Thermal Regime in the Hyporheic Zone of Pool-Riffle Streams

Interactions between surface flow dynamics and stream sinuosity drive significant hyporheic exchange and concomitant thermal advection within streambed sediments, processes critical to ecological and biogeochemical functioning. This study extends the laboratory meandering pool-riffle stream model established by Huang & Chui (2022) employing combined heat tracer experiments and coupled numerical simulations to investigate hyporheic zone thermal responses to in-stream temperature fluctuations. Numerical models, validated against laboratory data, elucidate the spatiotemporal complexity of the thermal regime within the HZ. Spatially, downwelling zones exhibit greater responsiveness to in-stream diel temperature variations relative to upwelling zones. Higher discharge amplifies the influence volume while preserving temporal response patterns. Temporally, hyporheic temperatures closely track in-stream fluctuations. Sinuosity substantially modulates the thermal regime by expanding regions experiencing pronounced temperature variations and extending heat residence times, thereby enhancing downstream thermal stability and enlarging zones conducive to biogeochemical activity. The intra-meander floodplain exhibits intensified thermal fluctuations, functioning as potential biogeochemical transformation hotspots due to heightened hydraulic connectivity. Collectively, stream sinuosity and discharge critically govern the heterogeneity of hyporheic thermal environments, with significant implications for ecological processes and stream restoration strategies.