Analysis of spatiotemporal rainfall variability and trend in Gilgel Gibe Watershed, Southwest Ethiopia: 1985–2017

This study aimed to investigate the spatio-temporal rainfall variability and trend of the Gilgel Gibe watershed. Rainfall variability was analyzed using coefficient of variation (CV), standard precipitation concentration index (PCI), and rainfall anomaly index (RAI). The rainfall trend was conducted using Man-Kendall (MK), Modified Mand-Kendall (MMK), Sen’s slope estimator, and innovative trend analysis (ITA). Based on the result, the monthly rainfall showed 51.2% variation, the highest rainfall variability. The respective CV value of winter, autumn, summer, and annual rainfall was 53.2%, 26.82%, 21.65%, and 16.84%, which indicates high, moderate, and low rainfall variation, respectively. The PCI and RAI of the Gilgel Gibe watershed varied from 9 to 17, and − 4.545 to + 5.94, respectively. Based on the Sen’s slope estimator of trend analysis, rainfall decreased by 10.873 mm/year during the summer, whereas from the ITA result, it was increasing by 4.143 mm/year. The Sen’s slope and ITA test for winter rainfall showed a 9.424-mm/year and 10.56-mm/year increasing rate, and the autumn rainfall showed a − 4.371-mm/year and − 4.613-mm/year decreasing rate, respectively. The Sen’s slope and SITA of the annual rainfall showed a 1.33-mm/year decreasing rate and a 9.931-mm/year increasing rate. The spatial rainfall trend analysis showed that the Gilgel Gibe watershed showed maximum and minimum spatial variation during autumn and summer. The rainfall trend analysis showed discrepancies among the trend investigating methods. However, the result of SITA was found to be more reliable. The finding of this study is vital for the preparation of local-scale adaptive strategies and efficient water resources management plans to reduce the adverse impacts of climate change.