# 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  
The robustness of the calculated SPEIs was verified by comparing them with the SPEI of the SPEIbase v2.5 products, and the results showed that the SPEI index calculated in this study was reliable (Figures S1 and S2). To investigate the correlation between NDVI and cumulative water deficit events on different timescales during the 2000-2020 growing season, the 5-year sliding window was employed to calculate the Spearman's rank. Additionally, the correlation between NDVI and soil moisture was also compared. The area with a significant positive correlation area (\(p<0.05\)) represents the VWD area, indicating that vegetation growth is promoted by an increase in water, while the areas with a significant negative correlation (\(p<0.05\)) represent excess water, indicating that vegetation growth is inhibited by an increase in water. No significant correlation indicates that vegetation growth is not affected by water in the area [53]. Figure 2 displays inconsistent trends of water deficit or water surplus area obtained at short timescales, whereas those calculated at timescales greater than 6 months are more consistent (Figure 2d-f). The relevant literature also suggested that short timescales were unsuitable for arid and semi-arid areas such as the Loess Plateau [3; 49; 54; 55]. Therefore, this study focused on the response characteristics of cumulative water deficit events with vegetation growth timescales greater than 6 months. The VWD area in the Loess Plateau exhibited a decreasing trend (\(p>0.05\)), which may be confounded by inconsistent regional results. Conversely, the vegetation water surplus area demonstrated an expanding trend (\(p<0.05\)), which may be attributed to the increasing annual precipitation. During 2000-2020, the maximum percentage of vegetation surplus area was 1.75%, and the maximum VWD area was 16.06%. Given the small area of vegetation water surplus in the Loess Plateau, this study focuses on the VWD. As depicted in Figure 2, the trend of VWD area varied across different climate zone, and the changes in VWD and vegetation water surplus area were analyzed using the SPEI07  
Figure 2: Area expansion trend of VWD in different climatic zones: (**a–c**) are the trends of VWD in arid, semi-arid, and semi-humid climate zones obtained by statistical analysis of the correlation between NDVI and SM, respectively; (**d–f**) are the trends of VWD in arid, semi-arid, and semi-humid climate zones obtained by statistical analysis of the correlation between NDVI and SPEI07, respectively. The major scale of the coordinate axis represents the starting year of the slide. (7-month cumulative water balance), which yielded results consistent the soil moisture and NDVI. Thus, SPEI07 proves to be a suitable tool for examining the response of vegetation growth to water deficit in the Loess Plateau. From a regional perspective, the VWD in arid and sub-humid climate zones exhibited a decreasing trend, while the VWD in the semi-arid climate zone showed no significant increase. The vegetation recovery areas were primarily located in the semi-arid climate zone, implying that vegetation growth contributed to the expansion of water deficit areas. In terms of temporal changes, nearly all correlation coefficients that passed the significance test were positive. The distribution of the VWD areas varied significantly across different periods (Figure 3), with notable differences between the two periods requiring separate analysis and comparison with various natural factors, particularly the average water conditions in each period. Figure 3: Correlation coefficients between NDVI and SPEI07 for the period of 2000–2020, calculated with a sliding window of 5 years. The top left graph represents the areas that passed the significance test (\(p<0.05\)).