The hyporheic zone, defined by shallow subsurface pathways through river beds and banks beginning and ending at the river, is an integral and unique component of fluvial systems. It hosts myriad ...hydrologically controlled processes that are potentially coupled in complex ways. Understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale. Thus, the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of Water Resources Research. This manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in Water Resources Research, from the birth of the concept of the hyporheic zone as a hydrologic black box (sometimes referred to as transient storage zone), to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view, to its mature emergence as an interdisciplinary research field that employs the wide array of state‐of‐the‐art tools available to the modern hydrologist. The field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth. There are exciting opportunities for scientists that are able to tightly link the allied fields of geology, geomorphology, hydrology, geochemistry, and ecology to tackle the many open problems in hyporheic zone science.
Key Points:
Hyporheic zone science has emerged as a mature field
Hyporheic zone science continues to grow
There are many important open problems remaining
Groundwater is projected to become an increasing source of freshwater and nutrients to the Arctic Ocean as permafrost thaws, yet few studies have quantified groundwater inputs to Arctic coastal ...waters under contemporary conditions. New measurements along the Alaska Beaufort Sea coast show that dissolved organic carbon and nitrogen (DOC and DON) concentrations in supra-permafrost groundwater (SPGW) near the land-sea interface are up to two orders of magnitude higher than in rivers. This dissolved organic matter (DOM) is sourced from readily leachable organic matter in surface soils and deeper centuries-to millennia-old soils that extend into thawing permafrost. SPGW delivers approximately 400-2100 m
of freshwater, 14-71 kg of DOC, and 1-4 kg of DON to the coastal ocean per km of shoreline per day during late summer. These substantial fluxes are expected to increase as massive stocks of frozen organic matter in permafrost are liberated in a warming Arctic.
The irregular planform morphology of rivers leads to formation of hyporheic zones along its banks. This study investigates how hyporheic exchange in stream banks, whose planform is idealized as ...sinusoidal, is affected by net gains from and net losses of water to the adjacent aquifer. These effects are studied via numerical modeling of groundwater flow adjacent to sinuous channels across a broad range of sinuosity and gain/loss magnitude. Hyporheic zone areas and fluxes both decrease exponentially with increasing magnitude of net gain or loss relative to the case where the stream has no net flux of water (neutral). Residence time through the hyporheic zone also decreases with gain/loss magnitude. The hyporheic zones become constrained near the apex of bends, indicating that these areas could be hot spots for mixing and biogeochemical processing. Hyporheic zones in channels with smaller sinuosity are more prone to hyporheic flux and area reduction while very sinuous channels are able to maintain a hyporheic zone even under largely losing or gaining conditions. Equations fitted to the suite of simulation results allow for prediction of hyporheic flux, area, and residence time on the basis of aquifer hydraulic conductivity, channel sinuosity, and the ratio of along-valley and across-valley mean head gradients.
Non‐Fickian transport ubiquitously occurs across all scales within fractured geological media. Detailed characterization of non‐Fickian transport through single fractures is thus critical for ...predicting the fate of solutes and other fluid‐borne entities through fractured media. Our direct numerical simulations of solute transport through two‐dimensional rough‐walled fractures showed early arrival and heavy tailing in breakthrough curves (BTCs), which are salient characteristics of non‐Fickian transport. Analyses for dispersion coefficients (DADE) using the standard advection‐dispersion equation (ADE) led to errors which increased linearly with fracture heterogeneity. Estimated Taylor dispersion coefficients deviated from estimated DADE even at higher Peclet numbers. Alternatively, we used continuous time random walk (CTRW) model with truncated power law transition rate probability to characterize the non‐Fickian transport. CTRW modeling markedly and consistently improved fits to the BTCs relative to those fitted with ADE solutions. The degree of deviation of transport from Fickian to non‐Fickian is captured by the parameter β of the truncated power law. We found that β is proportional to fracture heterogeneity. We also found that the CTRW transport velocity can be predicted based on the flow velocity. Along with the ability to predict β, this is a major step toward prediction of transport through CTRW using measurable physical properties.
Key Points
High‐resolution fracture map is used to examine non‐Fickian transport
The degree of non‐Fickian transport is proportional to fracture heterogeneity
CTRW transport velocity can be predicted based on mean flow velocity
Hyporheic exchange in riverbeds is driven by current‐bed topography interactions. Because riverbeds exhibit topographic roughness across scales, from individual grains to bedforms and bars, they can ...exhibit fractal patterns. This study analyzed the influence of fractal properties of riverbed topography on hyporheic exchange. A set of synthetic fractal riverbeds with different scaling statistics was used as inputs to sequentially coupled numerical simulations of turbulent channel flow and hyporheic flow. In the analysis, the maximum power spectrum (dune size) and the fractal dimension (topographic complexity) were considered as independent variables and we then investigated how interfacial fluxes and hyporheic travel times are functionally related to these variables. As the maximum power spectrum increases (i.e., dune height to flow depth ratio), the average interfacial flux increases logarithmically whereas it increases exponentially with an increase in fractal dimension. Hyporheic exchange is more sensitive to additional roughness (larger fractal dimensions) than to bedform size (larger maximum power). Our results imply that fractal properties of riverbeds are crucial to predicting hyporheic exchange. The predictive relationships we propose could be integrated with reduced complexity, large‐scale models. They can also be used to design artificial topographies that target hyporheic ecosystem services.
