# Evaluation of Clumping Effects on the Estimation of Global Terrestrial Evapotranspiration
## 5 Results  
### Simulations of Global Terrestrial ET  
Using the various land cover, meteorology, and soil inputs, a global annual ET map for 2015 is produced at one-degree resolution for each of the three cases, based on the hourly BEPS simulation. The global ET map of Case I is considered as representing the reality (Figure 5). The highest ET appeared in the tropics while low ET appeared in the northern high latitudes, high-altitude mountains, and arid regions (i.e., Australia, central  
Figure 4: A global CI map on 4–11 July 2015 derived from MODIS BRDF data at 500 m resolution as an example.  
Asia, the western US, and the Sahel). The mean ET variations with latitude are also shown in Figure 4. The highest ET (933.4 \(\pm\) 472.5 mm yr\({}^{-1}\)) occurs at the latitude of 1\({}^{\circ}\)N and ET gradually decreases with the increase of latitude to the North or the South (Figure 5). The variation of annual ET with latitude is large due to strong latitudinal variations in meteorological conditions, vegetation types, and densities.  
The ET averaged over the global land surface is 511.9 \(\pm\) 70.1 mm yr\({}^{-1}\), where the uncertainty \(\pm\) 70.1 mm yr\({}^{-1}\) is the sum of the uncertainty (\(\pm\)26.7 mm yr\({}^{-1}\)) resulting from the input vegetation data as well as model parameters, and the standard deviation (\(\pm\)43.4 mm yr\({}^{-1}\), Figure 8) of the annual global ET during 2013 to 2016 resulting from meteorological variations. The uncertainty from meteorological variations is calculated using the same model parameters and the same LAI and CI input dataset in 2015 but actual hourly meteorological data from 2013 to 2016. The uncertainty resulting from the input vegetation data and model parameters is approximated by the range of global terrestrial ET variation caused by LAI uncertainty of 20% (see Table 2) [23].  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **Case I a** & **Case II** & **Case III** & **Case II–Case I b** & **Case III–Case I b** \\ \hline Baseline c & 511.9 & 535.9 & 445.6 & 24.0 (4.7\%) & \(-\)66.3 (\(-\)13.0\%) \\
1.2*LAI & 541.6 (5.8\%) & 565.0 & 473.9 & 23.4 (4.3\%) & \(-\)67.7 (\(-\)12.5\%) \\
0.8*LAI & 476.6 (\(-\)6.9\%) & 501.1 & 413.3 & 24.6 (5.2\%) & \(-\)63.3 (\(-\)13.3\%) \\
1.2*\(\Omega\) & 521.6 (1.9\%) & 535.9 & 476.4 & 14.3 (2.7\%) & \(-\)45.3 (\(-\)8.7\%) \\
0.8*\(\Omega\) & 499.1 (\(-\)2.5\%) & 535.9 & 409.9 & 36.8 (7.4\%) & \(-\)89.2 (\(-\)17.9\%) \\
1.2*\(V_{max0}^{25}\) & 525.2 (2.6\%) & 551.6 & 458.7 & 26.4 (5.0\%) & \(-\)66.5 (\(-\)12.7\%) \\
0.8*\(V_{cmax0}^{25}\) & 494.5 (\(-\)3.4\%) & 515.9 & 429.1 & 21.4 (4.3\%) & \(-\)65.4 (\(-\)13.2\%) \\
1.2*RASM\({}_{\rm th}\) & 503.7 (\(-\)1.6\%) & 526.0 & 437.5 & 22.3 (4.4\%) & \(-\)66.2 (\(-\)13.1\%) \\
0.8*RASM\({}_{\rm th}\) & 522.6 (2.1\%) & 548.5 & 456.1 & 25.9 (5.0\%) & \(-\)66.6 (\(-\)12.7\%) \\ \hline \end{tabular}  
* The percent in the parentheses of the 2 nd column represents RD to the baseline ET. b The percent in the parentheses of the last two columns represents RD to ET of Case I. c The baseline is the ideal Case shown in Figure 7.  
\end{table}
Table 2: Sensitivities of global terrestrial ET (mm yr\({}^{-1}\)) to errors in key parameters such as LAI, \(\Omega\), \(V_{cmax0}^{25}\), and \(RASM_{th}\).  
Figure 5: Global ET map of Case I in 2015 simulated by the BEPS model.