A climatological assessment of drought impact on vegetation health index

The Vegetation Health Index (VHI) has been widely used for monitoring and characterising droughts. This index takes into account ecosystem features in terms of fluctuations between prescribed maxima and minima of NDVI (Vegetation Condition Index, VCI) and of Land Surface Temperature (LST; Thermal Condition Index, TCI), and is estimated as the weighted sum of these two contributions. Since there is no a priori knowledge about vegetation and temperature contributions, VHI is typically taken as the average of both contributions, i.e., a weight of 0.5 is assumed. In this work climatologies of NDVI and LST - spanning the period between 1982 and 2009 - are used to estimate VCI, TCI and VHI on a Mediterranean geographic window, which are then correlated with the multi scalar drought indicator SPEI (Standardized Precipitation-Evapotranspiration Index) with the aim of assessing the effect of drought on each contribution. Results of the correlations between VCI-SPEI and TCI-SPEI show that the relative contributions of VCI and TCI to vegetation health depend on vegetation cover: the effect of drought is more evident in the case of VCI in semiarid climate classes (regions where the limiting factor to vegetation growth is water); while the effect of drought is more obvious in TCI for moistier climate classes (regions where the limiting factor is solar radiation). This leads to the conclusion that by maximising the correlations between VHI and SPEI, over a climatological period, it is possible to evaluate the relative roles of VCI and TCI to VHI for different climate regions.