Rainfall Variability and Extremes in the Hamassa Sub‐Basin, Ethiopia: Past Trends and Future Risks

Abstract

ABSTRACT Rainfall variability and recurrent droughts pose escalating challenges for smallholder farmers in Ethiopia, where climate extremes threaten crop yields, water security, and livelihoods. This study assessed rainfall variability, extremes, and drought characteristics in the Hamassa sub‐basin (1986–2023) and evaluated future precipitation dynamics to capture evolving hydroclimatic risks. Observed station data, Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS v2.0), and Climate Models Intercomparison Project Phase Six (CMIP6) simulations were analyzed across multiple timescales. Modified Mann–Kendall (MMK) tests quantified rainfall trends; droughts were characterized using the Standardized Precipitation Evapotranspiration Index (SPEI), and spatial patterns were mapped using Geographic Information Systems (GIS). Teleconnections with El Niño‐Southern Oscillation indices (ENSO) were examined to identify seasonal climate controls. Observed records revealed moderate summer rainfall variability, while spring and annual rainfall exhibited moderate to very high variability. The sub‐basin experienced recurring moderate droughts and less frequent but prolonged extreme droughts lasting up to 26 months. Extreme rainfall indices indicated a significant decline in yearly rainfall at high elevations, contrasted by intensified and more frequent heavy storms in lowland areas, elevating flood and erosion risks. ENSO demonstrated weak annual but strong seasonal influence on rainfall anomalies. Among CMIP6 simulations, the multi‐model ensemble achieved the highest skill ( r = 0.75, KGE = 0.57, PBIAS = +4.21%, RMSE = 6.16 mm/year). Projections suggest rising winter rainfall and amplified seasonal fluctuations under warming‐enhanced moisture conditions and potential monsoon shifts. This implies that the sub‐basin is undergoing multifaceted hydro‐climatic transformations marked by intertwined risks of declining seasonal rainfall, intensifying extremes, and heightening vulnerability to droughts, floods, and ENSO‐driven anomalies. These stress the need for climate‐smart agriculture, early‐warning systems, and locally tailored adaptation strategies to strengthen smallholder resilience under increasing climate variability.