Exchange of water between streams and their hyporheic zones is known to be dynamic in response to hydrologic forcing, variable in space, and to exist in a framework with nested flow cells. The ...expected result of heterogeneous geomorphic setting, hydrologic forcing, and between‐feature interaction is hyporheic transit times that are highly variable in both space and time. Transit time distributions (TTDs) are important as they reflect the potential for hyporheic processes to impact biogeochemical transformations and ecosystems. In this study we simulate time‐variable transit time distributions based on dynamic vertical exchange in a headwater mountain stream with observed, heterogeneous step‐pool morphology. Our simulations include hyporheic exchange over a 600 m river corridor reach driven by continuously observed, time‐variable hydrologic conditions for more than 1 year. We found that spatial variability at an instance in time is typically larger than temporal variation for the reach. Furthermore, we found reach‐scale TTDs were marginally variable under all but the most extreme hydrologic conditions, indicating that TTDs are highly transferable in time. Finally, we found that aggregation of annual variation in space and time into a “master TTD” reasonably represents most of the hydrologic dynamics simulated, suggesting that this aggregation approach may provide a relevant basis for scaling from features or short reaches to entire networks.
Plain Language Summary
The exchange of water between streams and the shallow subsurface of their valleys is important to ecological functions, many of which depend upon the time it takes water to travel down the valley. For decades, researchers have focused on how river form affects this travel time, leaving a limited understanding of how travel times may change throughout the year due to different wetness conditions. Here we directly address this limitation by estimating travel times through a stream and its entire valley for a period of more than 1 year. We found that change in travel times caused by different wetness conditions throughout the year was smaller than variation caused by river form. Exceptions to this rule are during very dry times of year when streamflow is low or during very wet times of the year when streamflow is high such as those resulting from spring snowmelt flooding.
Key Points
Hyporheic transit time distributions are more variable in space than in time
Transit time distributions are not transferrable during periods of rapid changes in stream discharge
Relatively short study reaches may not capture representative spatial variation along a study reach
Rivers that cease to flow are globally prevalent. Although many epithets have been used for these rivers, a consensus on terminology has not yet been reached. Doing so would facilitate a marked ...increase in interdisciplinary interest as well as critical need for clear regulations. Here we reviewed literature from Web of Science database searches of 12 epithets to learn (Objective 1-O1) if epithet topics are consistent across Web of Science categories using latent Dirichlet allocation topic modeling. We also analyzed publication rates and topics over time to (O2) assess changes in epithet use. We compiled literature definitions to (O3) identify how epithets have been delineated and, lastly, suggest universal terms and definitions. We found a lack of consensus in epithet use between and among various fields. We also found that epithet usage has changed over time, as research focus has shifted from description to modeling. We conclude that multiple epithets are redundant. We offer specific definitions for three epithets (non-perennial, intermittent, and ephemeral) to guide consensus on epithet use. Limiting the number of epithets used in non-perennial river research can facilitate more effective communication among research fields and provide clear guidelines for writing regulatory documents.
Despite the growing interest in hyporheic exchange and the associated stream ecosystem processes, few studies consider restoration of hyporheic exchange as a design goal. Here we study the design of ...three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago. Vaux's designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream. In this preliminary analysis of subsurface structure design we use two‐dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities. Structure performance is evaluated on the basis of total streambed flux, physical extent of hyporheic flow paths created, and residence time distributions along flow paths modified by the structures. High hydraulic conductivity structures bend flow paths toward and through the structures themselves; performance is controlled by the structure hydraulic conductivity. Results show low hydraulic conductivity structure performance is insensitive to the structure material; hyporheic exchange is created by deflecting flow paths away from the structure itself. Time scales of simulated exchange are great enough to promote nitrification, denitrification, respiration, and thermal buffering in the subsurface, though these processes will also be controlled by site‐specific chemical and biological factors. General design recommendations for specific restoration objectives are presented. Results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities (e.g., clay and gravel deposits and bedrock knickpoints) and flow fields in creating hyporheic flow paths.
