## 4 Results  
### Accuracy Assessment  
The results showed that the AVPM had higher accuracy and significantly outperformed the MOD17 from 2012 to 2019. The AVPM significantly reduced the bias and made the fluctuation of the estimated NPP closer to the field sampling data. The average bias of the MOD17 was 120.2 gCm\({}^{-2}\)yr\({}^{-1}\), and the average bias of the AVPM was \(-9.8\) gCm\({}^{-2}\)yr\({}^{-1}\), which was reduced by 91.8%. The AVPM significantly reduced the MAPE, and the degree to which the estimated NPP deviates from the field sampling value was greatly reduced. The average MAPE of the MOD17 was 179.0%, and the average MAPE of the AVPM was 57.0%, which was reduced by 68.2%. Further, the AVPM improved the correlation between the observed and estimated data. The average \(r\) of the MOD17 was 0.3698, and the average \(r\) of the AVPM was 0.4809, which was improved by 30.0% (Table 1).  
The AVPM improved the year-to-year generalization of the model's estimation. The maximum bias of the MOD17 was 178.8 gCm\({}^{-2}\)yr\({}^{-1}\). The minimum bias of the MOD17 was 72.7 gCm\({}^{-2}\)yr\({}^{-1}\). The difference was 106.1 gCm\({}^{-2}\)yr\({}^{-1}\), while the maximum bias of the AVPM was 10.8 gCm\({}^{-2}\)yr\({}^{-1}\). The minimum bias of the AVPM was \(-27.7\) gCm\({}^{-2}\)yr\({}^{-1}\). The difference was 38.5 gCm\({}^{-2}\)yr\({}^{-1}\). The different range of the AVPM was only 36.3% of the MOD17. The maximum MAPE of the MOD17 was 256.6%. The minimum MAPE of the MOD17 was 135.5%, and the difference was 121.1%, while the maximum MAPE of the AVPM was 76.3%. The minimum MAPE of the AVPM was 49.7%, and the difference was 26.6%. The difference range of the AVPM was 19.6% of the MOD17. The maximum \(r\) of the MOD17 was 0.6195. The minimum \(r\) of the MOD17 was 0.3134, and the difference was 0.2061, while the maximum \(r\) of the AVPM was 0.5982. The minimum r of the AVPM was 0.3709, and the difference was 0.2273, which was similar to the MOD17 (Figure 2).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Year**} & \multicolumn{3}{c}{**Bias (gCm\({}^{-2}\)yr\({}^{-1}\))**} & \multicolumn{3}{c}{**MAPE (\%)**} & \multicolumn{3}{c}{**r**} & \multirow{2}{*}{**n**} \\ \cline{2-2} \cline{5-8}  & **MOD17** & **AVPM** & **MOD17** & **AVPM** & **MOD17** & **AVPM** & \\ \hline
2012 & 103.3 & 8.1 & 146.4 & 56.2 & 0.4550 & 0.5793 & 189 \\
2013 & 91.9 & \(-27.7\) & 149.9 & 49.7 & 0.5195 & 0.5982 & 188 \\
2014 & 72.7 & \(-18.8\) & 135.5 & 54.7 & 0.4274 & 0.5325 & 194 \\
2015 & 112.3 & \(-20.6\) & 178.5 & 51.0 & 0.3134 & 0.4600 & 199 \\
2016 & 178.8 & \(-0.7\) & 256.6 & 56.3 & 0.4466 & 0.5207 & 199 \\
2017 & 146.5 & \(-5.4\) & 202.6 & 54.0 & 0.3260 & 0.3709 & 196 \\
2018 & 126.8 & 10.8 & 181.0 & 76.3 & 0.3618 & 0.4491 & 178 \\
2019 & 127.1 & \(-23.6\) & 177.3 & 59.0 & 0.4091 & 0.4661 & 184 \\ Average & 120.2 & \(-9.8\) & 179.0 & 57.0 & 0.3698 & 0.4809 & 1527 \\ \hline \end{tabular}
\end{table}
Table 1: Accuracy assessment of the AVPM model and MOD17 NPP from 2012 to 2019 and their bias, MAPE, and r, respectively. MAPE: mean absolute percent error; r: [PERSON]’s correlation coefficient; AVPM: alpine vegetation photosynthesis model.  
Figure 2: The average bias, mean absolute percentage error (MAPE), and Pearson’s correlation coefficient (r) from 2012 to 2019 for the MOD17 and AVPM. The black arrows indicate the variation range of the MOD17’s accuracy index. The red arrows indicate the variation range of the AVPM’s accuracy index.