Spatio-temporal analysis of drought frequency and severity in the Bilate Watershed, Ethiopia: A multi-scalar SPI Approach

Drought is a recurring climate hazard in the Bilate Watershed of southern Ethiopia, affecting water availability, agricultural production, and ecosystem services. Yet, its variation across different time scales and locations within the watershed remains poorly understood. This study examines meteorological drought using the Standardized Precipitation Index (SPI) at 3-, 6-, and 12-month time scales. Monthly rainfall data from eight meteorological stations covering 1980–2022 were analyzed to assess drought frequency, severity, persistence, and spatial distribution. The results show that drought characteristics vary considerably across time scales and locations. Mild droughts were the most common, while severe and extreme droughts occurred less frequently but were associated with substantial rainfall deficits. Shorter SPI periods captured frequent short-term moisture fluctuations, whereas longer periods revealed more persistent deficits with greater implications for water-resource availability. Several widespread drought episodes affected multiple stations, although their timing and severity varied spatially. Spatial analysis further showed that the northern and northwestern highlands often experienced relatively severe drought conditions, highlighting the importance of geographic variability in drought assessment. Overall, this study provides a clearer understanding of drought dynamics in the Bilate Watershed and supports improved drought monitoring, targeted water-resource planning, catchment management, and climate adaptation in southern Ethiopia.