## 3 Results  
### Heterogeneity of Flux Footprint  
We calculated the footprint climatology for all 78 eddy covariance sites at 16-day time steps through the study period and considered the 80% contribution contour as each footprint's climatology extents. The footprint climatology had a relative symmetric geometry and the size and position of the footprint climatology varied with wind directions, measurement height, and atmospheric stability. We selected ten sites, one representing each vegetation type, to show how footprint size varied across vegetation types (Figure 2). Overall, the footprint areas ranged from 0.09 km\({}^{2}\) to 2.18 km\({}^{2}\) at the 16-day scale among the 78 sites encompassing ten biomes. Evergreen broadleaf forests, evergreen needleleaf forests, deciduous broadleaf forests, and mixed forests had the largest footprint areas, ranging from 0.74 km\({}^{2}\) to 2.18 km\({}^{2}\), while footprint areas in grasslands, wetlands, savannas, shrublands and croplands were relatively small, covering around 0.13 km\({}^{2}\) to 0.82 km\({}^{2}\). Figure 2: The contours of cumulative footprint climatology for a 16-day time step at 10 sites, representing each vegetation type. The sites (DE-Hai to US-Ne1) are given in Figure 3a. The biome types are as follows: (**a**) deciduous broadleaf forest (DBF); (**b**) evergreen broadleaf forest (EBF); (**c**) evergreen needleleaf forest (ENF); (**d**) mixed forest (MF); (**e**) grasslands (GRA); (**f**) savannas (SAV); (**g**) shrublands (SHR); (**h**) wetlands (WET); (**i**) croplands (CRO). We examined the heterogeneity of the flux footprint and calculated the mean footprint area, SD, and CV of footprint area for each of the 78 flux sites at the 30 m spatial resolution over 16-day intervals. The mean footprint area of the 78 sites varied from 0.09 to 2.18 km\({}^{2}\) (Figure 3b). The SD and CV for footprint area are illustrated in Figures 3c and 3d. Evergreen needleleaf forests, evergreen broadleaf forests, deciduous broadleaf forests, and mixed forests had a large mean footprint area with low SD and CV in footprint area. In contrast, grasslands, wetlands, savannas, and croplands had a relatively small mean footprint area with high SD and CV. For individual sites, grassland sites (e.g., IT-MBo, NL-Hor, US-ARc and US-ARb) and cropland sites (e.g., US-Ne1, US-Ne2 and US-Ne3) exhibited the largest SD among the 78 sites (Figures 3c and 4b). This indicated that the heterogeneity of the FLUXNET footprint varied across the sites and vegetation types. This also suggested that the FLUXNET footprint was highly heterogeneous in grasslands, wetlands, savannas, and croplands. To further examine how the footprint area and heterogeneity of flux footprint could be captured at 30 m and 500 m spatial resolution, we first calculated how many Landsat and MODIS pixels were needed to resolve each footprint area, and then computed the Landsat-footprint-NDVI and MODIS-footprint-NDVI. In general, the EC footprints were covered with only a few MODIS pixels, but needed thousands of Landsat pixels (Figure 4c). For example, the largest footprint areas were found for evergreen needle forests that covered 2.18 km\({}^{2}\). These were covered by a maximum of 8 MODIS 500 \(\times\) 500 m pixels. They required 2422 of the much smaller (30 \(\times\) 30 m) Landsat pixels to calculate NDVI. Cropland footprint areas with less than a 500 m radius were only covered by a single MODIS pixel, whereas there were 230 Landsat pixels available for calculating NDVI, enough to fully cover even this small footprint area. This demonstrated the huge difference in potential for spatial matches between the EC footprint and remote sensing pixels. The Landsat-footprint-NDVI was systematically higher than the MODIS-footprint-NDVI for forest ecosystems--but  
Figure 3: (**a**) Site names for each of the 78 sites. The site names listed in (**a**) correspond with the grids of (**b–d**), such that site CA-NS1 is in the top corner and site US- Ne1 is in the final grid column just above the white line.