## 4 Discussion  
### TVDI Drought Monitoring  
Our study employed the TVDI to assess aridity in Central Asia and identified 2012 as the driest year between 2000 and 2020, with 2019 ranking as the second driest year. These findings were corroborated by the investigation by [PERSON] et al. (2020) on drought monitoring across the Eurasian continent, which demonstrated a strong correlation between the TVDI and both the standardized precipitation evapotranspiration index (SPEI) and soil moisture [63]. Meanwhile, [PERSON] et al. (2019) applied TVDI to UAV imagery for estimating soil moisture, and the results demonstrated that this method performs well in complex mountainous terrains, generating soil moisture maps with a resolution of 50 cm, which showed strong correlation with field measurements (R\({}^{2}\) values of 0.55 and 0.76) [64]. [PERSON] et al. (2024) used TVDI to analyze drought across different climatic zones and vegetation types in the Yellow River Basin, identifying TVDI as an excellent drought index for monitoring the distribution and severity of agricultural droughts [65]. This validation provided substantial evidence for the effectiveness of the TVDI in drought monitoring and further confirmed that 2012 was the year most severely affected by drought, which is consistent with our research results.  
In 2012 and 2019, the southern region of Central Asia experienced severe drought, while the central region experienced relatively milder drought conditions. This difference in drought severity could be attributed to the use of the TVDI in monitoring aridity in Central Asia, which is constructed based on the triangular relationship between the normalized difference vegetation index (NDVI) and LST (Figure 3). The southern part of Central Asia, including Turkmenistan, Uzbekistan, Tajikistan, and some areas of Kyrgyzstan, is predominantly characterized by desert land cover with a small portion of shrubs and grasslands (Figure 1d).  
First, an increase in vegetation can lead to a reduction in air temperature [66]. This effect on surface temperature is achieved by simultaneously increasing surface roughness and conductivity, which in turn enhances the dissipation of turbulent energy flux, thereby lowering the surface temperature [67]. On the other hand, with the increase in vegetation, a greater amount of deeper soil moisture is utilized to replenish vegetation through transpiration, resulting in a decrease in surface temperature [68]. Therefore, the impact of  
Figure 14: The correlation between the EVI, FVC, GPP, LAI, RSVI, and TVDI in Central Asia is depicted in the figure. The black crosses indicate raster cells that achieved statistical significance through significance testing (\(p<0.05\)).  
sparse vegetation on surface temperature is limited, whereas regions with relatively more vegetation, such as Kazakhstan and certain parts of Xinjiang, China, tend to experience milder drought. The primary climate regions where drought and severe drought occur are arid, semiarid, and Mediterranean climates. This consensus is shared among numerous scholars, as these climate regions all have common characteristics, such as high summer temperatures, limited and unevenly distributed precipitation, and severe constraints on vegetation growth [69; 70; 71].