## 4 Evaluating Satellite-Retrieved Far-Red SiF with Data on Vegetation Activity
### Comparing Far-Red SiF with GPP Derived from Australian Flux Tower Observations  
Here, we compare SiF retrieved from GOME-2A based on the presented methodology and parameter settings with GPP derived from the regional Australian flux tower network (OzFlux--www.ozflux.org.au) [71; 72]. A full description of the network can be found in [PERSON] et al. [72]. We used 12 Australian flux towers (Table 1) available for the period January 2007 to June 2015. The tower sites cover different biomes in northern and southern Australia, from agriculture to woodlands and from pastures to savannah [73; 74]. The spatial footprint of flux tower observations is much smaller than the spatial sampling of a satellite instrument such as GOME-2, and this limits the spatial representativeness. However, by selecting flux towers in areas with homogenous land cover, the vegetation type in the flux tower's footprint will agree better with the dominant vegetation type in the satellite pixel. Since large homogeneous biomes are more common in Australia, we focus our comparison on Australian flux towers. Figure 4: GOME-2A SiF emissions (**left**); and MODIS NDVI (**right**) over North America for July 2013. At the flux tower sites, meteorological parameters, carbon, water and energy fluxes were measured and GPP was derived. For each flux tower we selected the 0.5\({}^{\circ}\) by 0.5\({}^{\circ}\) grid cell with centre coordinates closest to the tower site. We then averaged the tower-derived GPP at the overpass time of GOME-2 (time window of 2 h centred at 9:30 local time) to monthly values. In this way, we only take GPP into account for the same lighting conditions in which SiF is retrieved (e.g., instantaneous GPP). For each tower site, the derived time series of GPP and the corresponding SiF series, either from the KNMI or the NASA (v26) product, are shown in Figure 5. Subsequently, we have conducted a simple linear regression of monthly GPP data on monthly satellite-retrieved SiF values (Table 2). In Figure 6 we show two scatterplots for a flux site with a high and a low correlation between SiF and GPP with slopes, intercepts, and correlation values given in Table 2.  
Figure 5: Observed Sun-induced Fluorescence (SiF) time series for January 2007 to June 2015 from the KNMI (black line) and NASA (v26) (red line), and Gross Primary Productivity (GPP) derived from Australian flux towers (blue line). For comparison, the SiF and GPP data sets are scaled to be between zero and one. Despite large differences in spatial scales, SiF is related to the flux tower GPP data with \(R\)-values up to 0.84-0.87, although some tower sites show small correlations (Table 2). The SiF KNMI-GPP slopes and correlations are generally larger for agricultural biomes as opposed to more natural biomes such as savannah and woodland. Earlier reported observations confirm the higher correlation between SiF and GPP estimated at towers located near agricultural sites [53]. The slopes of the GPP-SiF relation for grass type biomes are very similar for European grasslands and US cropland [53] except for tropical grasslands. Interestingly, biomes with lower SiF-GPP slopes have also lower correlation and in general this happens at higher LAI values, which should be investigated further. Analysing differences in the SiF-GPP relationships for different biomes requires advanced canopy and SiF modelling using radiative transfer models such as SCOPE [44; 75]. By applying models, scale-related issues can be identified and separated from biophysical factors. Overall, both KNMI and NASA (v26) SiF products perform similarly, sometimes with either higher or lower correlations for each product. Slopes of the SiF-GPP regression are smaller for the KNMI product as compared with NASA (v26). This may be due to the fact that in general the KNMI SiF is higher as compared with the NASA (v26) product (see Figure 8). The vegetation onsets (Spring) and offsets (Fall) from the SiF time series broadly follow the GPP cycle.