2.2.1. FLUXNET Data

FLUXNET 2015 is the latest version of the FLUXNET dataset. Compared with previous datasets, FLUXNET v.2015 improves the protocols of data quality and the pipeline of data processing [28]. The Net Ecosystem Exchange (NEE) in the FLUXNET 2015 dataset was gap-filled with the marginal distribution sampling (MDS) method [29]. It was then partitioned into Ecosystem Respiration (RECO) and GPP using the daytime fluxes method (\_DT) [30] and the nighttime fluxes method (\_NT) [29]. The quality flags in FLUXNET 2015 are given values ranging from 0 to 1, indicating the percentage of high quality gap-filled and measured data [12]; 1 represents the highest quality and 0 represents the poorest quality [31].

As shown in Table 1, there are 12 sites in the FLUXNET datasets located in the study area, including 7 wetland sites, 2 forest sites, a grassland site, a shrublands site, and a permanent snow and ice site. This study used GPP (GPP\_NT\_VUT\_REF) estimated

from the night-time method with a daily temporal scale. The selection of GPP followed two criteria: (1) the quality flags were larger than 0.5; and (2) the difference between the GPP derived using the night-time method (GPP\_NT\_VUT\_REF) and the day-time method (GPP\_DT\_VUT\_REF) was lower than 50%. After filtering the data based on the two criteria, daily GPP were temporally aggregated to generate the 8-day averaged GPP, matching the temporal resolution of MOD17A2H.
