A semianalytical three-dimensional process-based model for hyporheic nitrogen dynamics in gravel bed rivers

We present a three-dimensional semianalytical process-based model of dissolved oxygen and dissolved inorganic nitrogen (DIN) transformation within the hyporheic zone of gravel bed rivers. Oxygen and multispecies solute transport is solved within a Lagrangian framework with transformation of DIN species modeled by linearized Monod’s kinetics, with temperature-dependent reaction rate coefficients derived from field experiments. Our solutions, which are obtained under the assumptions of sediments with uniform hydraulic properties and negligible local dispersion, highlight the importance of morphological characteristics of the streambed on DIN transformations within the hyporheic zone. By means of this model we explore the effects of streambed topography and relative abundance of ammonium and nitrate in stream waters on the reactive nitrogen cycle in the hyporheic zone of gravel bed rivers with a pool and riffle morphology. Our model shows complex concentration dynamics within the hyporheic zone that may act as a source or a sink of nitrogen depending on the residence time distribution, which can be parameterized in terms of streambed morphology, and the ratio between the in-stream concentrations of ammonium and nitrate. Application of the model to seven natural streams shows good agreement between predicted and measured nitrous oxide emissions from their hyporheic zone.