Seasonal and Annual Rainfall Variability and Trends in the Dawuro Zone, Ethiopia

Rainfall, a critical determinant of agriculture, water resources, and socioeconomic stability, was analyzed for seasonal and annual variability in the Dawuro Zone, Ethiopia, from 1991 to 2020. Using time series data from seven meteorological stations and four virtual stations, statistical methods such as the Coefficient of Variation (CV), Standard Anomaly Index (SAI), Sen’s Slope Estimator, and Modified Mann-Kendall test were employed, alongside Inverse Distance Weighted interpolation in ArcGIS for spatial analysis. The study revealed significant variability, with Tocha recording the highest mean annual rainfall (1998 mm) and Tercha the lowest (1399.5 mm). Tercha exhibited the least consistent rainfall, while Gasechere showed the highest variability (CV 31.82%, SD 600.37 mm). The Kiremt season (June-September) contributed the most to annual rainfall, with Tocha receiving 973.99 mm and Tercha 696.03 mm, while the Belg (February-May) and Bega (October-January) seasons showed lower contributions and higher variability, particularly at Gena Bossa and Gasechere. Spatially, the southwest, south, and southeast regions received higher rainfall, while the north and northeast experienced the lowest amount. This study investigates the onset, cessation, and duration of the rainy season in the Dawuro Zone, Ethiopia, from 1991 to 2020, with particular attention to the Kiremt season (June-September). The start of the rainy season, characterized by heavy rainfall, varies considerably, from mid-September to late September, with delays in years like 1994, 2000, and 2015 (around September 27-28) and earlier starts in 1992 and 2006 (around September 19-20). Cessation dates, occurring between mid-December and late December, show similar variability, with longer seasons ending around December 19-20 (e.g., 1995, 2005, 2015) and shorter ones around December 15-16 (e.g., 2000, 2010). The length of the growing season ranges from 90 to 110 days, reaching a maximum of 106-108 days in 1994, 2002, and 2016, and decreasing to 96-98 days in 1998 and 2012. This variability, driven by factors such as the Inter-Tropical Convergence Zone and El Niño-Southern Oscillation, presents challenges for agricultural planning by affecting crop selection and increasing the risk of failure due to shortened seasons or waterlogging in extended ones. The next study will consider the association between seasonal rainfall variability and global systems like El Niño-Southern Oscillation, Indian Dipole oscillation, and other global systems over the study area. The study recommends expanding meteorological stations, integrating findings into regional policies for sustainable agriculture and water management, and enhancing early warning systems to address the variable rainfall patterns across the Kiremt, Belg, and Bega seasons.