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  • 1
    In: Water Resources Research, January 2017, Vol.53(1), pp.779-798
    Description: In this study, we investigate the impact of single stream discharge events on water exchange, solute transport, and reactions in the hyporheic zone below a natural in‐stream gravel bar. We set up a reactive transport groundwater model with streamflow scenarios that vary by event duration and peak discharge. A steady ambient groundwater flow field is assumed that results in losing, neutral, or gaining stream conditions depending on the stream stage. Across the streambed dissolved oxygen, organic carbon, and nitrate are transported into the subsurface. Additional nitrate is received from upwelling groundwater. Aerobic respiration and denitrification are simulated for scenarios with different stream solute concentrations. Results show that hyporheic exchange flux, solute transport, and consumption increase during events. However, their intensities depend highly on the interplay between event characteristics and ambient groundwater conditions. During events where reversals in the hydraulic gradient occur stream water and solutes infiltrate deeper into the aquifer where they have more time to react. For those events, the reactive efficiency of the hyporheic zone (solute consumption as fraction of influx) for aerobic respiration and denitrification is up to 2.7 and 10 times higher compared to base flow conditions. The fraction of stream nitrate load consumed in the hyporheic zone increases with stream discharge (up to 150 mg/m/h), but remains below the value under base flow conditions for weak events. Events also increase denitrification of groundwater borne nitrate, but groundwater nitrate flux to the stream decreases by up to 33% due to temporary gradient reversals. Transient reactive transport groundwater model of the hyporheic zone for various single discharge events Aerobic respiration and denitrification occur at different timing and locations in the hyporheic zone Reversal in hydraulic gradient direction (gaining to neutral to losing) significantly increases reactive efficiency
    Keywords: Hyporheic Zone ; Discharge Events ; Transient Groundwater Model ; Reactive Transport ; Denitrification
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 2
    In: Water Resources Research, April 2015, Vol.51(4), pp.2243-2263
    Description: Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three‐dimensional numerical model of a natural in‐stream gravel bar (20 m × 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater‐borne nitrate when mixed with stream‐sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log‐space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures. CFD model of in‐stream gravel bar coupled to reactive transport model Losing and gaining conditions reduce aerobic respiration and denitrification Submergence controls skewness of residence time distribution and reactions
    Keywords: In‐Stream Gravel Bar ; Groundwater‐Surface Water Interaction ; Aerobic Respiration ; Denitrification ; Computational Fluid Dynamics ; Reactive Transport Model
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 3
    In: Water Resources Research, March 2018, Vol.54(3), pp.2317-2335
    Description: The analysis of transit/residence time distributions (TTDs and RTDs) provides important insights into the dynamics of stream‐water ages and subsurface mixing. These insights have significant implications for water quality. For a small agricultural catchment in central Germany, we use a 3D fully coupled surface‐subsurface hydrological model to simulate water flow and perform particle tracking to determine flow paths and transit times. The TTDs of discharge, RTDs of storage and fractional StorAge Selection (fSAS) functions are computed and analyzed on daily basis for a period of 10 years. Results show strong seasonal fluctuations of the median transit time of discharge and the median residence time, with the former being strongly related to the catchment wetness. Computed fSAS functions suggest systematic shifts of the discharge selection preference over four main periods: In the wet period, the youngest water in storage is preferentially selected, and this preference shifts gradually toward older ages of stored water when the catchment transitions into the drying, dry and wetting periods. These changes are driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths. Changes in the shape of the fSAS functions can be captured by changes in the two parameters of the approximating Beta distributions, allowing the generation of continuous fSAS functions representing the general catchment behavior. These results improve our understanding of the seasonal dynamics of TTDs and fSAS functions for a complex real‐world catchment and are important for interpreting solute export to the stream in a spatially implicit manner. Transit times of discharge strongly related to storage Strong seasonality in discharge selection preference Seasonally changing SAS functions are well captured by Beta distributions
    Keywords: Transit Time ; Subsurface Mixing ; Sas Functions
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 4
    In: Water Resources Research, September 2013, Vol.49(9), pp.5834-5850
