## 2 Materials and Methods
### Model Description and Data Sources  
The Community Land Model (CLM), developed by the National Center for Atmospheric Research (NCAR) in the United States, is capable of simulating biogeochemical processes, including radiative transfer, the vegetation-soil-hydroological cycle, the soil carbon-nitrogen cycle, and vegetation photosynthesis [34]. The Community Earth System Model (CESM) has been coupled with CLM models since version 4.0 and runs under the framework of CESM [35]. CLM uses a multi-layer nested subgrid hierarchy to represent the spatial heterogeneity of the land surface. Each grid cell consists of three sub-grids: land units, snow/soil columns, and plant functional types (PFTs). The number of land units can vary within each grid cell, with each land unit potentially containing a different number of snow/soil columns, and each column having multiple PFTs. CLM 5.0 has made significant improvements compared to CLM 4.5 in various aspects including soil and plant hydrology, snow density, river modeling, crop modeling, carbon and nitrogen cycling, and coupling processes. In terms of hydrology, CLM 5.0 now features increased soil vertical resolution (20 soil layers + 5 bedrock layers) and spatially variable soil depth (ranging from 0.4 to 8.5 m), as well as the removal of unconfined aquifer and no flux lower boundary condition. In terms of vegetation dynamics, CLM 5.0 has enhanced the plant hydraulic stress model for water transport through vegetation by replacing empirical soil moisture stress formulation and introducing hydraulic redistribution. Furthermore, in terms of carbon and nitrogen cycling processes, CLM 5.0 replaces dynamic NPP-based allocation schemes with flexible plant C:N ratios to eliminate instantaneous down-regulation of photosynthesis based on mineral N availability [36]. Most importantly, CLM 5.0 improves the permafrost physical processes compared to other process-based models [37]. Thus, it can be seen that CLM 5.0 includes carbon feedback and improves the accuracy of soil water-heating process simulation, especially in the permafrost zone. Therefore, CLM 5.0 is widely used for simulating carbon, water, and energy exchange between the global and regional scale land and atmosphere [38; 39; 40; 41], and it is well suited for simulating carbon, water and other variables in the Tibetan Plateau region [42; 43]. Figure 1: Geographic location map of the Tibetan Plateau. In CLM 5.0, the NPP is simulated with the biogeochemistry (BGC) model. The NPP can be calculated by subtracting plant respiration from gross primary productivity (GPP). The detailed process and formula are explained in [PERSON] et al. [36]. The \(NPP_{coh}\) is the cohort-specific Net Primary Productivity, which is calculated as follows:  
\[NPP_{coh}=GPP_{coh}-R_{plant,coh} \tag{1}\]  
GPP is calculated as follows:  
\[GPP_{coh}=12\times 10^{-9}\sum_{z=1}^{nz(con)}gpp_{Cl,ft,z}A_{ crown,coh}elai_{Cl,ft,z} \tag{2}\]  
where the \(GPP_{coh}\) is cohort-specific gross primary productivity, \(gpp_{Cl,ft,z}\) is the rate of gross photosynthesis, \(A_{ crown,coh}\) is the Crown area, and \(ela_{Cl,ft,z}\) is the sum of exposed leaf area index. Plant respiration is calculated as follows:  
\[R_{plant}=R_{g,coh}+R_{m,coh} \tag{3}\]  
where the \(R_{plant}\) is the plant respiration, and \(R_{g,coh}\) and \(R_{m,coh}\) is the growth and maintenance respiration. The simulation performance of land surface models can be influenced by several slow-changing parameters. The spin-up process is typically used to provide initial conditions for the model [44]. Spin-up makes the continuous adjustment of various model parameters, such as soil temperature and humidity, latent heat, and sensible heat, until the model reaches an equilibrium state [45]. This study uses the China Meteorological Forcing Dataset (CMFD) to complete the spin-up process for 400 years in order to ensure equilibrium. The climate-forcing dataset used in this study is the CMFD.