The opposite trends in precipitation total and extremes during two rain-seasons across Ethiopia, the Water Tower of Africa

Abstract

This study assesses trends in precipitation total and extremes across Ethiopia from 1980 to 2019, using datasets of daily gauge observations. Following quality control and homogenization, daily precipitation data from 110 stations are gridded onto a 1° × 1° latitude-longitude grid cells. Precipitation anomaly percentage (PAP) and the Expert Team on Climate Change Detection and Indices (ETCCDI) indices are applied to represent precipitation total and extremes, respectively. Regional time series are constructed using area-weighted averages derived from the grid-level data. The Theil-Sen estimator and the modified Mann-Kendall test are employed to evaluate the statistical significance of trends at the 5 % level. The results indicate an annual and Jun–Sep seasonal increase in both precipitation total and extremes, characterized by rising frequency and intensity of extreme events. The Theil-Sen slope estimates a regional annual PAP increase of 0.92 % per decade, with a more pronounced rise of 4.6 % per decade for the Jun–Sep season (main precipitation season). Significant upward regional trends are observed in extreme indices such as RX1day, R95p, R99p, R10, R25, and R40 during the forty years. Spatial analysis highlights central, northwestern, and northeastern Ethiopia as areas experiencing robust increases in precipitation total and extremes. The observed rise in precipitation total is predominantly driven by increases in precipitation extremes within the region, as demonstrated by spatial correlations between precipitation total and extremes. Conversely, the Feb–May season (secondary precipitation season) exhibits significant regional decreases in precipitation total and extremes, particularly in northeastern, eastern, and southern areas. This includes declines in frequency-related indices (R5, R10, R25) and prolonged dry spells as measured by CDD. The causes for the increase in frequency and intensity of annual and Jun–Sep seasonal precipitation in Ethiopia over recent decades need to be investigated, though it is in accordance with the expectation that anthropogenic global warming can result in a rise in precipitation extremes over most regions of the world. However, the significant changes in precipitation observed in both Jun–Sep and Feb–May seasons are a cause for concern, as they may exert a major impact on sectors and areas of the country where these two seasons hold critical importance.