## 3 Results
### SIF Response to Henan Drought in 2019
#### 3.2.2 Response of _Sif_ to Drought  
Because TROPOMI has a relatively short mission length, starting from October 2017, and TROPOSIF L2B is only available from May 2018 to December 2021 (at the time of the study), we did not calculate the anomalies of TSIF which generally require at least five years of data. We selected 2018 as the reference year to compute the relative difference (defined in Equation (1)) to characterize the drought event. Note that the wetter areas in the reference year will reduce the relative differences and the drier areas will increase the relative differences.  
As shown in Figure 17, the spatial and temporal patterns and magnitude of TSIF-Diff are consistent with TROPOSIF-Diff but with more detailed information. In general, TSIF-Diff shows negative values in most of Henan Province in July 2019, indicating that the drought limited the photosynthetic activity and resulted in lower _SIF_ compared with that in 2018. The drought observed by the relative differences (including TROPOSIF-Diff, TSIF-Diff, USIF-Diff, EVI-Diff, and NDVI-Diff) is consistent in spatial pattern with the drought observed by SPEI and PPT anomalies in July 2019. The results show that TROPOSIF-Diff, TSIF-Diff, USIF-Diff, EVI-Diff, and NDVI-Diff can capture the overall drought. Specifically, TSIF has an average decline of 14.8%, TROPOSIF by 13.2%, USIF by 4.7%, EVI by 10.4%, and NDVI by 9.8% on July 12 compared to 12 July 2018. TSIF and TROPOSIF have larger responses to drought, followed by EVI, NDVI, and USIF. USIF-Diff, EVI-Diff, and NDVI-Diff show a wetter southern Henan (Figure 17g,j,m), while the drought was captured by TROPOSIF-Diff and TSIF-Diff, which is likely due to the fact the universal model cannot capture the temporal variations well in TROPOSIF and EVI and NDVI has a long lag to drought. Some pixels corresponding to DBF located in the western part of Henan show positive differences, indicating that they did not suffer drought stress compared to 2018. Possible reasons for this are 1) the wetter June received sufficient precipitation and DBF have deep roots that can absorb water from deeper soil; 2) higher temperature enhances photosynthetic activity. On July 28, 2019, TSIF shows an average decline of 6.6%, TROPOSIF of 7.7%, USIF of 1.1%, EVI of 2.0%, and NDVI of 3.2%. TROPOSIF and TSIF  
Figure 16: Spatial and temporal patterns of drought characterized by precipitation anomalies for July (**a**,**c**) and August (**b**,**d**) of 2018 and 2019.  
also have the largest responses to drought, followed by NDVI, EVI, and USIF. As shown in Figure 17b,e,h,k,n, the drought area of DBF in western Henan increased as the drought continued, which can also be characterized by TROPOSIF-DIFF and TSIF-Diff; however, USIF-Diff shows a wetter western Henan (Figure 17h). The drought in southern Henan also intensified according to the relative differences between _SIF_ and EVI.  
On 13 August 2019, TSIF shows an average increase of 7.3%, TROPOSIF of 6.5%, USIF of 6.6% EVI of 0.35% and NDVI of 0.2% (Figure 17c,f,i,l,o). TROPOSIF-Diff, TSIF-Diff, USIF-Diff, EVI-Diff, and NDVI-Diff indicated a wetter August in Henan, but EVI and NDVI have the smallest increase, followed by TROPOSIF, USIF, and TSIF, indicating that _SIF_ is more sensitive to precipitation than EVI.  
These results suggest that TSIF-Diff has similar spatial and temporal patterns with TROPOSIF-Diff, and TSIF-Diff can capture well the spatial and temporal variations of drought in Henan compared to USIF-Diff and EVI-Diff. In general, TSIF is more sensitive to drought and precipitation than USIF and EVI.  
Figure 17: Spatial and temporal patterns of drought characterized by TROPOSIF-Diff (**a–c**), TSIF-Diff (**d–f**), USIF-Diff (**g–i**),EVI-Diff (**j–l**), and NDVI-Diff (**m–o**) for July and August of 2019.