Effect of hyporheic fluxes on streambed porewater temperature

Temperature is a key parameter in controlling stream water quality because of its influence on aquatic organism metabolism and biogeochemical reaction rates. Stream water downwells into the sediment and upwells into the stream due to hyporheic fluxes. Consequently, daily temperature fluctuations of downwelling stream waters influence pore-water temperature within the sediment due to hyporheic exchange. To analyze the effects of hyporheic flows on pore-water temperature, we develop a process-based model, which predicts temperature distribution within the hyporheic zone in rivers characterized by pool-riffle morphology. We solve the heat transport equation with a Lagrangian framework, assuming that the transverse dispersivity is negligible and that the thermal boundary condition at the streambed is the daily stream water temperature fluctuations, which have been modeled as a sinusoidal variation. Our model results show streambed temperature patterns changing through the day and night. Their distribution strongly relates to the stream water residence time into the hyporheic zone and consequently to the bed morphology and flow discharge. Therefore, we compare the hyporheic role of two end member streams: a small steep and a large low-gradient stream. Results show that the trend of variation of the mean hyporheic temperature has smaller daily amplitude in the former than in the latter stream because of the longer and more diversified residence times. Ratio between the hyporheic upwelling thermal flux and that of the stream show that the hyporheic zone plays a more important role in affecting stream water temperature in the small steep than in the large low-gradient stream.