# Vegetation Growth Response and Trends after Water Deficit Exposure in the Loess Plateau, China
## 3 Results
### Trends in the Vegetation Growth and Moisture Conditions
#### 3.1.1 Expansion Trend of VWD
Based on the Spearman's rank correlation analysis of NDVI and SPEI07 from 2000 to 2020 (as depicted in Figure 4), it was observed that the total VWD area accounted for 38.31% during 2000-2020. In particular, vegetation growth in the northwestern and central parts of the Loess Plateau was significantly constrained by water deficit, whereas in other areas, vegetation was either not limited, or not significantly limited, by water surplus and deficit (Figure 4a-d). Of the VWD area, which accounted for 38.31%, 23.67% was situated in the semi-arid climate zone, 7.94% in semi-humid climate zone, and 6.70% in the arid climate zone. Since soil moisture is the primary source of moisture for vegetation growth, the correlation between the vegetation index and soil moisture is more significant. Accordingly, there are more regions with a significant positive correlation between NDVI and surface soil moisture, accounting for 90.66% of the total area. From southwest to northeast, there was a gradual increase in the degree of correlation. The less significant regions were mainly located in the southern Loess Plateau. Figure 4: Spatial distribution of [PERSON]’s correlation coefficients between the NDVI and the water availability index during the growing season from 2000 to 2020: (**a,b**) indicate the spatial distribution of the [PERSON]’s correlation coefficients of NDVI with SPEI07 and SM, respectively; the black dots indicate a significant [PERSON]’s correlation at \(p<0.05\); (**c,d**) indicate the correlation coefficients and significance distributions of NDVI and SPEI07 in the climate zones. 3.1.2. Trends of the Vegetation Growth Index, the Water Availability Index, and the Meteorological Index  
Time trends were analyzed after standardizing the parameters with the min-max normalization. During the growing season, the vegetation area with an increasing trend (improvement area) accounted for 98.59%, while the decreasing trend (degradation area) accounted for only 1.41%. The Hurst index of annual mean growing NDVI was 0.46 on average, with 68.71% of the area exhibiting an index of less than 0.50, indicating that the NDVI changes were largely inversely persistent, with only a few areas being persistent. All three vegetation indices indicated that the vegetation cover increased during the growing seasons from 2000 to 2020, and the spatial distribution characteristics were consistent. In terms of the growth rate of the multi-year average growing season, NDVI showed a higher growth rate than EVI and SIF (Figure 5a-c). Figure 6 illustrates that the vegetation coverage rate in the Loess Plateau was low in the northwest and high in the southeast. The greening of vegetation was the most significant in the semi-humid climate zone, followed by the semi-arid climate zone, and was the least pronounced in the arid climate zone. The growth rate of vegetation cover in the semi-humid and semi-arid climate zone was similar, while it was the slowest in the arid climate zone. In arid and sub-humid climates, the average annual growing period regarding soil moisture and precipitation showed a downward trend, indicating vegetation degradation. In contrast, in semi-arid regions, the average annual growing period regarding soil moisture and precipitation increased, leading to significant improvement in vegetation (Figure 6). These findings suggest that vegetation growth is closely related to precipitation and soil moisture. Figure 5: The annual trend of different standardized indices during the growing period from 2000 to 2020. The solid lines in (**a–i**) indicate the normalized values of the average annual growing season representing NDVI, EVI, and SIF, SMwurf, SMroot, SPEII07 (the cumulative 7-month water deficit event), temperature, precipitation, and net solar radiation, respectively; the dashed lines are the relative fit lines, and the shaded area indicates the standard deviation.