## 4 Discussion
### Analysis of the Results of GPP Simulations of Typical Vegetation Types  
The interannual GPP spatiotemporal trends of change for typical vegetation types in the YRB were in the following order: evergreen needle leaf forests > two crops a year > deciduous broad leaf forests > one crop a year > shrubs > temperate grasses > alpine meadows. Forest vegetation generally exhibited a higher annual average GPP, having a stronger capacity for absorbing atmospheric CO\({}_{2}\). The GPP values of crop vegetation were mostly larger than those of shrubs and alpine meadows. And the GPP values of two crops a year were only smaller than those of evergreen needle leaf forests. The main reason is the cultivation of maize (a C4 crop) during the 180 to 300-day period. Compared to C3 plants, C4 plants (using the Hatch and Slack pathway) exhibit higher photosynthetic rates and greater water, nitrogen, and radiation use efficiency (WUE, NUE, and RUE, respectively) [47; 48]. Furthermore, C4 plants are better adapted to warm and arid climates than C3 plants. [49]. As a result, the annual average GPP of two crops a year was so high. The annual average GPP of shrubs was slightly higher than that of grassland vegetation. Due to climatic conditions, alpine meadows had consistently low GPP levels over multiple years. The vegetation in the Yellow River Basin was predominantly composed of temperate grasses, one crop a year, and alpine meadows, resulting in a relatively low average GPP for the entire region.  
By comparing the annual average simulated GPP of various vegetation types in this study with the results of previous research (Table 3), it could be found that the simulated values of temperate grass in this study were slightly higher than the annual average GPP of this vegetation type obtained by [50] based on Flux-net data. Ref. [51] indicated that the annual average GPP of the Tibetan Plateau was 312.3 g Cm\({}^{-2}\) y\({}^{-1}\). The spatial distribution pattern of the GPP on the Tibetan Plateau showed a gradual increase from west to east. Therefore, the annual average value of alpine meadows in the Yellow River Basin should be more than 312.3 g Cm\({}^{-2}\) y\({}^{-1}\). For other vegetation types in this study, such as shrubs, evergreen needle leaf forests, and deciduous broad leaf forests, the simulated annual average GPP values generally fell within the range of results from previous studies. The multi-year observational data from Flux-net sites on one crop a year and two crops a year in the study by [50] closely agreed with the simulated annual average GPP values of this study. By combining the separately simulated annual average GPP values for wheat and maize from [PERSON] et al. [43], it could be observed that their results also closely aligned with the results of this study. In summary, this paper argues that the simulated GPP values for various representative vegetation types from 2000 to 2021 in this study were reasonably accurate. The calibrated and validated vegetation parameters could effectively simulate the GPP of multiple typical vegetation types in the Yellow River Basin.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & \multicolumn{3}{c}{**Multi-Year Average GPP**} \\  & \multicolumn{3}{c}{**(g C m\({}^{-2}\) y\({}^{-1}\))**} \\ \hline \multirow{2}{*}{**Vegetation Type**} & **Simulation of This Study** & **Simulation of Other Studies** & **Reference** \\ \hline Temperate Grass & 658.10 & 542.0 & [50] \\ \hline \multirow{2}{*}{Alpine Meadow} & \multirow{2}{*}{471.80} & 312.3 & [51] \\  & & 461.6 & [52] \\ \cline{1-1}  & & 712.0 & [53] \\ \hline Shrub & 869.34 & 744.0–867.0 & [54] \\ \hline \hline \end{tabular}
\end{table}
Table 3: The comparison of results between this study and previous studies.