## 2 Study Area and Dataset
### Dataset  
#### 2.2.1 Meteorological Forcing and Land Surface Initial Conditions for the Biome-BGC Model  
In order to estimate the GPP of alpine grasslands at the regional scale, the Biome-BGC model needs spatially explicit forcing data and a vegetation distribution map. The forcing data used to run the Biome-BGC model includes daily meteorological data, land surface initialization data, and ecophysiological parameters. Among them, ecophysiological parameters need to be carefully calibrated to adapt the model to particular applications. For alpine meadows and alpine steppes, both of them are classified as C3 grass in the Biome-BGC model and contains a total of 28 ecophysiological parameters (Table 1). In this study, we used the EC-derived GPP to calibrate these ecophysiological parameters and evaluate the simulation performance of the Biome-BGC model. A detailed description of these data is as follows. In addition, all of the following spatially explicit data was resampled to 5 km \(\times\) 5 km resolution. Figure 1: Spatial distributions of grassland subtypes and elevations on the Tibetan Plateau (TP). (**a**) Grassland subtypes; (**b**) Elevation. (1) Daily meteorological data used to drive the Biome-BGC model mainly includes precipitation, temperature, shortwave radiation, day length, and vapor pressure deficit (VPD). They were calculated from the China Meteorological Forcing Dataset ([[http://westdc.westgis.ac.cn](http://westdc.westgis.ac.cn)]([http://westdc.westgis.ac.cn](http://westdc.westgis.ac.cn))) using the MT-CLIM tool. Among them, the China Meteorological Forcing Dataset has a 0.1\({}^{\circ}\) spatial resolution and a daily temporal resolution, spanning the period from 1982 to 2015, and the MT-CLIM tool is a program released with the Biome-BGC that can be used to estimate missing daily meteorological data that is not available from in situ measurements [33]. In this study, we used the MT-CLIM tool to estimate the day length and VPD that were not included in the China Meteorological Forcing Dataset. (2) Land surface initialization data mainly contains soil texture (percentage of sand, clay, and silt), soil depth, elevation, shortwave albedo, atmospheric CO\({}_{2}\) concentration, and atmospheric nitrogen deposition. Among them, soil texture and soil depth were obtained from Soil Map Based Harmonized World Soil Database (v1.2) provided by Cold and Arid Regions Sciences Data Center with a spatial resolution of 30 arc-second. Elevation data was acquired from STRM 90m Digital Elevation Database provided by CGIAR Consortium for Spatial Information (available at [[http://www.cgiar-csi.org](http://www.cgiar-csi.org)]([http://www.cgiar-csi.org](http://www.cgiar-csi.org))). Shortwave albedo data with a spatial resolution of 0.05\({}^{\circ}\times 0.05^{\circ}\) was obtained from the National Earth System Science Data Sharing Infrastructure, National Science & Technology Infrastructure of China (available at [[http://www.geodata.cn](http://www.geodata.cn)]([http://www.geodata.cn](http://www.geodata.cn))). Atmospheric CO\({}_{2}\) concentration data was obtained from the National Oceanic and Atmospheric Administration (NOAA) (available at [[ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/](ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/)]([ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/](ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/))). Atmospheric nitrogen deposition data at a 0.1\({}^{\circ}\times 0.1^{\circ}\) resolution was supplied by the Chinese Academy of Agricultural Sciences. (3) The EC flux data at two flux sites was obtained from the Chinese Flux Observation and Research Network (ChinaFLUX), which was used to calibrate the ecophysiological parameters corresponding to alpine meadows and alpine steppes, respectively. A detailed description of these two flux sites are given in Table 2, and information on the EC measurement system can be found in [34]. The initial measured fluxes (Net Ecosystem Exchange, NEE) were first processed by the ChinaFLUX CO\({}_{2}\) data processing system [35].