## 3 Results
### Spatial and Temporal Evolution Trend of Drought in YRB Based on VHlopt
#### 3.3.2 Spatial Distribution Analysis of Drought  
Evaluating the spatial characteristics of drought is essential for understanding its patterns and impacts. Figure 9 depicts the spatial distribution of VHIopt during the growing season across the YRB from 2001 to 2021. In 2001, drought conditions were most severe, affecting nearly the entire YRB, with the northern and central regions experiencing extreme drought. By 2002 and 2003, the drought area gradually decreased from north to south. Although the intensity lessened, some regions continued to experience drought. In 2004, the drought area expanded again, with a more concentrated distribution in the midstream. The drought reached a second peak in 2005, particularly in the central basin, with significant increases in areas classified as extreme, severe, and mild drought. Since 2005, drought has primarily concentrated in the middle, western, and downstream of the YRB, with a relatively small drought area and a tendency for minimal fluctuations. Especially in 2012, the drought area decreased to its minimum, indicating that the year was the wettest between 2001 and 2021, with areas of extreme, heavy, moderate, and mild drought nearly disappearing. In other years, drought predominantly affected the central and western regions and the downstream.  
Figure 8: Percentage of drought-affected area during the growing season from 2001 to 2021 in different vegetation types.  
To further investigate the spatial variation characteristics of VHIopt during the growing season in the YRB, this study employed Theil-Sen Median trend analysis and Mann-Kendall test methods to quantify and test the trends of VHIopt for each pixel, reflecting the spatial distribution of drought dynamics in the basin. Based on the positive and negative values of the VHIopt trend (Slope), the drought trends were classified into three categories:  
Figure 9: Spatial distribution maps of VHIopt from 2001 to 2021.  
intensifying drought trend, stable drought trend, and alleviating drought trend. Figure 10a shows that areas with an alleviating drought trend account for the majority of the YRB, reaching 87%; areas with a stable drought trend make up 2%; while areas with an intensifying drought trend constitute 11% of the basin, mainly distributed in the Hetao Plain, Ningxia Plain, Guanzhong Plain, and parts of the downstream, indicating a worsening drought condition in these areas. Furthermore, the Mann-Kendall test showed 76% of the areas with significant changes in agricultural drought trend (\(p<0.05\)) and 24% of the areas with no significant changes in drought in the vegetation coverage area of the YRB. Drawing on the results of the Theil-Sen Median trend analysis and the Mann-Kendall test, drought trends are divided into five main types: significant increase, insignificant increase, constant, insignificant decrease, and significant decrease, accounting for 5%, 6%, 2%, 16%, and 71% of the total area of the YRB, respectively. These results indicate that drought conditions have significantly worsened in certain areas, although drought conditions of the YRB are showing a decreasing trend, such as the Hetao Plain, Ningxia Plain, Guanzhong Plain, and parts of the downstream. The spatial distribution pattern is highly consistent with the linear trend analysis results of VHIopt.