Comparative Analysis of Drought Indices in the Horn of Africa

Successful drought identification and characterization are essential for effective drought risk assessment and management, requiring advanced characterization methods and the careful selection of drought indices and aggregation timescales capable of representing diverse drought features. Despite the wide range of existing drought indices, their general applicability is often constrained by dominant local conditions (climate regime, hydrology, land surface characteristics, and data availability) and the necessity to choose a suitable aggregation timescale for operational applications. This study aims to identify suitable drought indices to effectively characterize and monitor drought in the Horn of Africa (HoA). A combined cluster-area- and shape-based filtering approach, followed by three-dimensional (2D space and 1D time) connectivity, was employed to capture drought dynamics simultaneously in space and time. A range of drought indices with varying levels of complexity was evaluated and compared, including indices derived from single variables such as precipitation or soil moisture, as well as more complex multivariate indices based on combinations of multiple variables, including precipitation, potential evapotranspiration, soil moisture, normalized difference vegetation index (NDVI), and surface temperature. The performance of these indices was assessed against historical drought records reported by governmental and non-governmental organizations. The findings demonstrate that multivariate indices generally outperform univariate ones, with indices incorporating potential evapotranspiration showing high performance; however, no single index consistently excelled across all evaluation criteria. Considering both computational complexity and effectiveness in identifying drought-affected areas and capturing temporal characteristics, the combined use of the standardized precipitation evapotranspiration index (SPEI)–based indices, SPEI6 and SPEI9, is recommended for drought monitoring, planning, and management in the HoA, a region dominated by arid and semi-arid climates and recurrent, spatially extensive drought events.