## 2 Data and Methodology
### Research Methodology
#### 2.3.3 Vegetation NPP Simulation and Scenario Experiments  
The improved CASA (Carnegie-Ames-Stanford Approach) model, as refined by [PERSON] et al. [52; 53] through regional-specific parameter adaptations, was used to calculate the Net Primary Productivity (NPP) values to analyze the influence mechanism of drought stress and afforestation projects on the NPP. The model parameters adopted in this study are based on [PERSON] et al. 's refinements, which underwent rigorous validation against flux tower observations in their original work, confirming strong applicability and accuracy across China. The model substantially enhanced NPP simulation accuracy through implementation of vegetation-type-specific response functions coupled with a dynamic regional evapotranspiration correction algorithm [52]. The model core equation is expressed as:  
\[\text{NPP}(\text{x},\text{t})=\text{APAR}(\text{x},\text{t})\times\varepsilon( \text{x},\text{t}) \tag{5}\] \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multicolumn{1}{c}{**Dominant Driver Scenario**} & \multicolumn{1}{c}{\(\mathbf{S_{NDVI_{obs}}}\)} & \multicolumn{1}{c}{\(\mathbf{S_{NDVI_{pre}}}\)} & \multicolumn{1}{c}{\(\mathbf{S_{NDVI_{res}}}\)} & \multicolumn{1}{c}{
\begin{tabular}{c} **Relative** \\ **Contribution of** \\ **Climate Change** \\ \end{tabular} } & \multicolumn{1}{c}{\(\mathbf{Relative}\)} & \multicolumn{1}{c}{\(\mathbf{Relative}\)} \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{\(\mathbf{Contribution}\)} & \multicolumn{1}{c}{\(\mathbf{Contribution}\)} & \multicolumn{1}{c}{\(\mathbf{Contribution}\)} & \multicolumn{1}{c}{\(\mathbf{Contribution}\)} \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{\(\mathbf{Climate}\)-driven vegetation gain} & \(>\)0 & \(>\)0 & \(<\)0 & 100\% & 0\% \\ Anthropogenically promoted vegetation gain & \(>\)0 & \(<\)0 & \(>\)0 & 0\% & 100\% \\ Climate-anthropogenic & \(>\)0 & \(>\)0 & \(>\)0 & \(\left|\text{S}_{\text{NDVI}_{\text{pre}}}\right|/\left|\text{S}_{\text{NDVI}_{ \text{obs}}}\right|\) & \(\left|\text{S}_{\text{NDVI}_{\text{res}}}\right|/\left|\text{S}_{\text{NDVI}_{ \text{obs}}}\right|\) \\ Synergistic vegetation gain & \(<\)0 & \(<\)0 & \(>\)0 & 100\% & 0\% \\ Climate-driven vegetation & \(<\)0 & \(>\)0 & \(<\)0 & 0\% & 100\% \\ Anthropogenic disturbed vegetation degradation & \(<\)0 & \(>\)0 & \(<\)0 & 0\% & 100\% \\ Climate-anthropogenic & \(<\)0 & \(<\)0 & \(<\)0 & \(\left|\text{S}_{\text{NDVI}_{\text{pre}}}\right|/\left|\text{S}_{\text{NDVI}_{ \text{obs}}}\right|\) & \(\left|\text{S}_{\text{NDVI}_{\text{res}}}\right|/\left|\text{S}_{\text{NDVI}_{ \text{obs}}}\right|\) \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \end{tabular}
\end{table} 
Table 1: Scenarios for assessing driving mechanisms for the relative roles of climate change and human activities. where APAR(x,t) is the Photosynthetically Active Radiation (PAR) absorbed by image x in month t (gC\(\cdot\)m\({}^{-2}\)-month\({}^{-1}\)), and \(\varepsilon(x,t)\) denotes the actual light energy utilization of image x in month t (gC\(\cdot\)m\({}^{-1}\)) [53]. By designing four sets of control experiments (Table 2), the independent and interactive effects of drought and afforestation projects on NPP were systematically quantified. The baseline scenario S1 used actual observed precipitation (P), temperature (T), NDVI, and PAR. The S2 scenario replaced climatic parameters with climatic averages from 2000 to 2020 to eliminate extreme drought fluctuations. The S3 scenario fixed NDVI as the 2000-2005 base period mean to simulate the vegetation response to the termination.