Modeling Surface-Subsurface Exchange of Heat and Nutrients

Stream water continuously enters and exits streambed material because sediments are permeable and because of the presence of near‐bed pressure gradients. These fluxes, known as hyporheic fluxes, deliver surface water with its constituents, i.e. temperature and solutes, to the streambed environment, where reactive e solutes undergo biogeochemical reactions. These transformations depend on the amount of water downwelling into and the residence time of surface water within the hyporheic zone. They cause physicochemical gradients, which sustain a rich ecotone, and affect the quality of both surface and subsurface waters. Here, we present a set of analytical solutions to predict the mean hyporheic downwelling fluxes and the hyporheic residence time distribution induced by the interaction between stream flow and two important bedforms: dune‐like and pool‐riffle. We also introduce two dimensionless numbers to characterize the hyporheic zone redox and thermal conditions at the bedform scale. These numbers are easy to quantify because they are derived by commonly measured reach‐scale hydromorphological stream data, which include hydraulic depth, mean flow velocity, mean streambed slope and streambed grain size distribution.