    Description: Exchange of water and solutes across the stream‐sediment interface is an important control for biogeochemical transformations in the hyporheic zone (HZ). In this paper, we investigate the interplay between turbulent stream flow and HZ flow in pool‐riffle streams under various ambient groundwater flow conditions. Streambed pressures, derived from a computational fluid dynamics (CFD) model, are assigned at the top of the groundwater model, and fluxes at the bottom of the groundwater model domain represent losing and gaining conditions. Simulations for different Reynolds numbers () and pool‐riffle morphologies are performed. Results show increasing hyporheic exchange flows (m/d) for larger and a concurrent decrease in residence time (RT). Losing and gaining conditions were found to significantly affect the hyporheic flow field and diminish its spatial extent as well as rates of hyporheic exchange flow. The fraction of stream water circulating through the hyporheic zone is in the range of 1 × 10 to 1 × 10 per meter stream length, decreasing with increasing discharge. Complex distributions of pressure across the streambed cause significant lateral hyporheic flow components with a mean lateral travel distance of 20% of the longitudinal flow paths length. We found that the relationship between pool‐riffle height and hyporheic exchange flow is characterized by a threshold in pool‐riffle amplitude, beyond which hyporheic exchange flow becomes independent of riffle height. Hyporheic residence time distributions (RTD) are log‐normally distributed with medians ranging between 0.7 and 19 h. ambient groundwater flow significantly influences HZ exchange and 3D flow paths undular hydraulic jumps affect streambed pressures and bedform driven exchange threshold in pool riffle height, HZ exchange is independent of riffle height
    Keywords: Hyporheic Exchange ; Computational Fluid Dynamics ; Groundwater‐Surface Water Exchange ; Pool‐Riffle ; Residence Times ; Undular Hydraulic Jump
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 5
    In: Water Resources Research, February 2014, Vol.50(2), pp.1847-1855
    Description: This paper introduces the special section on “new modeling approaches and novel experimental technologies for improved understanding of process dynamics at aquifer‐surface water interfaces.” It is contextualizing the framework for the 27 research papers of the special section by firth identifying research gaps and imminent challenges for ecohydrological research at aquifer‐surface water interfaces and then discussing the specific paper contributions on (i) new developments in temperature/heat tracing at GW‐SW interfaces, (ii) new methods to capture the temporal and spatial variability of groundwater—surface water exchange, (iii) new approaches in modeling aquifer‐river exchange flow, and (iv) new concepts and advanced theory of groundwater—surface water exchange.
    Keywords: Groundwater ; Surface Water ; Hyporheic Zones ; Lacustrine Groundwater Discharge
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 6
    In: Water Resources Research, August 2018, Vol.54(8), pp.5856-5877
    Description: Hydrological water quality models have gained wide acceptance from environmental scientists and water managers to address deterioration of surface water quality. Higher spatiotemporal accuracy of such models is increasingly required for better understanding the functional heterogeneity of catchments and improving management decisions at different governance levels. However, balancing spatial representation and model complexity remains challenging. We present a new flexibly designed, fully distributed nitrate transport and removal model (mHM‐Nitrate) at catchment scale. The model was developed mainly based on the mesoscale Hydrological Model (mHM) and the Hydrological Predictions for the Environment (HYPE) model. The mHM‐Nitrate model was tested in the Selke catchment (Central Germany), which is characterized by heterogeneous physiographic and land‐use conditions, using adequate observed hydrological and nitrate data at three nested gauging stations. Long term (1997–2015) daily simulations showed that the model well reproduced the seasonal dynamics of biweekly nitrate observations in forested, agricultural and urban areas. High‐frequency measurements (2010‐2015) were additionally used to validate model performance of simulating short‐term changes in stream‐water concentrations that reflect changes in runoff partitioning and event‐based dilution effects. Uncertainty analysis confirmed the model's robustness. Moreover, model calculations showed that mean terrestrial nitrate input/output (in total 105 kg ha yr) and in‐stream removal (8% of mean nitrate load) were in comparable ranges with literature, respectively. The new mHM‐Nitrate model is capable of providing detailed spatial information on nitrate concentrations and fluxes, which can motivate more specific catchment investigations on nitrate transport processes and provide guidance on spatially differentiated agricultural practices and measures. New grid‐based catchment nitrate model (mHM‐Nitrate) with a flexible multi‐resolution structure Spatiotemporal validation with uncertainty analysis is conducted in a nested heterogeneous catchment using multi‐frequency observations The mHM‐Nitrate model provides detailed and reliable catchment‐wide spatial information of nitrate concentrations and fluxes
    Keywords: Process‐Based ; Multi‐Scale Nitrate Model ; Fully Distributed ; Spatiotemporal Validation ; Nitrate Concentrations And Fluxes
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 7
    In: Water Resources Research, April 2016, Vol.52(4), pp.2883-2900
    Description: Bioclogging in rivers can detrimentally impact aquifer recharge. This is particularly so in dry regions, where losing rivers are common, and where disconnection between surface water and groundwater (leading to the development of an unsaturated zone) can occur. Reduction in riverbed permeability due to biomass growth is a time‐variable parameter that is often neglected, yet permeability reduction from bioclogging can introduce order of magnitude changes in seepage fluxes from rivers over short (i.e., monthly) timescales. To address the combined effects of bioclogging and disconnection on infiltration, we developed numerical representations of bioclogging processes within a one‐dimensional, variably saturated flow model representing losing‐connected and losing‐disconnected rivers. We tested these formulations using a synthetic case study informed with biological data obtained from the Russian River, California, USA. Our findings show that modeled biomass growth reduced seepage for losing‐connected and losing‐disconnected rivers. However, for rivers undergoing disconnection, infiltration