A Multiscalar Standardized Vapor Pressure Deficit Index for Drought Monitoring and Impacts

Vapour pressure deficit (VPD) is a critical measure of the atmospheric demand for water and can be used to assess short-term and seasonal drought. To provide for probabilistic comparisons of VPD across space and time, we develop a Standardized Vapor Pressure Deficit Index (SVPDI). Similar to the way that other standardised drought indices are used, SVPDI allows for the analysis and comparison of changes in VPD across regions with different base level VPD values. It also should be useful for analysing impacts on vegetation that has varying levels of adaptation to high VPD. We use 1-, 3-, 6- and 12-month timescales for the development of SVPDI and show that the gamma distribution is superior to other zero-limited probability distributions for analysing VPD and, therefore, for calculating SVPDI. Then, focusing on the short-term variations at the 1- and 3-month timescales, we show how SVPDI has changed globally from 1958 to 2023 and how those changes differ from those of the commonly used Standardized Precipitation Evaporation Index (SPEI). We find that SVPDI shows more widespread drying conditions that also are larger in magnitude compared to those of SPEI. Although the two indices are moderately well correlated across the terrestrial surface, we discover that they are more decoupled in humid and arid regions compared to dry sub-humid and semi-arid regions. Using four locations that have recently experienced severe drought, we find that SVPDI generally showed longer drought duration and more severe drought events in the last decade when compared to SPEI.