# Adjustments to SIF Aid the Interpretation of Drought Responses at the Caatinga of Northeast Brazil
## 2 Materials and Methods
### Sun-Induced Fluorescence  
Among the available data sources, we have chosen to use SIF derived from GOME-2 instruments data due to the combination of several advantageous characteristics, namely: adequate spatial and temporal continuity, favorable measurement time (more in this same paragraph) and, availability of SIF\({}_{R}\) data [27; 28]. All other SIF data sources have had shorter sampling extent: OCO-2 has data for the period starting on December 2014 until the present [54], TROPOMI's data starts on November 2017 [55] and TanSat on February 2017 [56]. The instruments OCO-2, TROPOMI and TanSat sample SIFat 13:30 local solar-time [54; 55; 56] and GOSAT samples at 13:00 solar-time [57] and therefore, their data is likely biased by mid-day peaks on Vapor Pressure Deficit (VPD), further confounding its interpretation. SCIAMACHY's SIF data is also not ideal for our aims because of its circular footprint and spatial discontinuity [57]. For a more thorough comparison between the different instruments currently capable of measuring SIF from orbit, see [PERSON] et al. [3]. The GOME-2 SIF data we used were downloaded from NASA's AVDC (Aurora Data Validation Center) at version 2.8. This data was produced with methods described in [PERSON] et al. [27] and [PERSON] et al. [28]. The period we chose to study comprises most of GOME-2 SIF available data and it spans nearly eleven years, from February 2007 to December 2017. This data was converted from in its original gridded spatial resolution of 0.5\({}^{\circ}\) by 0.5\({}^{\circ}\) to a finer 0.05\({}^{\circ}\) by 0.05\({}^{\circ}\) to match the used MODIS products using the Nearest Neighbor method. There are two GOME-2 instruments in orbit on board satellites MetOP-A and MetOP-B from Eumetsat. Both are in Sun-synchronous orbits, with overpass time at approximately 9:30 solar-time but, although MetOP-A's GOME-2 data is available since January 2007 until now, MetOP-B data covers a shorter period, starting from March 2013. Unfortunately, the GOME-2 sensor on MetOP-A has suffered a continuous degradation since launch and an artificial negative trend of approximately \(-1.1\%\) per year has been consequently introduced into MetOP-A SIF data [28; 58]. While the degradation effects on source data were corrected [58], [PERSON] et al. [28] argue that this correction is likely imperfect due to the specifics of SIF signal disentangling. Accordingly, we have chosen to use MetOP-A data from February 2007 until February 2013, switching to MetOP-B's data for the rest of the period used in this work (i.e., from March 2013 until December 2017) since the GOME-2 instrument in MetOP-B has not presented a degradation problem [28]. Additionally, to remove this degradation-related negative trend, data analysis in this study was performed using data detrended through time-series decomposition with Locally Estimated Scatterplot Smoothing (LOESS) when adequate. Considering the previously mentioned dependence of SIF on the emitting vegetation's biochemistry and structure [15; 25] we have chosen to include alternative formulations of SIF aiming to adjust the sampled signal to better account for these effects. Firstly, beyond standard red and far-red GOME-2 SIF, we also included in our comparison SIF adjusted to account for variations in the Sun's zenith angle (SZA), already supplied with GOME-2 level 3 dataset. This SZA normalization (SIF-SZA) is obtained by calculating the quotient of the measured absolute SIF by the cosine of the Sun's zenith angle [27; 28], which has been proven to have a normalizing effect proportional to the seasonal variation in PAR incidence [29]. Secondly, we have also employed SIF-Yield. This term has been used in slightly different ways in the literature (e.g., [PERSON] et al. [59], [PERSON] et al. [11] and [PERSON] et al. [60]) but here, we have calculated it in a similar way as to the \"real fluorescence\" of [PERSON] et al. [26].