## 2 Materials and Methods  
### In Situ Meteorological Observations and Carbon Flux Data  
The fluxes and meteorological data used in this study are mainly based on a flux observation network located in a typical inland river basin: The Heihe River Basin (HRB) in the arid region of Northwest China. The HRB (37.7\({}^{\circ}\)-42.7\({}^{\circ}\) N, 97.1\({}^{\circ}\)-102.0\({}^{\circ}\) E), second largest inland river basin in China, is located in the middle part of the Hexi corridor and covers an area of approximately 1,432,000 km\({}^{2}\)[41]. The HRB is a unique region in China and can be viewed as an epitome of the arid region of western China for its varied distributed landscapes of alpine meadow, wetland-oasis-desert and natural oasis-desert ecosystems from upstream to downstream [42; 43]. We constructed a comprehensive flux observation network in the whole river basin to investigate the complexity of hydrological and ecological processes in the arid region (Figure 1). In this study, we compiled 12 EC flux sites covering 3 grassland sites, 3 desert grassland sites, 3 cropland sites (including a wetland site), and 3 forest sites, which almost covered the major plant function types (PFTs) and typical ecosystem types in the arid region of an inland river basin. Figure 2 shows the meteorological observations of all the flux tower sites over HRB including precipitation, air temperature (T), and vapor pressure deficit (VPD). A large variability of climate conditions exist within and across the species. The specific locations and related information of the sites are shown in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Site Code** & **PFTs** & \begin{tabular}{c} **Year** \\ **Used** \\ \end{tabular} & \begin{tabular}{c} **Vegegation** \\ **Cover** \\ \end{tabular} & **Longitude** & **Latitude** & \begin{tabular}{c} **MAT** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \
\begin{tabular}{c} **MAP** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **PET** \\ **(mm)** \\ \end{tabular} \\ \hline A’rou(ARZ) & Grassland & 2013-2016 & alpine grassland & 38.0473 & 100.4643 & \(-\)0.29 & 444.70 & 636.18 \\ Dashalong(DSL) & Grassland & 2013-2016 & alpine meadow & 38.8399 & 98.9406 & \(-\)3.91 & 314.43 & 698.07 \\ Yakov(YKZ) & Grassland & 2015-2016 & alpine meadow & 38.0142 & 100.2421 & \(-\)4.68 & 500.79 & 653.16 \\ Huazhaizi(HZZ) & Desert steppe & 2012-2016 & desert steppe & 38.76519 & 100.3186 & 8.89 & 139.68 & 590.93 \\ Gobi(GBZ) & Desert steppe & 2012-2015 & desert steppe & 38.91496 & 100.3042 & 9.07 & 102.25 & 575.72 \\ Luo(LDZ) & Desert steppe & 2013-2015 & desert steppe & 41.9993 & 101.1326 & 12.32 & 24.80 & 727.68 \\ Damam(DMZ) & Cropland & 2012-2016 & maize & 38.85551 & 100.3722 & 6.93 & 135.70 & 828.04 \\ Nongtian(NT2) & Cropland & 2012-2016 & cantaloupe & 42.0048 & 101.1338 & 9.39 & 35.55 & 727.68 \\ Shidi(SDZ) & Cropland & 2012-2016 & reed & 38.97514 & 100.4464 & 9.19 & 119.9 & 1249.35 \\ Huyanglin(HYL) & Forest & 2013-2015 & populus & 41.9928 & 101.1236 & 10.33 & 26.00 & 922.91 \\ Hunhelin(HHL) & Forest & 2013-2016 & mixed forest & 41.9903 & 101.1335 & 10.04 & 35.53 & 1043.34 \\ Sidao(SDQ) & Forest & 2013-2016 & tamark forest & 42.0012 & 101.1374 & 10.06 & 37.13 & 977.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the flux observation network sites used in this study. MAT (\({}^{\circ}\)C) represented mean annual air temperature, MAP (mm) represented mean annual accumulated precipitation, and PET (mm) represented mean annual potential evapotranspiration. Figure 1: Locations of the flux observation sites over Heihe River Basin (HRB). The open-path eddy covariance (OPEC) system was used to measure carbon and water vapor fluxes in the flux observation network. The OPEC system at each site consists of a 3D sonic anemometer (CSAT-3/[PERSON], Campbell Scientific Instruments Inc., USA/Gill, UK) and an open path infrared gas analyzer (Li-7500/7500A, Licor Inc., USA).