Spatial heterogeneity of flux sites

An essential prerequisite to justify upscaling from sites to the globe requires homogeneous site-level characteristics (i.e. soil and vegetation properties). However, site-level characteristics are in practice rarely completely homogeneous, which might introduce variations in the magnitude of observed fluxes depending on wind wind direction (Griebel et al., 2016). Thus, some portion of the variability between interannual carbon budgets may purely be attributable to inconsistencies in the prevailing wind directions between years, which cause systematic differences in the area that is being sampled.

Conceptual illustration outlining how annual carbon budgets vary in space and time at three sites with homogeneous, mixed and heterogeneous surface characteristics.
Griebel et al., (2020). Generating spatially robust carbon budgets from flux tower observations. Geophysical Research Letters, 47 (3).

As no standard method exists to quantify site homogeneity, we developed a Heterogeneity Index (HI) based on the range of EVI values (Enhanced Vegetation Index) across individual 30 x 30 m pixels to assess the spatio-temporal variation of vegetation characteristics around each flux tower (Griebel et al., 2020). Applying this to sites in the FLUXNET2015 data base identified that spatial homogeneity of site characteristics does not hold true for many flux tower locations.

map
Griebel et al., (2020). Generating spatially robust carbon budgets from flux tower observations. Geophysical Research Letters, 47 (3).

Our proposed space‐time‐equitable budgets (Griebel et al., 2020) reduce uncertainty related to heterogeneities, allow for more accurate attribution of physiological variations in productivity trends, and provide more representative grid cell averages for linking fluxes with gridded data products.