## 3 Results and Discussion
### Monthly Scale  
#### 3.2.1 Evapotranspiration (ET)  
Figure 3 illustrates the multi-year average seasonal cycle of simulated and observed monthly ET, along with the standard deviation across sites for each month. The model generally captures the monthly variation well. Most vegetation types exhibit a \"single-peak\" seasonal distribution (seasons defined by the Northern Hemisphere: Spring MAM, Summer JJA, Autumn SON, Winter DJF; the Southern Hemisphere shifted by 6 months), \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**IGBP** & **Site-Year \({}^{\ast}\)** & **Observed** & **Modelled** & **Absolute** & **Relative** & **Correlation** & **RMSE/** & **\(p\)-Value** \\
**Types** & **Site-Year \({}^{\ast}\)** & **GPP/** & **GPP/** & **Error/** & **Eror/** & **Coefficient** & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **\(p\)-Value** \\  & & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\)** & **Error/** & **Coefficient** & **RMSE/** \\ \hline ENF & 229 & 1412.73 & 1256.74 & 155.99 & 11.04 & 0.73 & 410.1 & \textless{}0.001 \\ EBF & 45 & 1798.20 & 2558.12 & 759.92 & 42.26 & 0.58 & 969.62 & \textless{}0.001 \\ DNF & 3 & 1136.76 & 1553.96 & 417.2 & 36.70 & \(-\)0.09 & 418.91 & 0.9428 \\ DBF & 58 & 1675.78 & 1764.98 & 89.20 & 5.32 & 0.13 & 363.54 & 0.3342 \\ MF & 35 & 1586.20 & 1681.39 & 95.19 & 6.00 & 0.14 & 334.56 & 0.4111 \\ CSH & 22 & 922.32 & 1107.31 & 184.99 & 20.06 & 0.97 & 240.9 & \textless{}0.001 \\ OSH & 4 & 472.18 & 727.10 & 254.92 & 53.99 & \(-\)0.72 & 284.51 & 0.2793 \\ WSA & 45 & 1128.52 & 1299.12 & 170.60 & 15.12 & 0.9 & 366.37 & \textless{}0.001 \\ SAV & 34 & 831.80 & 748.25 & 83.55 & 10.04 & 0.79 & 349.79 & \textless{}0.001 \\ GRA & 143 & 1090.49 & 1008.17 & 82.32 & 7.55 & 0.73 & 573.77 & \textless{}0.001 \\ WET & 24 & 893.20 & 947.08 & 53.88 & 6.03 & 0.81 & 159.54 & \textless{}0.001 \\ CRO & 56 & 1161.88 & 1665.09 & 503.21 & 43.31 & 0.58 & 634.11 & \textless{}0.001 \\ All & 698 & 1295.48 & 1346.85 & 51.37 & 3.97 & 0.67 & 500.62 & \textless{}0.001 \\ \hline \hline \end{tabular} \({}^{\ast}\)Site-Year: Total number of observation years summed across all sites within each vegetation type.  
\end{table} 
Table 3: Quantitative evaluation of annual GPP simulations against FLUXNET observations across 12 vegetation types defined in Section 2.2.  
with peaks typically occurring in summer (June-August) and troughs in winter (December-February). The simulated seasonal fluctuations align reasonably well with the observations, suggesting the model responds appropriately to the seasonal dynamics of climate drivers (mainly radiation, temperature, and precipitation). However, for many vegetation types (e.g., WSA, CRO, and ENF), the simulation bias for summer ET is generally larger than for winter ET. This indicates that the model's ability to capture inter-site ET heterogeneity decreases during seasons with high energy input, leading to greater uncertainty under high radiation and strong transpiration conditions. The standard deviation (represented by bar height in Figure 3) also shows seasonal differences. For most types, the standard deviation among sites is larger in summer than in winter, implying greater variation in simulation performance across different locations during the peak growing season. EBF is an exception, showing larger standard deviation in winter. Figure 4 presents scatter plots comparing simulated and observed monthly mean ET across sites for each vegetation type. The correlation for seasonal ET variation is generally high, with R values above 0.8 (often >0.9) for all vegetation types except ENF. For OSH and WSA, simulated monthly means are consistently lower than observed means (points below the 1:1 line).