Flow and temperature dynamics in the hyporheic zones

Hyporheic zones (HZs) are key compartments in river-aquifer systems, characterized by continuous exchange of water, solutes, nutrients and bacteria between stream water and aquifers. Flow in the HZs is driven and controlled by interplay between bedform topography, stream stage fluctuations, temperature oscillations in stream water temperature, hydraulic conductivity, channel gradients and other sediment properties. Resultant hyporheic exchange fluxes affect biogeochemical turnover and is a determinant of biogeochemical hot-spots and hot-moments. While previous studies have improved our understanding of hyporheic exchange processes for steady state flow and temperature conditions, little attention has been paid to the transient behaviour of HZs. To evaluate the dynamic development of HZ characteristics in response to transience in surface-water flow and temperature, a two-dimensional flow and transport model with a time-varying boundary condition at the sediment-water interface is used. The impacts of single peak-flow events, repeated and superimposed events and diurnal variations in surface-water temperature are systematically explored. Moreover, we quantify the adequacy of reduced-order models to represent the hyporheic characteristics under transient flow conditions by comparing our numerical results with field-observations. Peak-flow events cause the expansion and contraction of the HZ, which is controlled by events yet is modulated by ambient groundwater flow. Discharge of older hyporheic water is observed for higher bedform aspect ratio (i.e. ratio between the bedform amplitude and wavelength) and lower channel slopes. Variations in residence times during peak-flow events lead to the development of larger areas of potential nitrification and denitrification in the HZ for longer duration. However, when superimposed and repeated peak-flow events were taken into account, we observed that separation between the occurrence of two events largely determines the discharge of older or younger hyporheic waters. Increased time-lag between two events show higher variability in residence time distribution. Moreover, flatter numerical breakthrough curves for events with increased separation and longer duration of the events are observed. Also, when temperature-dependent flow is considered in ripple-like bedforms, significant variations in hyporheic exchange fluxes are highlighted. Temperature signals leaving the HZs are cooled and lagged which are dependent on the parameters determining the heat transport time scales (i.e. hydraulic conductivity and channel slopes). Furthermore, it is shown that the reduced-order models can adequately predict the dynamics of HZs for a step-pool bedform, however there are minor deviations for a low-gradient run when compared to field-observed data. The presented findings illustrate the importance of consideration of transience in hyporheic exchange processes and potential implications for river management and restoration. These findings also suggested that mechanistic knowledge derived from the reduced-order models should be considered with caution as there are limitations in our ability to upscale hyporheic exchange processes