Comparative evaluation of Mann–Kendall and innovative trend analysis for spatiotemporal rainfall variability in the Upper Blue Nile Basin

Abstract

The Upper Blue Nile Basin (UBNB) in Ethiopia is a critical hydrological region where rain-fed agriculture supports over 80% of employment, making assessment of long-term rainfall trends essential for water resources management and food security. This study integrates the Mann–Kendall (MK), Modified MK (mMK), Sen's slope, Innovative Trend Analysis (ITA), and Percent Bias to evaluate annual and seasonal rainfall variability across 41 stations in the UBNB from 1981 to 2024, using National Meteorology Agency records supplemented by CHIRPS data for gap-filling. The framework is extended with Pettitt change-point and Sequential Mann-Kendall (SQMK) tests to capture trend evolution, and Benjamini–Hochberg false discovery rate (FDR) testing to assess whether locally significant station trends constitute genuine basin-wide signals. Results show about 75% of stations exhibit increasing annual rainfall, with significant increases at Asendabo (9.22 mm/yr) and Nekemte (7.35 mm/yr) and significant decreases at Mota (−4.11 mm/yr) and Debre Tsige (−2.55 mm/yr). Summer and autumn rainfall show dominant increasing trends, while winter shows widespread declines across ~65% of stations. ITA consistently detects subtle sub-trends across low-, medium-, and high-rainfall regimes overlooked by MK/mMK, with strong inter-method correlations (R² up to 0.8774). Change-point analysis identifies significant shifts at 34% of stations, clustered around 2002–2005 and 2011–2012, while FDR testing confirms the annual and autumn increases as genuine field-significant signals, whereas the widespread spring increase does not survive correction for multiple testing and spatial dependence. These findings provide a more rigorously validated basis for climate adaptation and water resource management in the basin.