Key Points
Restoration structures located in the subsurface can restore hyporheic exchange
Spatial and temporal scales of flow paths created are ecologically relevant
Subsurface heterogeneities interact with gradients to create complex flow paths
The hydrologic connectivity between streams and their valley bottoms (stream corridor) is a critical determinant of their ecological function. Ecological functions are known to be spatially and ...temporally variable, but spatial dimensions of the problem are not easily quantified and thus they are usually overlooked. To estimate the spatial patterns of connectivity, and how connectivity varies with changes in discharge, we developed the hyporheic potential model. We used the model to interpret a series of solute tracer injections in two headwater mountain streams with contrasting valley bottom morphologies to estimate connectivity in the stream corridor. The distributions of flow path origination locations and the lengths of hyporheic flow paths appear to vary with base flow recession, even in cases where transport timescales are apparently unchanged. The modeled distribution of origination locations further allowed us to define a spatial analog to the temporal window of detection associated with solute tracer studies, and enables assessment of connectivity dynamics between streams and their corridors. Altogether, the reduced complexity hyporheic potential model provides an easy way to anticipate the spatial distribution and origination locations of hyporheic flow paths from a basic understanding of the valley bottom characteristics and solute transport timescales.
Plain Language Summary
The manuscript details a simple method to assess the spatial connectivity of streams and their riparian zones. While the timescales of exchange in the river corridor have been broadly studied, the complimentary spatial dimension (i.e., the geometry of exchange flowpaths) remains largely unknown. The major challenge in assessing the spatial dimensions of exchange is the limited information available in the subsurface. Here, we develop a reduced complexity model of valley bottom transport to overcome these information limitations. With this model, relatively simple field site characterization and solute tracer data are combined to assess the spatial distribution of downwelling along a headwater mountain stream. We validate the model with a numerical experiment, and demonstrate its application in two watersheds of contrasting geology, repeated through baseflow recession.
Key Points
Hyporheic flow path geometry varies with discharge, even in cases where transport times remain unchanged
In‐stream discharge and along‐stream morphology cannot be used to identify flow path origination locations
Observations of solute tracers have a spatial window of detection in addition to the more broadly recognized temporal window of detection
Stream solute tracers are commonly injected to assess transport and transformation in study reaches, but results are biased toward the shortest and fastest storage locations. While this bias has been ...understood for decades, the impact of an experimental constraint on our understanding has yet to be considered. Here, we ask how different our understanding of reach‐ and segment‐scale transport would be if our empirical limits were extended. We demonstrate a novel approach to manipulate experimental conditions and observe mass that is stored at timescales beyond the traditional reach‐scale window of detection. We are able to explain the fate of an average of 26% of solute tracer mass that would have been considered as “lost” in a traditional study design across our 14 replicates, extending our detection limits to characterize flowpaths that would have been previously unmeasured. We demonstrate how this formerly lost mass leads to predicting lower magnitudes of gross gains and losses in individual reaches, and ultimately show that the network turnover we infer from solute tracers represents an upper limit on actual, expected behavior. Finally, we review the evolution of tracer studies and their interpretation including this approach and provide a proposed future direction to extend empirical studies to not‐before‐seen timescales.
Key Points
With careful design, solute tracer studies can be used to assess the fate of solute mass along flowpaths that would not normally be detected
Extended mass recovery allows us to understand the formerly unknown fate of more than 26% of all lost tracer mass
Accounting for mass recovery beyond the typical window of detection reduced inferred channel water turnover, changing interpreted spatial sources of gains
Solute transport along riparian and hyporheic flow paths is broadly expected to respond to dynamic hydrologic forcing by streams, aquifers, and hillslopes. However, direct observation of these ...dynamic responses is lacking, as is the relative control of geologic setting as a control on responses to dynamic hydrologic forcing. We conducted a series of four stream solute tracer injections through base flow recession in each of two watersheds with contrasting valley morphology in the H.J. Andrews Experimental Forest, monitoring tracer concentrations in the stream and in a network of shallow riparian wells in each watershed. We found hyporheic mean arrival time, temporal variance, and fraction of stream water in the bedrock‐constrained valley bottom and near large roughness elements in the wider valley bottom were not variable with discharge, suggesting minimal control by hydrologic forcing. Conversely, we observed increases in mean arrival time and temporal variance and decreasing fraction stream water with decreasing discharge near the hillslopes in the wider valley bottom. This may indicate changes in stream discharge and valley bottom hydrology control transport in less constrained locations. We detail five hydrogeomorphic responses to base flow recession to explain observed spatial and temporal patterns in the interactions between streams and their valley bottoms. Models able to account for the transition from geologically dominated processes in the near‐stream subsurface to hydrologically dominated processes near the hillslope will be required to predict solute transport and fate in valley bottoms of headwater mountain streams.
