hydrological regime shifts and functional river typologies in south-western nigeria

Rivers in tropical regions and specifically west africa are experiencing core hydrological changes under the twin-impacts of climate variability and intense land use change. this study analysed daily discharge for six rivers in south-western nigeria (oba, ogbese, ogun, osun, owena, yewa) over 1983–2023 (~14,600 observations per river). methods combined mann–kendall with sen’s slope (annual means), flow-duration curves (q10/q50/q90 and log-slope), descriptive statistics, percentile-based extremes (q95/q5), weibull flood-frequency, colwell’s predictability (p, c, m), and pca–k-means clustering. no river showed a significant monotonic trend (p > 0.05), yet variability was large (cv range ≈ 1.03–2.10). ogun carried the largest high flows (q10 = 488 m³ s⁻¹) and, with yewa (cv ≈ 1.03), exhibited more baseflow support; oba and ogbese were flashy (cv = 2.10 and 1.37). predictability was low across basins (p ≤ 0.246; c = 0), indicating unstable seasonality. extremes were frequent: each river recorded ~296–749 flood days and a similar number of drought days over the record; 100-year peaks in ogun exceeded 3,000 m³ s⁻¹. pca–k- means separated ogun from the other five rivers, supporting a functional typology for management. these findings argue for regime-oriented, nonstationary planning: linking operations, urban green infrastructure, and monitoring to variability, thresholds, and detected shifts rather than historical means. rivers in tropical regions and specifically west africa are experiencing core hydrological changes under the twin-impacts of climate variability and intense land use change. this study analysed daily discharge for six rivers in south-western nigeria (oba, ogbese, ogun, osun, owena, yewa) over 1983–2023 (~14,600 observations per river). methods combined mann–kendall with sen’s slope (annual means), flow-duration curves (q10/q50/q90 and log-slope), descriptive statistics, percentile-based extremes (q95/q5), weibull flood-frequency, colwell’s predictability (p, c, m), and pca–k-means clustering. no river showed a significant monotonic trend (p > 0.05), yet variability was large (cv range ≈ 1.03–2.10). ogun carried the largest high flows (q10 = 488 m³ s⁻¹) and, with yewa (cv ≈ 1.03), exhibited more baseflow support; oba and ogbese were flashy (cv = 2.10 and 1.37). predictability was low across basins (p ≤ 0.246; c = 0), indicating unstable seasonality. extremes were frequent: each river recorded ~296–749 flood days and a similar number of drought days over the record; 100-year peaks in ogun exceeded 3,000 m³ s⁻¹. pca–k- means separated ogun from the other five rivers, supporting a functional typology for management. these findings argue for regime-oriented, nonstationary planning: linking operations, urban green infrastructure, and monitoring to variability, thresholds, and detected shifts rather than historical means.