Diagnosing apparent linear recession signatures using hyperbolic and phase-dependent models in tropical Island catchments

Master Recession Curves (MRCs) exhibiting apparent semi-logarithmic linearity are commonly interpreted as evidence of homogeneous linear-reservoir behaviour. However, physically based groundwater theory predicts that recession dynamics may become nonlinear as storage, transmissivity, and hydraulic connectivity evolve during catchment depletion. Whether apparent MRC linearity represents a structurally invariant catchment property or an emergent signature of aggregated recession processes remains unresolved, particularly in tropical island catchments characterised by steep terrain, rapid runoff generation, and heterogeneous subsurface flow pathways. This study addresses this question using 217 streamflow recession events from five tropical monsoonal catchments on Ambon Island, eastern Indonesia. A hierarchical multi-diagnostic framework integrated global exponential recession modelling, hyperbolic recession analysis, phase-specific recession-factor estimation, residual diagnostics, continuous segmented regression, information-theoretic model comparison, and uncertainty-aware interpretation of the Recession Steepness Index ( 𝑅 𝑆 𝐼 = − l n ⁡ 𝑘 ) . Although global exponential models consistently achieved excellent descriptive performance (R 2 = 0.979–0.987), all catchments exhibited systematic phase dependence 𝑘 𝑒 𝑎 𝑟 𝑙 𝑦 < 𝑘 𝑙 𝑎 𝑡 𝑒 , temporally organised residual curvature, and substantially stronger support for segmented models. Statistically supported breakpoints clustered between approximately 7 and 10 days, indicating reproducible transitions in effective recession behaviour. These findings demonstrate that high global semi-logarithmic goodness of fit may conceal internally evolving depletion dynamics and that a single recession constant does not necessarily provide a structurally complete representation of catchment recession. Because groundwater observations, transmissivity measurements, tracer evidence, evapotranspiration estimates, and coastal boundary effects were not independently quantified, the inferred mechanisms remain process-consistent hypotheses rather than direct hydrogeological verification. Overall, this study provides a reproducible, uncertainty-aware framework that explicitly distinguishes descriptive model performance, structural recession behaviour, and physical process interpretation, thereby strengthening the interpretation of low-flow persistence and storage–discharge dynamics in tropical island catchments.