considering or ignoring clumping are still robust since errors of key model parameters will move in the same directions for all cases.  
This study highlights the importance of considering leaf clumping in estimating global terrestrial ET, which would be helpful to improve the quality of global terrestrial ET products derived from various land surface models. Both LAI and CI are needed for accurately estimating sunlit and shaded LAI as well as ET by the two-leaf model. This two-leaf separately ET estimation is important because stomatal conductance controlling transpiration is coupled with photosynthesis at leaf-level, of which sunlit and shaded leaves are controlled by different biochemical processes.  
For better serving the global water research, the remote sensing community should produce more accurate global CI products based on data from new satellite sensors. Physical retrieval methods should also be explored to produce high-resolution global CI products. Finally, more field measurements are also needed for the validation of global CI products, especially in tropical regions.  
Conceptualization, J.M.C. and S.W.; methodology, X.L. and B.C.; validation, B.C.; formal analysis, B.C.; investigation, B.C.; data curation, B.C., F.J., M.A.A., J.C. and X.W.; writing--original draft preparation, B.C.; writing--review and editing, J.M.C., S.W., H.F. and Y.L.; visualization, B.C. and Z.L.; supervision, S.W.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Research and Development Project of Scientific Instruments and Equipment of the Chinese Academy of Sciences, grant number YJKYYQ20190042; the Second Tibetan Plateau Scientific Expedition and Research Program (STEP), grant number 2019 QZKK1006; the National Key Research and Development Program from Ministry of Science and Technology, China, grant number 2017 YFC05038-03; and National Natural Science Foundation of China, grant number 41871342.  
The datasets analyzed in this study are managed by the Institute of Geographic Sciences and Natural Resources Research, the Chinese Academy of Sciences, and are available upon request from the corresponding author.  
We acknowledged support from SHARCNET for the computational resources.  
The authors declare no conflict of interest.