## 4 Discussion  
We analyzed the spatiotemporal patterns of drought from 2000 to 2022, with a particular focus on 2022, using TVDI to assess drought. It also explored the differences in the responses of vegetation ecosystems to drought between karst and non-karst areas, using three vegetation indices: NDVI, LAI, and GPP. Among these indices, NDVI determines the health status of vegetation in drought-affected areas by monitoring vegetation greenness, LAI is used to analyze the canopy structure of vegetation in drought-prone areas, and GPP helps to assess the ecological resilience of vegetation under drought stress. The study found that the karst areas were more severely impacted, exacerbating the effects of drought  
Figure 10: Differences in the NDVI (**a**), LAI (**b**), and GPP (**c**) values in the karst and non-karst areas between the 2022 drought period and the baseline year (2020–2021). on vegetation ecosystems. These indices provide strong theoretical support for drought assessment based on TVDI. TVDI, which combines the land surface temperature (LST) and vegetation cover (NDVI), can be used to assess the degree of drought stress. Numerous studies have suggested that a combination of NDVI and LST can reveal information on regional drought conditions [43; 44; 45; 46; 47]. We found that TVDI demonstrated a similar trend to that in the SPI and SPEI. Furthermore, TVDI is derived from the simplified NDVI-LST space to assess drought conditions using remote sensing datasets (such as MODIS in our study), enabling coverage of large areas, particularly those with few meteorological stations. The Guilin region has complex karst topography, and low-resolution remote sensing data may struggle to accurately observe this fragmented landscape. Therefore, we believe that TVDI, with its higher accuracy, can better reveal drought conditions in karst areas compared to SPI and SPEI, which primarily rely on in situ climate data measurement. However, given the complex topography, uneven vegetation distribution, and significant spatial heterogeneity of karst landscapes, the application of higher spatial resolution remote sensing data is crucial for more accurate detection and analysis of drought conditions in these regions. According to the spatiotemporal patterns of the TVDI, the 2022 drought began in July and lasted until December in Guilin city. From August to November, Guilin city exhibited moderate to severe drought (with severe drought occurring in September). [PERSON] et al. [34] and [PERSON] et al. [31] indicated that mostly single and compound extreme temperature-drought events occurred in 2022, setting records for the hottest and driest summer in history, and were primarily concentrated in southeastern, southwestern, and central China, as well as the Tibetan Plateau. The timing of drought occurrence in 2022 in Guilin city was similar to that reported by [PERSON] et al. [48]. The spatiotemporal patterns of the TVDI across Guilin city indicated that the karst areas were the most severely affected by drought. From a karst geological perspective, the extremely low water retention capacity of the soil in karst areas is a major factor contributing to drought stress in karst ecosystems. Recent studies suggest that drought, commonly referred to as karst drought, is a common phenomenon in karst areas [49]. However, this type of drought differs from that in deserts or plateau areas, which are typically caused by scarce rainfall. Due to the uneven seasonal distribution of rainfall and the unique surface-underground dual-layer spatial structure in karst areas, surface water is prone to leakage [50]. As a result, the karst ecosystems in southwestern China are particularly sensitive to climate change [51]. Specifically, during drought events, the water retention capacity in the karst area of Guilin is only 8% [52], with most of the water concentrated in groundwater. Vegetation with shallow root systems in karst areas struggles to access sufficient water.