Key Points:
Hyporheic transport in constrained valleys is controlled by geologic setting
Hyporheic transport near large in‐stream features is not variable with discharge
Transport in broad riparian zones is controlled by dynamic hydrologic forcing
Stream and river systems transport and process substantial amounts of dissolved organic matter (DOM) from terrestrial and aquatic sources to the ocean, with global biogeochemical implications. ...However, the underlying mechanisms affecting the spatiotemporal organization of DOM composition are under-investigated. To understand the principles governing DOM composition, we leverage the recently proposed synthesis of metacommunity ecology and metabolomics, termed ‘meta-metabolome ecology.’ Applying this novel approach to a freshwater ecosystem, we demonstrated that despite similar molecular properties across metabolomes, metabolite identity significantly diverged due to environmental filtering and variations in putative biochemical transformations. We refer to this phenomenon as ‘thermodynamic redundancy,’ which is analogous to the ecological concept of functional redundancy. We suggest that under thermodynamic redundancy, divergent metabolomes can support equivalent biogeochemical function just as divergent ecological communities can support equivalent ecosystem function. As these analyses are performed in additional ecosystems, potentially generalizable concepts, like thermodynamic redundancy, can be revealed and provide insight into DOM dynamics.
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•Assemblages of metabolites were analyzed using the ‘meta-metabolome ecology’.•Surface and pore water metabolite assemblages were significantly divergent.•Divergent biochemical transformations partially drove surface/pore water differences.•Despite differences, surface and pore water have common thermodynamic properties.•We describe the phenomenon of ‘thermodynamic redundancy’.
The Raccoon River Basin is the primary source for drinking water in Iowa's largest city and plays a major role in the Mississippi River Basin's high nutrient exports. Future climate change may have ...major impacts on the biological, physiological, and agronomic processes imposing a threat to ecosystem services. Efforts to reduce nitrogen (N) loads within this basin have included local litigation and the implementation of the Iowa Nutrient Reduction Strategy, which suggest incorporating bioenergy crops (i.e., miscanthus) within the current corn–soybean landscape to reach a 41% reduction in nitrate loads. This study focuses on simulating N export for historical and future land use scenarios by using an agroecosystem model (Agro‐IBIS) and a hydrology model (THMB) at the 500‐m resolution, similar to the scale of agricultural fields. Model simulations are driven by CMIP5 climate data for historical, mid‐century, and late‐century under the RCP 4.5 and 8.5 warming projections. Using recent crop profit analyses for the state of Iowa, profitability maps were generated and nitrogen leaching thresholds were used to determine where miscanthus should replace corn–soybean area to maximize reductions in N pollution. Our results show that miscanthus inclusion on low profit and high N leaching areas can result in a 4% reduction of N loss under current climate conditions and may reduce N loss by 21%–26% under future climate conditions, implying that water quality has the potential continue to improve under future climate conditions when strategically implemented conservation practices are included in future farm management plans.
Agroecosystem and hydrology models were used to simulate nitrogen loss under multiple climate scenarios with and without the inclusion of profit‐based miscanthus integration at the 500 m resolution for the Raccoon River Basin in Iowa. Under future climate conditions, cropping system productivity and water quality were significantly reduced when no miscanthus is incorporated. The inclusion of miscanthus on low profit and high N leaching areas can result in a 4% reduction of N loss under current climate conditions and may reduce N loss by 21%–26% under future climate conditions.
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