Hydraulic and thermal effects of in-stream structure-induced hyporheic exchange across a range of hydraulic conductivities

In-stream structure-induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel-spanning weir (represents log dams, boulder weirs) to field data and varied K from 10(-7) to 10(-2) m/s (silt to gravel). Surface water stopped cresting the weir for K>10(-3) m/s. Non-Darcy hyporheic flow was also prevalent for K > 10(-3) m/s, and velocity errors using non-CFD models ranged up to 32.2%. We also modeled weir-induced heat transport during summer. As K increased from 10(-7) to 10(-3) m/s, weir-induced hyporheic heat advection steadily increased. Cooling and buffering along hyporheic flow paths decreased with increasing K, particularly above K=10(-5) and 10(-4) m/s, respectively. Vertical heat conduction between surface water and groundwater near the weir decreased with increasing K, particularly for K>10(-5) m/s. Conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow. Downstream surface water cooling by hyporheic advection increased steadily with K as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths. Yet such effects were small (0.016 degrees C) even at K=10(-3) m/s. The largest thermal effect of weir-induced exchange was therefore spatial expansion of subsurface diel variability (particularly for K>10(-5) m/s) which affects benthic habitat and chemical reactions. The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread.