## 3 Results and Discussion  
### Annual Scale  
#### 3.1.1 Evaptotranspiration (ET)  
As shown in Table 2 and Figure 2a, the model generally underestimates annual mean ET. The overall simulated annual mean ET is 405.39 mm yr\({}^{-1}\), compared to the observed mean of 476.67 mm yr\({}^{-1}\), resulting in an absolute error of 71.28 mm yr\({}^{-1}\) and a relative error of 14.95%. The overall correlation coefficient (R) is 0.66, indicating a moderate positive linear relationship, but the RMSE of 188.99 mm yr\({}^{-1}\) suggests considerable spread. With the exception of DBF, MF, CSH, and CRO, the simulated values are lower than the observations for most vegetation types. The largest underestimation occurs for ENF, with a relative error of 33.26%. This bias may arise from inadequate representation of vegetation transpiration processes, potentially linked to errors in the parameterization of canopy radiation scattering, canopy interception, or the rooting depth of plants [32]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**IGBP** & **Site-Year \({}^{\star}\)** & **Observed** & **Modelled** & **Absolute** & **Relative** & **Correlation** & **RMSE/** **mm yr\({}^{-1}\)** & **\(p\)-Value** \\
**Types** & **Site-Year \({}^{\star}\)** & **ET** & **ET** & **Error/** & **Error/\%** & **Coefficient** & **mm yr\({}^{-1}\)** & **\(p\)-Value** \\ \hline ENF & 225 & 483.50 & 322.71 & 160.79 & 33.26 & 0.21 & 237.78 & 0.0019 \\ EBF & 45 & 716.77 & 699.05 & 17.72 & 2.47 & 0.89 & 207.27 & -0.001 \\ DNF & 3 & 302.13 & 396.04 & 93.91 & 31.08 & \(-\)0.37 & 100.61 & 0.7555 \\ DBF & 55 & 406.94 & 408.41 & 1.47 & 0.36 & 0.44 & 82.74 & -0.001 \\ MF & 35 & 340.62 & 405.27 & 64.65 & 18.98 & 0.24 & 113.26 & 0.1676 \\ CSH & 22 & 463.09 & 435.19 & 27.90 & 6.02 & 0.93 & 109.39 & -0.001 \\ OSH & 4 & 340.13 & 294.32 & 45.81 & 13.47 & 0.45 & 64.42 & 0.545 \\ WSA & 45 & 664.80 & 500.85 & 163.95 & 24.66 & 0.95 & 284.89 & -0.001 \\ SAV & 34 & 536.71 & 401.12 & 135.59 & 25.26 & 0.90 & 196.18 & -0.001 \\ GRA & 149 & 425.90 & 384.26 & 41.64 & 9.78 & 0.72 & 134.56 & -0.001 \\ WET & 24 & 394.16 & 344.54 & 49.62 & 12.59 & 0.54 & 119.49 & 0.0068 \\ CRO & 56 & 417.06 & 503.64 & 86.58 & 20.76 & 0.18 & 140.32 & 0.1862 \\ All & 697 & 476.67 & 405.39 & 71.28 & 14.95 & 0.66 & 188.99 & -0.001 \\ \hline \end{tabular} \({}^{\star}\)Site-Year: Total number of observation years summed across all sites within each vegetation type.  
\end{table} 
Table 2: Quantitative evaluation of annual ET simulations against FLUXNET observations across \(12\) vegetation types defined in Section 2.2. Performance varies significantly across vegetation types. CSH shows the best performance with a high correlation (R = 0.93, \(p\) < 0.001) and low relative error (6.02%), indicating excellent agreement. DNF performs the worst, with a negative correlation (R = \(-\)0.37, \(p\) = 0.7555, not significant), significant overestimation (observed 302.13 mm yr\({}^{-1}\) vs. simulated 396.04 mm yr\({}^{-1}\), relative error 31.08%), and high variability. The lack of significance for OSH (\(p\) = 0.545) and DNF might be due to small sample sizes. For most other types (DBF, CSH, WSA, SAV, GRA, WET, CRO), the \(p\)-values are <0.001, indicating statistically significant confidence levels. The significant underestimation of ET in ENF, with a relative error of 33.26%, likely stems from inaccuracies in the LAI simulations for ENF by the default Dynamic Vegetation (DVEG) module within the Noah-MP model. As a critical parameter governing canopy transpiration and interception, biases in DVEG's LAI simulations are directly propagated into the ET estimates. Moreover, ENFs are characterized by complex canopy structures. Deficiencies in the model's simulation of intra-canopy radiation transfer (specifically under RAD option 1, a three-dimensional canopy morphology scheme) and energy partitioning may further contribute to the underestimation of both canopy interception evaporation and vegetation transpiration [33]. Conversely, ET simulations for CSH exhibited the highest accuracy.