# Response of NDVI and SIF to Meteorological Drought in the Yellow River Basin from 2001 to 2020
## 3 Results
### Spatial and Temporal Evolution Characteristics of Vegetation in YRB
#### 3.2.2 Spatial Evolution Characteristics of NDVI and SIF  
In the past 20 years, NDVI and SIF in different seasons in YRB have shown obvious spatial heterogeneity (Figure 7). On the whole, the vegetation increases gradually from north to south throughout the year and in different seasons. The spatial distribution of annual NDVI and annual SIF shows that the south is higher than the north. The pixels with NDVI \(>0.5\) account for 54.05%, and the pixels with SIF \(>0.3\) account for 23.35%, which are mainly distributed in the southeast and southwest of the study area. The temperature rises in spring, and the vegetation is in the embryonic stage. NDVI and SIF have monitored that the growth condition in the southeast of the study area is better than that in the north. The vegetation growth in summer is significantly better than that in spring and autumn, with NDVI \(>0.5\) accounting for 47.74% and SIF \(>0.3\) accounting for 22.83%, which are mainly distributed in the southwest of the study area. The vegetation growth in autumn monitored  
Figure 5: Spatial characteristics of SPEI variation trend (**a**: Annual; **b**: Spring; **c**: Summer; **d**: Autumn) and significance (**e**: Annual; **f**: Spring; **g**: Summer; **h**: Autumn) in annual and different seasons (\(p<0.05\)).  
by NDVI and SIF was significantly lower than that in summer. From the spatial distribution of different seasons, the spatial distribution trend of vegetation growth monitored by NDVI and SIF is consistent.  
Combined with Theil-Sen median trend analysis and Mann-Kendall significance test, the significant change trend of vegetation on the pixel scale in YRB from 2001 to 2020 is obtained (Figure 8). In the past 20 years, the vegetation has shown a significant increasing trend, and the significantly increased area is significantly higher than the significantly degraded area. According to the annual change trend, the significant improvement areas of NDVI and SIF are mainly distributed in the middle of the study area, and the significant  
Figure 6: The temporal variation trend of NDVI and SIF in YRB from 2001 to 2020 (**a–h**). (**a**) Annual NDVI; (**b**) Spring NDVI; (**c**) Summer NDVI; (**d**) Autumn NDVI; (**e**) Annual SIF; (**f**) Spring SIF; (**g**) Summer SIF; (**h**) Autumn SIF.  
improvement areas of SIF (66.49%, \(p<0.05\)) are higher than NDVI (50.7%, \(p<0.05\)). In spring, the vegetation monitored by NDVI and SIF has a significant degradation trend in the northern part of the study area, accounting for 2.47% and 1.99% respectively. NDVI monitored that the vegetation showed a significant improvement trend in spring (35.12%, \(p<0.05\)), summer (40.20%, \(p<0.05\)), and autumn (36.56%, \(p<0.05\)). The vegetation monitored by SIF also showed a significant improvement trend in spring (61.75%, \(p<0.05\)), summer (66.06%, \(p<0.05\)), and autumn (64.39%, \(p<0.05\)). The significant change trends of NDVI and SIF are consistent as a whole; however, the significant improvement areas of vegetation monitored by SIF are higher than NDVI.