### SEVIRI's Detection Performance Stratified by Land Cover and Tree Cover  
It is possible to argue that fire behaviour could be homogeneous across the CAR, and that other non-fire factors are influencing geographic variations in the SEVIRI and MODIS observations of fire activity. Whilst we cannot account for all spatial variables, we stratified SEVIRI's omission rate and MODIS measurements of FRP by land cover using the Global Land Cover 2000 (GLC2000) database [64], and also by percent tree cover using the Global Land Cover Facility (GLCF) Version 3 of the Collection 4 Vegetation Continuous Field (VCF) product [65].  
Despite stratifying active fire pixels into GLC2000 classes, differences in SEVIRI's omission rate were found between the ROIs even within the same land cover class (Table 3). Meaningful results are limited here to the \"Deciduous woodland\" since this class occupies 83% of all ROIs, varying from 100% in ROI 1% to 67% in ROI 4. SEVIRI's omission rate in the \"Deciduous woodland\" ranged from a minimum of 25% in ROI 1 to a maximum of 72% in ROI 4, and coherent with SEVIRI's omission rate, the median FRP measured by MODIS in the \"Deciduous woodland\" decreased from 41.8 MW in ROI 1 to 21.8 MW in ROI 4 (Table 3).  
SEVIRI's detection performance in the \"Deciduous woodland\" was further decomposed based on percent tree cover. Distributions of VCF percent tree cover in the \"Deciduous woodland\" (degraded to \(\sim\)3 km resolution) are shown for each ROI in Figure 9a. Note that 93% of all SEVIRI active fire pixels detected in the \"Deciduous woodland\" were found under 20%-50% tree cover. SEVIRI's omission rate under this range of tree cover remained relatively constant within each ROI, but differed between the ROIs (Figure 9b). In addition, Figure 9c demonstrates that the median FRP measured by MODIS under 20%-50% tree cover in the \"Deciduous woodland\" remained relatively constant within each ROI (\(\pm\)6 MW), but differed between the ROIs.  
Figure 8: **(a)** Histogram of the per-pixel fire radiative power (FRP) measured by MODIS in each of the four ROIs depicted in Figure 1a between 1 October 2008 and 31 April 2009. A shift in the distributions from high FRP pixels to low FRP pixels between ROI 1 and ROI 4 is coherent with an increase in SEVIRI’s omission rate. **(b)** Histogram of the cluster size of MODIS active fire pixels in each of the four ROIs between 1 October 2008 and 31 April 2009. A shift in the distribution from larger to smaller cluster sizes between ROI 1 and ROI 4 is also coherent with an increase in SEVIRI’s omission rate.  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c} \hline  & & ROI 1 & & ROI 2 & & & ROI 3 & & & ROI 4 & \\ \hline  & & \(\phi_{Farr}=0.75\) & & & \(\phi_{Farr}=0.62\) & & & \(\phi_{Farr}=0.50\) & & & \(\phi_{Farr}=0.37\) & \\ \hline \multirow{2}{*}{(GLC2000)} & \multirow{2}{*}{Area} & SEVIRI & \multirow{2}{*}{Median} & \multirow{2}{*}{Area} & \multirow{2}{*}{Median} & \multirow{2}{*}{Area} & \multirow{2}{*}{SEVIRI} & \multirow{2}{*}{Median} & \multirow{2}{*}{Area} & \multirow{2}{*}{SEVIRI} & \multirow{2}{*}{MODIS} & \multirow{2}{*}{Area} & \multirow{2}{*}{SEVIRI} & \multirow{2}{*}{MODIS} & \multirow{2}{*}{Area} & \multirow{2}{*}{MODIS} \\  & & & MODIS & &  
Based on this evidence we suggest that it is possible for SEVIRI's detection performance and MODIS measurements of FRP to vary substantially within similar vegetation types and canopy structures across the CAR, indicating that spatial variations in vegetation characteristics are not the primary cause of variations in the FTA detection performance with respect to MODIS.