## 3 Results  
### SIF Correlates Better with Drought Than EVI and NDVI  
Although drought impacts vegetation production throughout the growing season [26], the sensitivity of plants to water stress varies according to their phenology [27; 28]. Vegetation photosynthesis is strongest during the peak greenness season and weaker during the senescence period, so vegetation is more sensitive to drought in the peak growing months [4]. Therefore, the peak growth period was chosen as the study period for correlation analysis in this section. The correlation analysis in this section is calculated on a monthly basis for the peak growth period from May to September 2009-2011, for a total of 15 months. The spatial distributions of SIF, EVI, NDVI and sc_PDSI during the study period are shown in Figure 2, and the correlation distributions and significance test percentages of the three indices with sc_PDSI are shown in Figures 3 and 4. Figures 3 and 4 show that the mean value of correlation and the proportion of positive correlation between SIF and sc_PDSI are the largest among the three indices, indicating that the correlation between SIF and sc_PDSI is the best. Figure 2: Distribution of monthly average values in Yunnan Province from 2009 to 2011: (**a**) SIF; (**b**) EVI; (**c**) NDVI; (**d**) sc_PDSI. The correlation between SIF and sc_PDSI in Yunnan Province (Figure 3a) shows a spatial distribution pattern of \"low in the west and high in the east\". According to Figure 3, the mean value of the correlation between SIF and sc_PDSI is 0.42, which shows a significantly positive correlation overall. The significantly positive correlation area (correlation \(>\) 0 and \(p<\) 0.05) accounted for about 52.79% of the province's area, which is mainly distributed in parts of Central and Northeastern Yunnan, and the sc_PDSI in these areas ranged from \(-\)3.4 to \(-\)0.87. The annual mean value of SIF in this area is about 0.892, which is 17.33% less than the 2008 (non-drought year) mean SIF value of 1.079. The insignificant correlation area (\(p>\) 0.05) accounted for about 34.07% of the province's area and is mainly distributed in some areas such as Southern and Western Yunnan, where the drought index ranged from \(-\)4.02 to \(-\)0.41. The annual mean value of SIF in these areas is 1.133, which is 9.14% less than the 2008 mean value of 1.247. The significantly negative correlation area (correlation \(<\) 0 and \(p<\) 0.05) accounted for about 13.14% of the province's area, mainly in Western Yunnan. The drought index in these areas ranged from \(-\)3.26 to \(-\)0.27. The annual mean value of SIF in these areas is 1.167, which is 5.90% more than the 2008 mean value of 1.102. The spatial correlations of EVI and NDVI with sc_PDSI in Yunnan Province are shown in Figure 3b,c. The overall spatial pattern is similar to the distribution pattern of correlations between SIF and sc_PDSI. In addition to the lower overall correlation than SIF, the main differences are in Southwest Yunnan. The drought situation in Southwest Yunnan is mild to moderate, and the annual average values of SIF, EVI and NDVI in Southwest Yunnan from 2009 to 2011 are lower than those in 2008. The annual mean values of SIF, EVI and NDVI in Southwest Yunnan from 2009 to 2011 are 0.94%, 5.18% and 12.25% lower than those in 2008, respectively. It is evident that drought has less influence on Southwest Yunnan and the correlation between each index and sc_PDSI should be relatively low. However, EVI and NDVI were significantly and positively correlated with sc_PDSI, which may be related to  
Figure 4: Percentage of SIF, EVI, NDVI and sc_PDSI significance tests in Yunnan Province, 2009–2011. Figure 3: Correlation distribution of monthly average values in Yunnan Province from 2009 to 2011: (a) sc_PDSI & SIF; (b) sc_PDSI & EVI; (c) sc_PDSI & NDVI. (The numbers below the title are the range of values for this raster plot and the mean value). the characteristics of vegetation indices.