Rainfall homogeneity, variability, and trends across unimodal and bimodal regimes in the Wami Basin, Tanzania

ABSTRACT The graphical abstract links rainfall data, statistical tests, spatial outputs, and hydrological implications for the Wami basin. Rainfall regime coexistence influences hydrological variability in semi-arid regions, but is rarely treated explicitly in basin-scale assessments. This study evaluates rainfall homogeneity, variability, anomalies, seasonality, and trends across unimodal and bimodal regimes in Tanzania's Wami basin from 1985 to 2014. Results from Pettitt, Buishand, and SNHT indicated that more than 80% of monthly, seasonal, and annual series were homogeneous. Most changepoints occurred in dry months and at the start/end of rainy seasons, and were rarely confirmed by all three tests. Bimodal regime exhibited a moderate coefficient of variation (CV; 20–30%) in wet months but a very high CV during October-December (>40%). Unimodal regime CV was low from November to April and high from May to October. Rainfall anomaly analysis showed alternating wet, dry, and mixed years, with dry conditions evident in 1992–1993, 2003, and 2005. Rainfall was highly seasonal, especially at unimodal stations, where PCI values frequently exceeded 20. Mann-Kendall/Sen's slope and Innovative Trend Analysis revealed spatially uneven trends, slopes range −5.6 to +2.3 mm yr−1. Significant changes, including unimodal wet-season contraction, were localized across regimes. Annual rainfall totals obscure seasonal and regime-based differences, providing basis for hydrological assessment and water planning in semi-arid basins where contrasting rainfall regimes coexist.