# Vegetation Growth Response and Trends after Water Deficit Exposure in the Loess Plateau, China
## 3 Results
### Response Characteristics of Vegetation to Water Deficit  
#### 3.2.1 The Most Sensitive Time Scale of Vegetation to Water Deficit  
The timescale at which vegetation was most affected by the water deficit, determined by the timescale corresponding to the maximum positive correlation coefficient (Figure 7a,b), was used to analyze the relationship between vegetation growth and water deficit in the Loess Plateau from 2000 to 2020. The results showed that the average response time was 6.43 months, with 7.53 months in the arid climate zone, 6.28 months in the semi-arid climate zone, and 6.19 months in the semi-humid climate zone. Vegetation growth was the most impacted by water deficit events lasting 6-8 months, making 7 months the reasonable timescale to investigate the moisture deficit of vegetation. Regions with a response time greater than 7 months were mainly located in the northwestern section, with a few in the north and south-southeast. For the multi-year average growing season, the decreasing and increasing trends in response time accounted for 37.15% and 26.84% of the region's total area, respectively. An increasing response time indicated that the vegetation in the region was more drought-resistant and less sensitive to drought, while a decreasing response time indicated the opposite, requiring that attention be paid to the adverse effects of water deficit on vegetation growth. The characteristics of the response of vegetation to drought showed apparent spatial and temporal heterogeneity. The areas where the response time decreased were mainly in the northern part of the Loess Plateau, mostly in the semi-arid climate zone, while the areas where the response time increased were mainly distributed in the southwestern part of the arid climate zone and semi-arid climate zone, as well as the southeastern part of the semi-humid region.  
2.2 The Timescale Showing the Most Sensitive of Periods of Vegetation at Different Growth Stages to Water Deficit  
To examine the impact of water availability on vegetation during different stages of growth, this study divided the growing season into three sub-stages: the early growing season (April-June), the mid growing season (June-August), and the late growing season (August-October). The value of the maximum correlation coefficient between NDVI and SPEI (Rmax) indicated the extent to which vegetation growth was affected by water deficit and the duration of the vegetation growth response to water surplus and deficit. On average, the Rmax value for the entire growing season is 0.38, with values of 0.26, 0.35, and 0.29 for April-June, June-August, and August-October, respectively, highlighting that vegetation growth is the most impacted by water deficit in the mid growing season. The maximum response of the semi-humid and arid climate zones to water deficit was not  
Figure 7: Multi-year average (**a**) and slope (**b**) of the response time of vegetation growth to water deficit.  
significantly different, and the response for both of these areas was higher than that in the semi-arid climate zone. June to August show the highest vegetation response to water deficit. The average response scale of vegetation growth to water deficit in the growing season was 7.8 months, with an average response timescale of 9 months in April-June, 5.7 months in June-August, and 6.5 months in August-October. Therefore, vegetation growth was found to be the most sensitive to water deficit in June-August, indicating the need to pay close attention to the impact of water deficit on vegetation growth during this period.