Time-series temperature analyses indicate conduction and diffusion are dominant heat-transfer processes in fine sediment, low-flow streams

Highlights: • Thermal conduction is dominant in fine-sediment streambeds. • Thermal conduction makes thermal methods challenging in fine-sediment streambeds. • Numerical and analytical thermal methods are reliable tools fine-sediment, low flow streams. • The seasonal-trend decomposition method is not suitable for fine-sediment streambeds. • The seasonal-trend decomposition method is likely suitable where advection dominates. Stream-groundwater exchange has been investigated in a wide range of hydrologic settings, though very few studies have focused on fine-sediment streambeds. Well-established thermal methods (i.e., analytical and numerical solution of time-series temperature depth-profiles) in combination with Darcy's and electrical resistivity (ER) evaluations were implemented to improve understanding of processes dominating flow and transport in a low permeability and low-flow coastal stream such as Oso Creek, Texas. The seasonal-trend decomposition using Loess (STL) is tested as a potential means to differentiate between advection and conduction and is validated against groundwater fluxes derived from the other well-established thermal methods. The numerical and analytical solutions indicate groundwater upward discharge was 9 mm d⁻¹ for summer and 3.5 mm d⁻¹ for winter, corresponding to the region's extreme drought conditions. These types of low flow conditions are usually accompanied by hyporheic flow, limiting the vertical flow assumption. While the numerical and analytical methods provide good insight into streambed hydrology for a low-permeability and low-flow stream in a semiarid coastal area, there are limitations associated with the STL method. The analytical and numerical thermal methods employed herein confirm that conduction and diffusion are the dominant processes of heat and solute transfer in fine-sediment streambeds, providing an improved understanding of process-based groundwater-stream interaction and water resources in this type of settings.