## 4 Results  
### Response of SIF to Drought  
Figures 3-5 show the correlation between GOME-2 SIF and the multi-time-scale SPI. Two adjacent CDs with similar land cover were grouped together to increase the number of samples used in the regression. We found significant positive correlations (\(p<0.05\)) between SIF and all the drought indices for almost every CD from June to September, but the correlation coefficients varied by month and for different SPI time scales. The correlation coefficients between SIF and SPI-1 were no lower than those between SIF and SPI-2 or SPI-3, but were even higher for CDs 1401/1404 and 3901/3905, indicating that SIF was influenced by the precipitation of the past one to three months. The correlation coefficients between SIF and SPI-1 for CDs 1406/1409 and CDs 2501/2502 were lower. These CDs were near the Mississippi Basin, which has a moist soil [31], and the meteorological drought was eased by ground water. There were no significant correlations between SIF and SPI-2 or SPI-3 in June for CDs 3901/3905, which were located at a higher latitude. However, for CDs at a lower latitude (CDs 1401/1406 and 2501/2502), the correlation coefficients between SIF and SPI-3 in June were higher than in other months. The phenology of plant growth at higher latitudes was later than that for plants at lower latitudes [6]. Thus, SIF responded more significantly to precipitation in April, May, and June, when the grass at lower latitudes started to grow, while grass at higher latitudes was not yet growing. The correlation coefficients between SIF and SPI in September were generally lower than in other months. Although drought impacts vegetation production throughout the growing season [37], the sensitivity of plants to water stress varies according to their phenology [38; 39]. Photosynthesis is most intensive during the peak greenness season (July and August), but is weaker in the senescence period (September). Thus SIF is more sensitive to drought in the peak growing months. Figure 3: Relationship between the one-month standard precipitation index (SPI) and Global Ozone Monitoring Instrument 2 (GOME-2) solar-induced chlorophyll fluorescence (SIF). The “r” in the figure represents the correlation coefficient, while “**a” and “***” represent the significance levels \(p<0.05\) and \(p<0.01\), respectively. and SIF are lower than those between the SPI-1 and NDVI. The correlation coefficients between the SPIs and NDVI or NDWI increased significantly when the time scale of the SPI was expanded, which concurred with a previous study [6], indicating that VIs respond more strongly to precipitation over a longer historical period. For SIF, the correlation coefficients increased slightly as the SPI time scale was expanded in subfigures (a) and (b), while in subfigures (c) and (d), the strongest relationships were found for SPI-2 and SPI-1, respectively. This means that the relationship between SIF and historical precipitation remained relatively stable over the past one to three months, but that SIF responds more strongly to more recent precipitation. It is reasonable to conclude that significant positive relationships were found between GOME-2 SIF and multi-time-scale SPIs. Figure 7 shows the relationship between SIF and PDSI. The PDSI indicates the current drought condition for vegetation considering both heat and water stress. We found a significant positive relationship for all CDs in June, July, and August, while in September the relationship was weaker. Water and heat stress both restrained the photosynthetic rate [40; 41]. [PERSON] _et al._[41] found an increase in the minimum level of chlorophyll fluorescence when the heat stress was aggravated, which was assumed to result from an increase in non-photosynthetically-used APAR. Recent ground and aerial-based experiments have shown that SIF tends to decrease under drought stress [42], and the drought-induced reduction of SIF is considered a result of the increase in heat dissipation [43] and a decline in the LUE and APAR [25].