# Evaluation of Clumping Effects on the Estimation of Global Terrestrial Evapotranspiration
## 5 Results
### Leaf Clumping Effects on ET Estimation and Its Components for Various PFTs  
Based on Case I, leaf clumping effects on estimations of global terrestrial ET as well as its components have been assessed for various PFTs (Figure 8). The discrepancy of ET is 18.3 mm yr\({}^{-1}\) (4.7%) between Case II and Case I and \(-\)48.3 mm yr\({}^{-1}\) (\(-\)13.0%) between Case III and Case I (Figure 8). ET of Case II is always larger than those of Case I for all PFTs (Figure 8). Evergreen needleleaf forests (ENF) have the largest relative differences (RD) between Case II and Case I because they are most clumped with the lowest CI on average (Figure 8). In contrary to Case II, ET is underestimated for all PFTs in Case III. The reason is that the effective LAI of a highly clumped canopy is much smaller than its true LAI. Deciduous broadleaf forests (DBF) have the largest RD between Case III and Case I because DBF has both high LAI and low CI compared to other PFTs (Figure 8). If effective LAI is used ignoring clumping, shaded LAI is much smaller in Case III than that in Case I, especially for high LAI and low CI, while the sunlit LAI is unchanged (Equation (9)). Thus, the ET of shaded leaves in Case III is lower than those in Case I so that the total ET is underestimated for Case III. ET of Case III is underestimated considerably due to the large fractions of shaded T\({}_{\text{o}}\) in ET. The fractions of shaded T\({}_{\text{o}}\) to total ET are 23.2%, 17.0%, 17.3%, 18.7%, 20.1%, 4.1%, 9.4%, 4.4% and 8.3% to ET for ENF, DNF, DBF, EBF, MF, grass, crop, shrubs and others. The discrepancy in ET between Case III and Case I is more than that between Case II and Case I because ET is more biased due to the underestimation of shaded LAI in Case III than that caused by the overestimation of sunlit LAI in Case II. This is consistent with what [PERSON] et al. [14] found by assessing leaf clumping effect on ET estimates over eight forest sites in North America. Figure 7: The spatial distribution of bias errors in global terrestrial ET estimates for Case III. Leaf clumping effects on ET components of the various PFTs were further investigated in Figure 9. Overstory sunlit transpiration (T\({}_{o\_sun}\)) values of Case II are always higher than those of Case I (Figure 9a) because overstory sunlit LAI is overestimated in Case II. Overstory shaded transpiration (T\({}_{o\_sha}\)) values of Case II are all close to those of Case I (Figure 9a). This is because the overestimation of leaf-level T\({}_{o\_sha}\) is offset by the underestimation of overstory shaded LAI in Case II. T\({}_{o\_sun}\) values of Case III are almost the same as those of Case I (Figure 9a) because both leaf-level T\({}_{o\_sun}\) and overstory sunlit LAI are unchanged in Case III. T\({}_{o\_sha}\) values of Case III are always lower than those of Case I (Figure 9b) because the underestimation of overstory shaded LAI is more significant than the overestimation of leaf-level T\({}_{o\_sha}\) in Case III. E\({}_{o}\) values of Case II are always higher than those of Case I while E\({}_{o}\) values of Case III are always lower than those of Case I (Figure 9c). T\({}_{u}\) values of Case II and Case III are both lower than those of Case I for most of the cases. Figure 8: BEPS simulated ET of the various PFTs (**a**); the relative differences (RD) of Case II and Case III were both compared to Case I (**b**). CI and LAI of various PFTs (**c**). Case I: the true LAI and CI are used; Case II: the true LAI is used while clumping is ignored; Case III: effective LAI is used ignoring clumping. ENF: evergreen needleleaf forests; DNF: deciduous needleleaf forests; DBF: deciduous broadleaf forests; EBF: evergreen broadleaf forests; MF: mixed forests. the PFTs (Figure 9d). \(\mathrm{E_{u}}\) and ES values of Case II and Case III are both lower than those of Case I for all the PFTs (Figure 9e,f).