## 2 Data and Methods
### Data Sources
#### 2.2.2 Satellite Data  
By comprehensively comparing the SIF satellite products that are currently the most widely used and have been proven to have good applicability [13; 16; 17; 18; 25], we have preliminarily selected the SIF satellite product generated based on the two most widely used satellite platforms (OCO_2 and Sentinel-5P). These SIF satellite products have a complete time cycle, time/spatial resolution, and coverage range, which can perfectly match the research area and time period. It mainly includes four satellite products as follows: CSIF [19; 20], RTSIF [21; 22], GOSIF [23], and SIF_OCO_005 [24] (Table 2).  
The leaf area index (LAI) data for this study were obtained from the HIQ-LAI satellite product data set on the Google Earth Engine platform. This data set was created by [PERSON] et al. [37], using the spatiotemporal informative component analysis (STICA) algorithm, which reanalyzed nearly 22 years of MODIS C6.1 LAI products. The data set has an 8-day temporal scale, and each year includes 46 TIFF format files at a resolution of 500 m.  
The land use data for this study were sourced from the Chinese Academy of Sciences Resource and Environment Science Data Registration and Publishing System. This data set was produced by [PERSON] et al. [38], through manual visual interpretation of Landsat 8 remote sensing images to generate the data set. The data set comprises 25 secondary types and offers raster data at resolutions of 1000 m, 100 m, and 30 m, with this study utilizing data at a resolution of 30 m.  
This study used the observation data from the aforementioned satellites products in 2020.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station Name** & **Abbreviation** & **Coordinates** & **Elevation** & **Region** & **Underlying Surface** \\ \hline \begin{tabular}{c} Ulan Usu Land- \\ Atmosphere Interaction \\ Observation Station \\ \end{tabular} & Ulan Usu Station & \begin{tabular}{c} 44\({}^{\circ}\)16\({}^{\prime}\)59\({}^{\prime\prime}\) N \\ 85\({}^{\circ}\)49\({}^{\prime}\)00\({}^{\prime\prime}\) E \\ \end{tabular} & 406.9 m & \begin{tabular}{c} The northern slope \\ of Tianshan \\ Mountains \\ \end{tabular} & \begin{tabular}{c} Cultivated and \\ farmland area [32] \\ \end{tabular} \\ \begin{tabular}{c} Central Tianshan Land- \\ Atmosphere Interaction \\ Observation Station \\ \end{tabular} & \begin{tabular}{c} Ulastai \\ Station \\ \end{tabular} & \begin{tabular}{c} 43\({}^{\circ}\)28\({}^{\prime}\)55\({}^{\prime\prime}\) N \\ 87\({}^{\circ}\)12\({}^{\prime}\)50\({}^{\prime\prime}\) E \\ \end{tabular} & 2036 m & \begin{tabular}{c} The central \\ hinterland of the \\ Tianshan Mountains \\ \end{tabular} & \begin{tabular}{c} Pasture grassland \\ area [33; 34] \\ \end{tabular} \\ \begin{tabular}{c} Kelameili Land- \\ Atmosphere Interaction \\ Observation Station \\ \end{tabular} & \begin{tabular}{c} Kelameili Station \\ 87\({}^{\circ}\)35\({}^{\prime}\)00\({}^{\prime\prime}\) E \\ \end{tabular} & 531 m & \
\begin{tabular}{c} Gurbantunggut \\ Desert \\ \end{tabular} &
\begin{tabular}{c} Desert vegetation \\ area [35; 36] \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic information of different observation stations.