Key Points
A series of numerical simulations were conducted to explore how the fractal properties of bedforms are related to hyporheic exchange
The results show that hyporheic exchange flux increases exponentially with respect to the bed topography fractal dimension
Fractal properties of riverbeds are crucial to predicting hyporheic exchange
Snap‐off is an important dynamic multiphase flow phenomenon which occurs in porous media. It plays a dominant role in the residual trapping and mobilization/immobilization of nonwetting fluids such ...as hydrocarbons or CO2. Current studies, applications, and threshold criteria of snap‐off are mostly based on static or equilibrium conditions. Thus, the dynamics of snap‐off which is relevant for many real world applications has rarely been systematically studied. While a static criterion indicates the snap‐off potential for nonwetting fluids, the competition between the time required for snap‐off and the local pore throat capillary number determines whether snap‐off actually occurs. Using a theoretical model to couple the wetting film thickness to the local capillary number at the pore throat, we analyzed the dynamics of the wetting/nonwetting interface instability in sinusoidally constricted capillary tubes. The influence of dynamic factors as encapsulated by the effect of local capillary number on nonwetting fluid snap‐off time were investigated for varying pore throat to pore body aspect ratio and pore body distances. The analysis showed that snap‐off can be inhibited by a sufficiently large local capillary number even in cases where the static snap‐off criterion has been met.
Key Points:
Large local capillary number can inhibit snap‐off occurrence
Snap‐off time is calculated against controlling parameters
Snap‐off process is coupled with film thickness
We know little regarding how geomorphological features along the surface‐groundwater interface collectively affect water quality and quantity. Simulations of surface water‐groundwater exchange at ...increasing scales across bed forms, bars and bends, and basins show that groundwater has a power‐law transit time distribution through all these features, providing a purely mechanistic foundation and explanation for temporal fractal stream chemistry. Power‐law residence time distributions are almost always attributed to spatial variability in subsurface transport properties‐ something we show is not necessary. Since the different geomorphological features considered here are typical of most landscapes, fractal stream chemistry may be universal and is a natural consequence of water exchange across multifaceted interfaces.
In cold regions where soils freeze and thaw annually, the ground surface deforms due to the density difference between groundwater and ground ice. Here we mapped thaw subsidence and frost heave ...signals over the Toolik Lake area on the North Slope of Alaska using 12 ALOS PALSAR Interferometric Synthetic Aperture Radar (InSAR) scenes (2006–2010). For the first time, we jointly analyzed InSAR observations with a large number of soil measurements collected within ~ 100 km of the Toolik Field Station. We found that the InSAR-observed deformation patterns are mainly related to soil water content and the seasonal active layer freeze-thaw (FT) cycle. We did not observe any substantial long-term subsidence trend outside the 2007 Anaktuvuk River Fire scar. This suggests that the magnitude of the maximum annual thaw subsidence did not change much outside the fire zone during the study period. The joint analysis of InSAR and field observations allows us to show that the amplitude of the seasonal thaw subsidence is proportional to the total amount of ice that has melted into liquid water at any given time. We note that topography influences the spatial distribution of soil water content, and the availability of soil water influences the type of vegetation that can grow. As a result, we found that the average seasonal thaw subsidence increases along a geomorphic-ecohydrologic transect with heath vegetation on the drier ridge-tops, tussock tundra on hillslopes, and sedge tundra at the wet lowland riparian zones. In addition, we detected a net uplift between late July and early September, mostly in the wetter riparian zone that experienced a larger seasonal thaw subsidence. Toolik Field Station in-situ records suggest that the air temperature fluctuated around or below freezing in early September during the ALOS PALSAR data acquisition times (at ~ 12 am local time). In this scenario, ice can be formed at the top of the soil, which leads to frost heave in saturated soils. Our results highlight how InSAR can improve our understanding of active layer freeze-thaw and water storage dynamics in permafrost environments.
•InSAR data were analyzed with 220 in-situ Arctic Foothills soil samples.•Maximum seasonal thaw subsidence is proportional to the active layer soil water content.•Topography and vegetation covers influence the magnitude of thaw subsidence.•No long-term subsidence was detected outside the 2007 Anaktuvuk River fire zone.•InSAR and temperature data suggest that frost heave occurred in early September.
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