Damping of stream temperature time series by hyporheic exchange
Kraseski, Kristin Ann
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Stream temperature is an important water quality attribute that influences instream processes, reaction rates, and species distributions. Solar radiation has the largest effect on water temperatures, but heat exchange also occurs at the streambed interface. Hyporheic exchange is a type of streambed interaction that occurs when channel water advects through the streambed. Studies of thermal regimes suggest that mean temperature of hyporheic and surface waters are similar on daily timescales, but that patterns and timing extreme values are sufficiently different that this exchange is a common contributor to thermal heterogeneity within streams. This study seeks to investigate how temperature changes may occur based on the rates of heat transfer in hyporheic pathways between water and hyporheic sediments. An extensive literature review identified reported ranges of common hyporheic characteristics that affect movement and heat transfer within hyporheic flowpaths. A laboratory experiment was used to measure heat transfer rates between water and sediment. Four bed sediments (sand, fine gravel, coarse gravel, and a mix) were initially different temperatures and then added to room temperature water. Water-sediment temperatures were measured every half second during the resulting interaction. The majority of heat transfer, or conduction, consistently occurred within the first few seconds of sediment addition. This, combined with the slow velocities in most hyporheic flow paths, would imply near-instantaneous conduction for these systems. This information was used to develop two spreadsheet models: one that subjects stream temperature time series to a set of calculated “damping coefficients” based on travel time in the hyporheic zone, and another that explicitly calculates heat transfer terms based on common literature values. Results showed that heat transfer in the hyporheic zone damps temperature patterns, and water present in a flow path for four daily cycles would have temperature patterns almost indistinguishable from the average temperature. Assuming near-instantaneous conduction, this model should be applicable to most stream systems where hyporheic characteristics and travel times can be estimated.