A review of widely used drought indices and the challenges of drought assessment under climate change

Under climate change, drought assessment, which can address nonstationarity in drought indicators and anthropogenic implications, is required to mitigate drought impacts. However, the development of drought indices for a reliable drought assessment is a challenging task in the warming climate. Thus, this study discusses factors that should be considered in developing drought indices in changing climate. Inconsistent drought assessment can be obtained, depending on the baseline period defined in developing drought indices. Therefore, the baseline period should represent the contemporary climate but should also correspond to long enough observations for stable parameter estimation. The importance of accurate potential evapotranspiration (PET) for drought indices becomes higher under a warming climate. Although the Penman-Monteith method yields accurate PET values, depending on the climate and vegetation cover, other suitable PET formulas, such as the Hargreaves method, with fewer hydrometeorological data can be used. Since a single drought index is not enough to properly monitor drought evolution, a method that can objectively combine multiple drought indices is required. Besides, quantifying anthropogenic impacts, which can add more uncertainty, on drought assessment is also important to adapt to the changing drought conditions and minimize human-induced drought. Drought is expected to occur more frequently with more severe, longer, and larger areal extent under global warming, since a more arid background, which climate change will provide, intensifies land-atmosphere feedback, leading to the desiccation of land and drying atmosphere. Thus, an accurate drought assessment, based on robust drought indices, is required.

Satellite-based application in drought disaster assessment using terra MOD13Q1 data across free state province, South Africa

Terra MOD13Q1 satellite data were used to assess drought disaster events and its spatiotemporal patterns over the Free State Province, South Africa between 2001 and 2019. Moderate Resolution Imaging Spectroradiometer (MODIS) products and climate data downloaded from Application for Extracting and Exploring Analysis Ready Samples" (AppEEARS) and NASA: Prediction of Worldwide Energy Resource databases, respectively were used in the study area. After acquiring MODIS data with the area of interest extracted using field sample and the layers of interest: using Enhanced Vegetation Index (EVI) and pixel reliability and defining the output as GeoTIFF with geographic projection, R programming was used for the analysis. This study also evaluated Standardized Precipitation Index (SPI) to further identify meteorological drought, computed at various time scales utilising information acquired from the Global Drought Observatory database. The results show and identify the years that are water-stressed in the study area, which indicated that low vegetation abundance and high temperature in the Free State Province occurred in 2000, 2008, and 2009. The result also shows that the summer season over large parts of the study region is characterized by moderate to extreme drought while winter seasons have light drought conditions during the same time. Seasonal and inter-annual comparisons of drought events outlined in the Vegetation Condition Index (VCI) provided a useful tool for analysing the temporal and spatial evolution of regional drought and for estimating potential environmental impacts. Drought events occurrence in the study area is less frequent and milder in winter months while summer droughts of different years were more severe. The results of this study can also be used as a tool for monitoring droughts and support for decision making in the evaluation and management of regional drought, for disaster preparedness.