A New Vegetation Health Index Based on Vegetation Optical Depth: Development and Application in Vegetated Australia

Abstract

A solely human‐centered approach to drought assessment is inadequate. Monitoring drought from an ecological perspective is essential for developing scientifically grounded response measures, particularly in drought‐prone countries such as Australia. However, the Vegetation Health Index (VHI) may be limited in accurately representing vegetation conditions because of shortcomings in the Normalized Difference Vegetation Index (NDVI). To address this, we developed a Vegetation Optical Depth (VOD)‐based VHI (VHI _VOD ) by integrating long‐term harmonized VOD data sets into the conventional VHI framework. Vegetated Australia was used as a case study to evaluate the performance of VHI _VOD during 1990–2024. Based on Spearman correlations with Gross Primary Production and soil moisture, VHI _VOD outperformed the NDVI‐based VHI across over 70% of vegetated Australia at both national and biome levels. Although no significant long‐term trend in annual VHI _VOD was detected, a significant breakpoint occurred in 2002. During 1990–2001, annual VHI _VOD increased across more than 78% of vegetated Australia, with 44% showing significant increases, whereas only 19% showed significant increases during 2002–2024. Regarding vegetation drought indicators, including severity, intensity, duration, and area percentage, both vegetated Australia and most biomes experienced weakening drought conditions during both sub‐periods. Over 70% of vegetated areas experienced declines in severity, intensity, and duration, with significant weakening observed in over 39% of vegetated Australia during 1990–2001 and over 20% during 2002–2024. These findings highlighted the improved capability of VHI _VOD in representing vegetation conditions and demonstrated its potential for vegetation drought monitoring under changing hydroclimatic conditions.