Abstract
Hydrologic exchange flows (HEFs) have environmental significance in riverine ecosystems. Key river channel factors that influence the spatial and temporal variations of HEFs include river stage, riverbed morphology, and riverbed hydraulic conductivity. However, their impacts on HEFs were often evaluated independently or on small scales. In this study, we numerically evaluated the combined interactions of these factors on HEFs using a high-performance simulator, PFLOTRAN, for subsurface flow and transport. The model covers 51 square kilometers of a selected river corridor with large sinuosity along the Hanford Reach of the Columbia River in Washington, US. Three years of spatially distributed hourly river stages were applied to the riverbed. Compared to the simulation when riverbed heterogeneity is not ignored, the simulation using homogeneous riverbed conductivity underestimated HEFs, especially upwelling from lateral features, and overestimated the mean residence times derived from particle tracking. To derive a surrogate model for the river corridor, we amended the widely used transient storage model (TSM) for riverine solute study at reach scale with reactions. By treating the whole river corridor as a batch reactor, the temporal changes in the exchange rate coefficient for the TSM were derived from the dynamic residence time estimated from the hourly PFLOTRAN results. The TSM results were evaluated against the effective concentrations in the hyporheic zone calculated from the PFLOTRAN simulations. Our results show that there is potential to parameterize surrogate models such as TSM amended with biogeochemical reactions while incorporating small-scale process understandings and the signature of time-varying streamflow to advance the mechanistic understanding of river corridor processes at reach to watershed scales. However, the assumption of a well-mixed storage zone for TSM should be revisited when redox-sensitive reactions in the storage zones play important roles in river corridor functioning.
Highlights
The concept of river corridors that integrate in-stream transport with lateral and vertical exchange across interfaces (Harvey and Gooseff, 2015) has been increasingly recognized due to their valuable ecological functions
Studies accounting for the dynamic hydrologic processes on hyporheic exchange and biogeochemical reactions have started to emerge in recent years (Larsen et al, 2014; Rahimi et al, 2015; Gomez-Velez et al, 2017; Dwivedi et al, 2018; Liu and Chui, 2018; Singh et al, 2019; Zheng et al, 2019; Kruegler et al, 2020)
We investigate how mechanistic understanding from high resolution three-dimensional (3D) simulations of flow and reactive transport induced by multiple years of transient stream discharge in a large gravel bed river corridor can be used to inform the development of reach scale reduced order models, such as the transient storage models
Summary
The concept of river corridors that integrate in-stream transport with lateral and vertical exchange across interfaces (Harvey and Gooseff, 2015) has been increasingly recognized due to their valuable ecological functions. Studies accounting for the dynamic hydrologic processes on hyporheic exchange and biogeochemical reactions have started to emerge in recent years (Larsen et al, 2014; Rahimi et al, 2015; Gomez-Velez et al, 2017; Dwivedi et al, 2018; Liu and Chui, 2018; Singh et al, 2019; Zheng et al, 2019; Kruegler et al, 2020) These studies mainly focused on the spatial and temporal patterns of HEFs and biogeochemistry for synthetic individual bedform or two-dimensional cross-sections of single geomorphic features along the river corridor. Scaling these observed complex patterns from small scale localized sampling to reach scale and basin scale investigation is challenging as they are influenced by geomorphic and hydrologic dynamics (Helton et al, 2012), channel morphology, bed roughness, and permeability (Triska et al, 1989; Hassan et al, 2015), hydrologic connectivity (Datry et al, 2008), and vegetation feedbacks (Magliozzi et al, 2019)
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