Heat transport and tracing within the hyporheic zone of a pool-riffle-pool sequence

Hyporheic water is thought to infiltrate at the head of a riffle which in turn is complemented by upwelling back to the stream at the tail of the riffle in a pool-riffle-pool (PRP) sequence. Heat tracing is a potentially useful method to characterize these hyporheic flow paths and quantify associated fluxes. Temperature was monitored within a PRP sequence for several days. Temperature in the hyporheic zone reflected the diel temperature change in the river but not uniformly. The observed thermal pattern exhibited deeper penetration of thermal oscillations below the head pool and shallower penetration below the tail pool. This pattern is consistent with the conceptual model of hyporheic exchange over a PRP sequence. One-dimensional analytical heat transport models were used at different points below the PRP sequence to estimate distributed vertical fluid fluxes. The calculated fluxes exhibit a trend that follows the expected distribution for a PRP sequence but modified for a losing stream. Deviation of both magnitude and distribution of fluxes from the conceptual ‘downwelling-to-upwelling’ model is partly due to the dominantly losing conditions at the study site but the trends are consistent with a losing stream undergoing hyporheic exchange. Violation of the assumptions in the analytical models most likely adds error to flux estimates. For this study, flux estimation methods using a temperature time series amplitude analysis more closely matched field measurements than phase methods.