## 4 Evaluating Satellite-Retrieved Far-Red SiF with Data on Vegetation Activity
### GOME-2A Retrieved SiF Time Series Analysis over Australian Flux Tower Sites: A Case Study  
The GOME-2A retrieved SiF (January 2007 to June 2015) over Australia covers a period with interesting climatological contrasts. The 2007-2010 time range is part of the Australian decade-long \"Millennium Drought\", the most severe drought period since instrumental records began in the 1990s [76]. This drought ended in the Australian summer of 2010/2011 due to the strong 2010-2012 La Nina phenomenon [77]. La Nina is typically associated with increased rainfall in northern and eastern Australia (both years experienced rainfall well above the long-term average of 465 mm-703 mm in 2010 and 708 mm in 2011). During La Nina, winter and spring daytime temperatures are below average across southern Australia, but are above average in the northern parts, while summer daytime temperatures are below average across most of Australia [77].  
The relatively large timespan and spatial coverage of the GOME-2A or any other satellite-retrieved SiF series, increases the ability to study anomalies and trends in vegetation dynamics over areas with limited surface data. In addition, GPP data derived from flux tower measurements may suffer from data gaps as not all the sites provide continuous data records (see for example Figure 5). SiF data may be used as a proxy for GPP (as shown in Section 4.1, Table 2 and Figure 5) to examine trends in the carbon cycle.  
Across all the Australian flux tower sites in this study, a strong positive anomaly in the de-seasonalized SiF time series (computed by subtracting from each monthly value the eight-year average for that specific month and dividing by that value to obtain percentages) was observed in 2010/2011 (blue area in Figure 7). In contrast, strong negative anomalies were observed at the start and especially at the end of the de-seasonalized time series (red areas in Figure 7). This resulted in an increase (+2.7% to +17.2% year\({}^{-1}\); on average 10.3% year\({}^{-1}\)) of de-seasonalized SiF from January 2007 to April 2011. This increase is higher over southern sites (Riggs, Wallaby, Whoo, Wombat, Yanco Jaxa: +15.7% to +17.2% year\({}^{-1}\)) compared to the northern ones (Sturt Plains, Daly Uncleared, Adelaide River, Daly Pasture, Dry River, Fogg Dam, Howard Springs: +2.7% to +7.5% year\({}^{-1}\)). For the period May 2011 to June 2015 a decrease is observed (+0.7% to \(-\)10.3% year\({}^{-1}\); on average \(-\)6.0% year\({}^{-1}\)). This decrease is stronger over the southern sites (\(-\)5.9% to \(-\)10.3% year\({}^{-1}\)) as compared with the northern sites (+0.7% to \(-\)9.5% year\({}^{-1}\)). We attribute the increasing SiF values until 2010/2011 and the decreasing SiF values afterwards to the meteorological extremes occurring during that period: drought at the start of the time series indicated by the negative anomalies, the severe rainfall in 2010 and 2011 connected to La Nina indicated by the positive anomalies, and again the drier period at the end of the time series [77; 78; 79; 80]. The small differences in responses (in magnitude, not sign) between northern and southern sites can perhaps be explained by the somewhat different impact of La Nina on precipitation and temperature in both parts of Australia [77].  
This short time series analysis illustrates the strong potential of SiF data to monitor vegetation activity in relation with meteorological anomalies [81; 82]. Future research should further investigate the (complex) relation between satellite-retrieved solar-induced fluorescence and derived parameters, such as the fluorescence yield, and photosynthetic activity.