Comparative evaluation of the Vegetation Dryness Index (VDI), the Temperature Vegetation Dryness Index (TVDI) and the improved TVDI (iTVDI) for water stress detection in semi-arid regions of Iran

This study aims at developing appropriate methods to detect water stress in the semi-arid regions of Iran. To do this, the Vegetation Dryness Index (VDI) concept, originally developed for forest fire detection, was applied to detect vegetation/soil water stress. A modified approach towards the Temperature Vegetation Dryness Index (TVDI) concept, incorporating air temperature and a Digital Elevation Model (DEM) to develop the improved TVDI (iTVDI) is also introduced and the results are compared with the original TVDI and VDI through verification by precipitation and soil moisture data. Evaluation of the VDI in the study area showed that there was no significant relationship between the VDI values and precipitation or soil moisture indicating its inappropriateness to be used for water stress detection. Compared with the TVDI, results indicated that there were more statistically significant relationships between the iTVDI and recent precipitation and soil moisture in the four land cover types in the study area. This indicates that the iTVDI is highly influenced by recent precipitation during the summer and can therefore estimate water status. It is concluded that the iTVDI can be successfully used for vegetation/soil water stress monitoring in the semi-arid regions of Iran.