Assessment of long term hydrological variability and flow regime alterations in the Subarnarekha River basin using non parametric trend analysis and wavelet approaches

Understanding long-term hydrological variability in monsoon-dominated tropical rivers requires integrating trend detection with multi-scale variability analysis. This study assesses spatial and temporal flow variability across eight gauging stations of the Subarnarekha River using the Mann-Kendall test, Sen’s slope estimator, Pettitt change-point detection, and Morlet wavelet analysis. Results reveal strong spatial heterogeneity rather than a uniform basin-wide trend. A statistically significant increasing trend is observed at Gopiballavpur (Z = 2.024, p = 0.043; + 472.86 MCM yr⁻¹), while Adityapur shows a near-significant increase. Other stations exhibit positive but statistically insignificant trends, whereas Jamshedpur and Muri indicate weak declining tendencies, suggesting relatively stable discharge conditions in the upper and industrial reaches. Pettitt tests identify significant regime shifts ( P < 0.001) across all stations, indicating non-stationary discharge behaviour within the basin. Wavelet analysis highlights dominant intra-annual variability, with synoptic-scale fluctuations (2–10 days) contributing more than 50% of spectral energy at several stations, indicating strong short-term discharge variability. Ghatsila exhibits comparatively balanced synoptic and annual variability, suggesting stronger monsoonal coherence, while inter-annual variability remains weak across most stations. Longitudinal analysis indicates increasing dominance of high-frequency variability from upstream to downstream reaches. The transitional mid-reach retains comparatively balanced multi-scale variability, whereas downstream sections exhibit greater short-duration discharge fluctuations and higher discharge magnitudes. Overall, the results indicate spatially heterogeneous discharge variability, significant regime shifts, and dominant short-term hydrological fluctuations across the Subarnarekha River basin.