## 3 Results
### Spatial and Temporal Evolution Trend of Drought in YRB Based on VHlopt  
#### 3.3.1 Analysis of Drought Time Variation  
Analyzing the temporal evolution of drought is essential for understanding the dynamics of drought changes. The annual mean VHlopt is employed to examine the temporal variation of droughts in the YRB. Figure 7a shows that VHlopt fluctuated between 0.3 and 0.65 in the YRB from 2001 to 2021. The index displayed significant changes between 2001 and 2005, as well as from 2011 to 2013. In contrast, the changes were insignificant from 2006 to 2009 and from 2019 to 2020. Overall, the VHlopt trend in the YRB displayed a positive slope of 0.0094, indicating a weakening of drought conditions. Figure 7b-d present the VHlopt time series for the upstream, midstream, and downstream of the YRB, respectively. All three regions demonstrated substantial volatility in VHlopt, with the downstream showing the greatest fluctuation. Notably, the minimum values of VHlopt in the upstream and midstream align with those in the YRB, both occurring in 2012. The VHlopt trends in the upstream and midstream showed upward slopes of 0.0083 and 0.0114, respectively, indicating a weakening of drought conditions in these areas. Conversely, the VHlopt trend in the downstream displayed a downward slope of \(-0.0031\), suggesting a worsening of drought conditions in the downstream, which can be ascribed to factors including rapid urban expansion, the greenhouse effect, and rising temperatures, which further exacerbate agricultural drought [80; 81; 82]. Significantly, VHlopt values for the YRB and its upstream, midstream, and downstream increased significantly in 2003, indicating a marked alleviation of drought conditions in that year. This improvement may be associated with various factors, such as increased precipitation, temperature changes, and human interventions in water resource management and ecological restoration measures [83; 84; 85].  
Figure 5: Spatial distribution maps of VHlopt, SPEI03, and SMA during the growing season in 2001.  
Figure 6: Time series plots of VHlopt, SPEI03, and SMA in 2001.  
Arid area is not only a significant characteristic of drought but also a vital indicator for evaluating the severity of drought conditions. To further understand the temporal evolution of drought, we calculated the percentage changes in drought areas at the monthly scale during the growing season for different vegetation zones and the YRB. Figure 8b-e shows that the percentage changes in drought areas for forests, grasslands, croplands, and changing vegetation types, respectively. Among them, forests exhibited the greatest fluctuation in drought areas, especially during the growing seasons of 2005, 2007, and 2021. The extreme and severe drought areas in forests were mainly concentrated between 2001 and 2007, as well as in 2009, 2011, and 2017, with most years dominated by moderate and mild drought, indicating a weaker sensitivity to drought conditions. From 2001 to 2021, the variability in drought area was smaller for grassland, cropland, and variable vegetation types compared to forests. Extreme and severe droughts occurred in grassland, cropland, and variable vegetation types, with similar proportions of extreme and severe drought areas across these three types. Extreme and severe droughts occurred in all three vegetation types, with similar proportions of extreme and severe drought areas. As illustrated in Figure 8a, the YRB exhibited the smallest fluctuation in drought areas, with extreme and severe drought occurring throughout the period from 2001 to 2021, similar to the patterns in grasslands, croplands, and changing vegetation types. Additionally, the total no drought area across forests, grasslands, croplands, changing vegetation types, and the YRB showed increased fluctuation from 2001 to 2021, indicating that drought conditions in the YRB have generally weakened.  
Figure 7: Temporal evolution of VHIopt and the red dashed lines in all subfigures represent the lines of linear fit.