declines occurred only after the system was fully disconnected. Before full disconnection, biologically induced permeability declines were not significant enough to offset the infiltration gains introduced by disconnection. The two effects combine to lead to a characteristic infiltration curve where peak infiltration magnitude and timing is controlled by permeability declines relative to hydraulic gradient gains. Biomass growth was found to hasten the onset of full disconnection; a condition we term ‘effective disconnection’. Our results show that river infiltration can respond dynamically to bioclogging and subsequent permeability declines that are highly dependent on river connection status. Riverbed bioclogging is a key control on infiltration in losing rivers River infiltration gains from disconnection can offset riverbed permeability declines caused by bioclogging Permeability reduction can hasten the onset of disconnection
    Keywords: Bioclogging ; Disconnection ; Dynamic Riverbed Permeability ; Time‐Variable Parameters ; Riverbank Filtration ; Groundwater Pumping ; Infiltration ; Biofilm ; Losing Rivers ; Arid Climates ; Heterotrophic Bacteria ; Hyporheic Zone ; Vadose Zone
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 8
    In: Water Resources Research, April 2016, Vol.52(4), pp.3227-3245
    Description: Improved understanding of stream solute transport requires meaningful comparison of processes across a wide range of discharge conditions and spatial scales. At reach scales where solute tracer tests are commonly used to assess transport behavior, such comparison is still confounded due to the challenge of separating dispersive and transient storage processes from the influence of the advective timescale that varies with discharge and reach length. To better resolve interpretation of these processes from field‐based tracer observations, we conducted recurrent conservative solute tracer tests along a 1 km study reach during a storm discharge period and further discretized the study reach into six segments of similar length but different channel morphologies. The resulting suite of data, spanning an order of magnitude in advective timescales, enabled us to (1) characterize relationships between tracer response and discharge in individual segments and (2) determine how combining the segments into longer reaches influences interpretation of dispersion and transient storage from tracer tests. We found that the advective timescale was the primary control on the shape of the observed tracer response. Most segments responded similarly to discharge, implying that the influence of morphologic heterogeneity was muted relative to advection. Comparison of tracer data across combined segments demonstrated that increased advective timescales could be misinterpreted as a change in dispersion or transient storage. Taken together, our results stress the importance of characterizing the influence of changing advective timescales on solute tracer responses before such reach‐scale observations can be used to infer solute transport at larger network scales. Advection is the primary control on observed stream solute tracer responses The influence of spatial heterogeneity in morphology is muted by advection Interpretation of solute transport requires consideration of tracer timescales
    Keywords: Stream Solute Transport ; Transient Storage ; Conservative Tracer ; Storm Event ; Statistical Moments ; Advective Timescale
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 9
    In: Water Resources Research, August 2017, Vol.53(8), pp.6359-6376
    Description: The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often “hot spots” of ecological, biogeochemical, and hydrological processes and may provide refuge for biota during extreme events. Ecohydrological interfaces can have significant impact on global hydrological and biogeochemical cycles, biodiversity, pollutant removal, and ecosystem resilience to disturbance. The organizational principles (i.e., the drivers and controls) of spatially and temporally variable processes at ecohydrological interfaces are poorly understood and require the integrated analysis of hydrological, biogeochemical, and ecological processes. Our rudimentary understanding of the interactions between different drivers and controls critically limits our ability to predict complex system responses to change. In this paper, we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analyzing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their nonlinear process dynamics, ecohydrological interfaces need to be conceptualized as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems. The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces that we introduce here as a new concept to support the quantitative analysis of nonlinear system behavior stimulated by the complex and multifacetted interactions of hydrological, biogeochemical, and ecological processes across system boundaries. Ecohydrological interfaces are defined here as the dynamic transition zones that may develop at ecosystem (or subsystem) boundaries and control the movement and transformation of organisms, water, matter, and energy between adjacent systems. In this paper, we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analyzing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their nonlinear process dynamics, ecohydrological interfaces need to be conceptualized as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems. Ecohydrological interfaces are dynamic transition zones, changing in space and in time Ecohydrological interfaces are defined by their specific functioning often supporting process hot spots and hot moments Interface ecohydrological, biogeochemical, and ecological processes often differ from their neighboring ecosystems
    Keywords: Ecohydrological Interface ; Boundary ; Biogeochemical Transformation ; Interdisciplinary ; Hot Spot
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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