## 2 Method
### GPP Product  
As mentioned in the introduction, since large inconsistencies exist in current GPP products, five sets of GPP products were collected to ensure the robustness of our research. The available data included the optimum light use efficiency (LUEopt) product, the GPPNIRv product, the Global Land Surface Satellite (GLASS) product, the FluxCom product, and the global Orbiting Carbon Observatory-2 solar-induced chlorophyll fluorescence (GOSIF) product. The individual product information for each product used in this study can be found in Table 1, as detailed below. The LUEopt product is obtained from The Oak Ridge National Laboratory ([[https://earthdata.nasa.gov/](https://earthdata.nasa.gov/)]([https://earthdata.nasa.gov/](https://earthdata.nasa.gov/)), accessed on 17 October 2021). Based on the light use efficiency (LUE) models, this product is improved by the optimum LUE originating from the global FLUXNET network [37] and is proven to obtain a better understanding of the response of primary productivity to climatic variation [38]. [PERSON] et al. [10] analyzed and compared 45 global GPP products and found that the estimated value of LUEopt on the seasonal scale was closer to the average value of the 45 sets of products compared with the other products we collected. Considering that analysis on a seasonal scale is an important part of this study, and LUEopt itself has relatively high spatio-temporal resolution, it was mainly used for analysis. The spatial distribution of the annual average GPP of this product during the period 1982-2015 is shown in Figure 1b. Among the remaining GPP products for verifying the reliability of the analysis results, the GPPNIRv product was obtained from the National Tibetan Plateau Data Center ([[http://data.tpdc.ac.cn/en/](http://data.tpdc.ac.cn/en/)]([http://data.tpdc.ac.cn/en/](http://data.tpdc.ac.cn/en/)), accessed on 17 October 2021). This dataset provides global long-term GPP based on the strong correlation between near-infrared reflectance (NIRv) and GPP, demonstrating the ability of NIRv in capturing the long-term trends of GPP [39]. The GLASS product was provided by the National Earth System Science Data Center ([[http://www.geodata.cn/](http://www.geodata.cn/)]([http://www.geodata.cn/](http://www.geodata.cn/)), accessed on 25 October 2021). With the characteristics of satisfactory precision, high resolution, and a wide range of spatial-temporal coverage, this product provides powerful assistance for the study of global ecological and environmental changes [40]. The FluxCom product was downloaded from the FLUXCOM initiative ([[http://fluxcom.org/](http://fluxcom.org/)]([http://fluxcom.org/](http://fluxcom.org/)), accessed on 29 October 2021). It combines remote sensing and meteorological data based on a variety of machine learning algorithms, such as artificial neural network and  
Figure 1: The spatial distribution of (**a**) vegetation type and (**b**) annual mean of GPP from 1982 to 2015 in each river basin. Vegetation-type data are available from NASA LP DAAC ([[https://lpdaac.usgs.gov/](https://lpdaac.usgs.gov/)]([https://lpdaac.usgs.gov/](https://lpdaac.usgs.gov/)), accessed on 11 October 2021). The box-plot in (**b**) represents the interquartile range (box), median (line within the box), and whiskers of the annual mean GPP in different basins. The nan value is displayed in white. random forest, and generates global flux tower GPP products through eddy covariance technology [41]. The GOSIF product is available from the Global Ecology Group ([[https://globalecology.unh.edu/data/GOSIF.html](https://globalecology.unh.edu/data/GOSIF.html)]([https://globalecology.unh.edu/data/GOSIF.html](https://globalecology.unh.edu/data/GOSIF.html)), accessed on 2 November 2021). This product is generated based on the solar-induced chlorophyll fluorescence (SIF) retrieved from the Orbiting Carbon Observatory-2 (OCO-2). SIF is regarded as an indicator of the functional state of actual plant photosynthesis [42].