# 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
Leaf Clumping Effects on the Calculation of Key Biophysical Parameters Controlling Transportation Simulations  
In this section, we explored leaf clumping effects on the calculation of key biophysical parameters controlling transpiration (T) estimation, which is the major component of ET, such as leaf-level absorbed photosynthetic photon flux density (PPFD), net radiation (\(\mathrm{R_{n}}\)), stomatal conductance to water vapor (\(\mathrm{g_{sw}}\)) and LAI. The leaf clumping effects on the calculation of these biophysical parameters were investigated for sunlit and shaded leaves in the overstory and understory canopies, respectively (Figures 10 and 11). If true LAI is used in transpiration estimation and clumping is ignored, leaf-level absorbed PPFD, Rn, \(\mathrm{g_{sw}}\), and T of the representative sunlit leaf in the overstory canopy would be negatively biased (Figure 10a,b,d,e). In contrast, leaf-level absorbed PPFD, \(\mathrm{R_{n}}\), \(\mathrm{g_{sw}}\), and T of the shaded leaf in the overstory canopy would be positively biased when true LAI is used in transpiration calculation and clumping is ignored (Figure 10a,b,d,e). For instance, in a typical conifer forest, where LAI is 4.0 and \(\Omega\) is 0.5, downward short-wave radiation above the canopy is 500 W m\({}^{-2}\) and relative humidity is 80%, leaf-level absorbed PPFD, \(\mathrm{R_{n}}\), \(\mathrm{g_{sw}}\), and T of the representative sunlit leaf in the overstory canopy  
Figure 9: Leaf clumping effects on ET components of the various PFTs. **(a)**\(\mathrm{T_{o\_sun}}\): overstory sunlit transpiration; **(b)**\(\mathrm{T_{o\_sha}}\): overstory shaded transpiration; **(c)**\(\mathrm{E_{0}}\): evaporation from intercepted liquid water in overstory canopy; **(d)**\(\mathrm{T_{u}}\): understory transpiration; **(e)**\(\mathrm{E_{u}}\): evaporation from intercepted liquid water in understory canopy; **(f)** ES: soil evaporation. 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. Gra: grass; Cro: crop; Shr: shrub; Oth: others. would be 663.7 umol m\({}^{-2}\)s\({}^{-1}\), 234.3 W m\({}^{-2}\), 4.15 mm s\({}^{-1}\) and 0.0391 g H\({}_{2}\)O m\({}^{-2}\) s\({}^{-1}\) when clumping is considered (Figure 9). However, they will be 641.0 umol m\({}^{-2}\)s\({}^{-1}\), 229.8 W m\({}^{-2}\), 4.12 mm s\({}^{-1}\), and 0.0384 g H\({}_{2}\)O m\({}^{-2}\) s\({}^{-1}\) when clumping is ignored (Figure 9). Thus, the relative errors (RE) in the estimations of leaf-level absorbed PPFD, Rn, gsw, and T for the representative sunlit leaf in the overstory canopy are \(-\)3.4%, \(-\)1.9%, \(-\)0.6%, and \(-\)1.8% when clumping is ignored. In this case, leaf-level absorbed PPFD, Rn, gsw and T of the representative shaded leaf in the overstory canopy would be 75.8 umol m\({}^{-2}\)s\({}^{-1}\), 24.3 W m\({}^{-2}\), 1.76 mm s\({}^{-1}\) and 0.0053 g H\({}_{2}\)O m\({}^{-2}\) s\({}^{-1}\) when clumping is considered (Figure 10). However, they will be 91.9 umol m\({}^{-2}\)s\({}^{-1}\), 27.9 W m\({}^{-2}\), 1.98 mm s\({}^{-1}\), and 0.0061 g H\({}_{2}\)O m\({}^{-2}\) s\({}^{-1}\) when clumping is ignored (Figure 10). Thus, the relative errors in the estimations of leaf-level absorbed PPFD, Rn, gsw, and T for the representative shaded leaf in the overstory canopy are 21.2%, 14.7%, 12.8%, and 15.1% when clumping is ignored. The positive biases in these variables for the shaded leaf in the overstory are much higher than the negative biases for the sunlit leaf in the overstory. In this case, leaf-level absorbed PPFD, Rn, gsw, and T for understory sunlit and shaded leaves would be negatively biased when true LAI is used in transpiration estimation and clumping is ignored (Figure 10). Meanwhile, understory sunlit LAI would be negatively biased while understory shaded LAI would be positively biased when true LAI is used in transpiration calculation and clumping is ignored (Figure 10). In this case, canopy-scale understory sunlit and shaded T would be both negatively biased (Figure 10f).