## 2 Materials and Methods
### Data Sources  
#### 2.2.1 Ground Observation Data  
There were three sources of GPP ground observation data: the first source was literature data [50]; the second was the American Flux Network dataset ([[https://fluxnet.fluxdata.org/data/fluxnet2015-dataset/](https://fluxnet.fluxdata.org/data/fluxnet2015-dataset/)]([https://fluxnet.fluxdata.org/data/fluxnet2015-dataset/](https://fluxnet.fluxdata.org/data/fluxnet2015-dataset/)) (accessed on 23 November 2023)), specifically selecting the value of \"GPP_DT_VUT_REF\"; and the third source was the China Flux Observation and Research Network ([[http://www.chinaflux.org/](http://www.chinaflux.org/)]([http://www.chinaflux.org/](http://www.chinaflux.org/)) (accessed on 31 December 2023)), which provided monthly data. The ground observation data of ET came from three sources: the American Flux Network dataset, the China Flux Observation and Research Network, and the Space Time Tripodal Data Center in China ([[https://poles.tpdc.ac.cn/](https://poles.tpdc.ac.cn/)]([https://poles.tpdc.ac.cn/](https://poles.tpdc.ac.cn/)) (accessed on 9 April 2024)). The per 30 min ET value is calculated using Formula (1), and the daily mean ET value is obtained using Formula (2) [51].  
\[ET_{i}=\frac{LE_{i}\times 60\times 30}{(2.501-0.002361\times T_{i})\times 10^{6}} \tag{1}\]  
\[ET=\frac{\sum_{i=1}^{N}ET_{i}\times 48}{N} \tag{2}\]  
where \(ET_{i}\) is the actual evapotranspiration of the flux station every 30 min (mm), \(LE_{i}\) is the latent heat flux every 30 min (W/m\({}^{2}\)), \(T_{i}\) is the air temperature (\({}^{\circ}\)C),the latent heat of vaporization is equal to 2.501 \(-\) 0.002361 \(\times\)\(T_{i}\). This equation highlights a light dependence of the latent heat of vaporization to temperature, which is expressed through the 0.002361 coefficient, and \(N\) is the number of valid data per day. In order to avoid errors caused by missing values as much as possible, only \(N\geq\) 46 valid measurement data per day are selected to participate in the calculation as the real data of ground observation.  
Finally, the GPP data of the QTP are sorted out, including observations from 22 stations over 498 station-months, while the ET data comprise 12 stations covering 302 station-months, spanning the period from 2002 to 2018. The spatial distribution of GPP and ET observation sites is relatively uniform, as illustrated in Figure 1a. These sites encompass a diverse range of vegetation types, including steppes, meadows, shrubs, and wetlands, ensuring that the observational data are representative of the study area's ecological characteristics.  
Figure 1: Schematic diagram of the geographical location of the QTP (**(a)** shows the spatial distribution of the main vegetation types on the plateau, with the black solid circle representing the GPP flux observation station and the red triangle representing the ET flux observation station; Region I represents the humid zone, Region II represents the semi-humid zone, Region III represents the semi-arid zone, and Region IV represents the arid zone; **(b)** shows the geographical location of the QTP in China, indicating the altitude of the plateau area).