Climate Variability and Trends in the Tana and North Gojjam Subbasins, Upper Blue Nile Basin, Ethiopian Highlands

Abstract

Climate change is a serious threat to rain-fed agriculture in Ethiopia, particularly in the Upper Blue Nile Basin (UBNB), where climatic patterns are not yet accurately understood. To contextualize this knowledge gap, this study explores the rainfall and temperature trend and spatiotemporal variability in Tana subbasin (TaSB) and North Gojjam subbasin (NoGSB) in Ethiopia in 1989–2021 using records for 13 meteorological stations. Methodologically, we employed the autocorrelation before analysis, the use of the Mann–Kendall (M–K) test, and Sen’s slope estimator for trend identification, in addition to the coefficient of variation (CV) and the Precipitation Concentration Index (PCI) for characterization of variability. Missing values were handled using the multivariate imputation by chained equations (MICE) algorithm, whereas rigorous schemes for homogenization were employed to ensure a high level of confidence in the information. Main results show a marked rise in annual rainfall in both subbasins (TaSB: 5.79 mm/year; NoGSB: 7.95 mm/year), caused mainly by wetter conditions within the main rainy season (Kiremt). Conversely, the small rainy season (Belg) had large variability (CV >80% at some stations) and no definitive patterns, further evidencing its unreliability for crop production. The PCI also shows irregular rainfall distribution within the TaSB, making agro-planning problematic, while a more regular precipitation distribution is shown for the NoGSB. Spatial analysis identified strong orographic controls over rainfall, with southern parts receiving greater precipitation based on local topography and the presence of the intertropical front (ITF). Surprisingly, gridded datasets (e.g., NOAA Climate Prediction Centre [CPC]) were unable to portray localized trends found in station records, further demonstrating a need for high-resolution observations. Warming signals in temperature indicate a strong and significant rise throughout both subbasins, with a greater general increase found in the NoGSB relative to the TaSB. There is an important pattern throughout our study area: minimum temperatures (Tmin) are increasing at a greater rate compared to maximum temperatures (Tmax). Asymmetrical warming was especially marked in the dry season (Bega), a pattern that shows severe consequences for supplying water and for ecosystem integrity. The results pinpoint the paramount requirement for localized adaptation at the climatic level. Higher Kiremt rainfall would support rain-fed crops but also raises the risk for floods, while a warmer temperature and erratic Belg rains compromise crop yields as well as sources of water. These are paramount in developing resilient agricultural plans, water resource allocation, as well as policymaking in Ethiopia, like in any agroecological zone in Africa.