Evaluating drought responses of optical and microwave-based vegetation indices in China

Under global warming, the increasing frequency of drought has significantly impacted vegetation dynamics. Compared to optical vegetation indices, vegetation optical depth (VOD) derived from passive microwave remote sensing is less affected by atmospheric conditions, providing a novel approach for monitoring vegetation dynamics under drought stress. Based on NDVI, X-band VOD, and standardized precipitation evapotranspiration index (SPEI) data across China from 2013 to 2022, this study systematically evaluated the differential responses of various vegetation types to drought through the case studies of typical drought events in the Yangtze River basin, while comparing the capabilities of NDVI and VOD in reflecting drought-induced vegetation responses. The key findings include: ① X-band VOD exhibits higher sensitivity to drought than NDVI, particularly in dense-canopy ecosystems (e.g., forests), where it better captures earlier signals of vegetation water deficit; ② Due to different rates of drought development, vegetation indices exhibit distinct responses to extreme drought events. Compared to NDVI, VOD remains effective in capturing vegetation water stress under sustained high-intensity drought conditions. Our results demonstrate that VOD serves as a valuable complement to NDVI for drought monitoring. VOD is particularly useful in drought monitoring for ecosystems with complex canopy and compound extreme drought events.