Abstract
AbstractHyporheic exchange is a crucial control of the type and rates of streambed biogeochemical processes, including metabolism, respiration, nutrient turnover, and the transformation of pollutants. Previous work has shown that increasing discharge during an individual peak flow event strengthens biogeochemical turnover by enhancing the exchange of water and dissolved solutes. However, due to the nonsteady nature of the exchange process, successive peak flow events do not exhibit proportional variations in residence time and turnover, and in some cases, can reduce the hyporheic zones' biogeochemical potential. Here, we used a process‐based model to explore the role of successive peak flow events on the flow and transport characteristics of bedform‐induced hyporheic exchange. We conducted a systematic analysis of the impacts of the events' magnitude, duration, and time between peaks in the hyporheic zone's fluxes, penetration, and residence times. The relative contribution of each event to the transport of solutes across the sediment‐water interface was inferred from transport simulations of a conservative solute. In addition to temporal variations in the hyporheic flow field, our results demonstrate that the separation between two events determines the temporal evolution of residence time and that event time lags longer than the memory of the system result in successive events that can be treated independently. This study highlights the importance of discharge variability in the dynamics of hyporheic exchange and its potential implications for biogeochemical transformations and fate of contaminants along river corridors.
Highlights
IntroductionRiver discharge and stage are characterized by significant temporal variability and frequent peak flow events (Bernard‐Jannin et al, 2016; Gomez‐Velez et al, 2017; Krause et al, 2011; Mojarrad et al, 2019; Salazar et al, 2014; Sawyer et al, 2009; Singh et al, 2019; Trauth & Fleckenstein, 2017; Vivoni et al, 2006; Wu et al, 2018)
A process‐based model was developed in this study and used for performing model simulations following an approach of six major steps (1) setup of simulation framework including streambed sediment geometry and parameterization of properties; (2) generation of successive peak flow event stage hydrograph; (3) inclusion of flow in porous media model; (4) solute transport model to track the response of hyporheic exchange and dynamic hyporheic zone development; (5) implementation and simulation of residence time model; and (6) simulation of numerical breakthrough curves and development of model scenarios
This research mainly focuses on variations in hyporheic flux velocities, residence times, and transport of conservative solutes under dynamic flow conditions.Our results indicate that high‐flow events caused an expansion of the hyporheic zones and increase hyporheic fluxes, indicating that events have the potential to enhance downwelling of surface water rich in oxygen, dissolved organic matter, and other nutrients, being delivered into the hyporheic zone at high concentrations into greater depths and larger streambed areas (Gomez‐Velez et al, 2017; Hester & Doyle, 2008; Singh et al, 2019)
Summary
River discharge and stage are characterized by significant temporal variability and frequent peak flow events (Bernard‐Jannin et al, 2016; Gomez‐Velez et al, 2017; Krause et al, 2011; Mojarrad et al, 2019; Salazar et al, 2014; Sawyer et al, 2009; Singh et al, 2019; Trauth & Fleckenstein, 2017; Vivoni et al, 2006; Wu et al, 2018). In addition to river stage fluctuations and pressure variations, continuous exchange of water, solutes, and energy between the water column and the hyporheic zone is controlled by interactions between geomorphological settings, channel gradient, hydraulic conductivity, sediment heterogeneity, and spatial variability in heads at the sediment‐water interface (SWI) and preferential flow paths (Gomez‐Velez et al, 2014; Gomez‐Velez & Harvey, 2014; Lotts & Hester, 2020; Marzadri et al, 2016; Menichino & Hester, 2015; Tonina & Buffington, 2011). The main impacts of peak flow events relate to the activation of deeper subsurface flow paths, enhanced
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