The Effect of Sand-Bedded Stream Heterogeneity on Nitrous Oxide Emissions from Hyporheic Zone of Dune-Like Bedforms

Heterogeneity of the streambed fine sediment hydraulic property, K, may have important implication to nitrous oxide emissions from streams as it may affect hyporheic residence times. Here, we systematically investigate the effect of heterogeneity, which is the typical formation for sand bedded streams, and anisotropy of K on nitrous oxide emissions from the hyporheic zone induced by flow-dune interaction with a set of numerical simulations based on Monte Carlo realizations. We quantify K-heterogeneity impact on hyporheic hydraulics and provide a new interpretation of its effect at the reach scale for hyporheic processes. Our main result supports the use of the homogenous K-field assumption for quantifying both hyporheic hydraulics and processes for sand bedded streams with dune-like bedforms for low heterogeneity values, which are typical values for this formation, smaller than σ²_lnK=0.6 regardless of anisotropy. A major result is that the K-field heterogeneity compresses the hyporheic flow cell near the streambed surface and anisotropy reduces uncertainty. The hyporheic zone morphological change reduces the extent of the downwelling zone, decreases the spatially averaged downwelling flux and increases the median residence time due to increase length of pathlines due to tortuosity.