Statistical analysis of the return period and probability distribution of annual maximum rainfall in southern Ethiopia

ABSTRACT Estimating the return period of extreme rainfall is crucial for understanding hydroclimatic hazards and supporting infrastructure design, agricultural planning, and flood risk management. This study evaluates return periods and identifies best-fit probability distributions for annual maximum rainfall (AMR) in southern Ethiopia, a region with limited prior research. Twenty-five years (1987–2011) of rainfall data from four meteorological stations – Addis Ababa, Arba Minch, Hawassa, and Mega – were obtained from the Ethiopian National Meteorological Service Agency. Return periods and probabilities were estimated using Weibull's plotting position formula. Three statistical distributions (Normal, Log-normal, and Gumbel) were applied and assessed using root mean square error (RMSE). The Gumbel distribution provided the most accurate representation across all stations. Maximum AMR values for a 26-year return period were 351 mm (Addis Ababa), 307 mm (Arba Minch), 255 mm (Hawassa), and 468 mm (Mega). These results provide essential information for flood risk management and agricultural planning and offer ground-based data to support the validation of satellite-based precipitation products and hydrological models. Combining statistical rainfall analysis with Earth observation techniques, this study contributes to enhancing climate resilience and water resource management in southern Ethiopia.