The roles of NDVI and Land Surface Temperature when using the Vegetation Health Index over dry regions

Dry lands are expected to cover about half of the terrestrial surface in the near future due to climate change. Drought events, which are recurrent over dry lands, are also projected to increase in both frequency and severity. There is a strong need to better monitor droughts over dry regions, and satellite-based indicators such as the Vegetation Health Index (VHI) have been operationally used worldwide in the last two decades. VHI is traditionally defined as the simple average of two components, the Temperature Condition Index (TCI) and the Vegetation Condition Index (VCI) respectively derived from information on the thermal and visible bands. However, the weights of VCI and TCI depend on landcover because of the different contributions of moisture and temperature to the vegetation cycle. By systematically comparing VHI with the Standardised Precipitation-Evapotranspiration Index (SPEI), a multi-scalar drought index, we demonstrate that is possible to disentangle the role played by VCI and TCI on vegetation health. Here we propose a methodology that allows estimating optimal weights for the two components of VHI and we show that VHI is persistently dominated by VCI over dry lands. Results obtained indicate that severe drought episodes over dry lands are better identified when using the proposed methodology. This may be an asset for operational monitoring, paving the way to more efficient social and political responses aiming to mitigate drought impacts. This work is also expected to contribute to the development of optimal sets of VCI and TCI weights that take into consideration expected changes in the land surface based on information from future climate scenarios.