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  • 1
    Language: English
    In: Journal of Hydrology, October 2015, Vol.529, pp.969-979
    Description: Coupling surface and subsurface water flow in fully integrated hydrological codes is becoming common in hydrological research; however, the coupling of surface–subsurface solute transport has received much less attention. Previous studies on fully integrated solute transport focus on small scales, simple geometric domains, and have not utilised many different field data sources. The objective of this study is to demonstrate the inclusion of both flow and solute transport in a 3D, fully integrated catchment model, utilising high resolution observations of dissolved organic carbon (DOC) export from a wetland complex during a rainfall event. A sensitivity analysis is performed to span a range of transport conditions for the surface–subsurface boundary (e.g. advective exchange only, advection plus diffusion, advection plus full mechanical dispersion) and subsurface dispersivities. The catchment model captures some aspects of observed catchment behaviour (e.g. solute discharge at the catchment outlet, increasing discharge from wetlands with increased stream discharge, and counter-clockwise concentration–discharge relationships), although other known behaviours are not well represented in the model (e.g. slope of concentration–discharge plots). Including surface–subsurface solute transport aids in evaluating internal model processes, however there are challenges related to the influence of dispersion across the surface–subsurface interface, and non-uniqueness of the solute transport solution. This highlights that obtaining solute field data is especially important for constraining integrated models of solute transport.
    Keywords: Solute Transport ; Surface–Subsurface Coupling ; Integrated Modelling ; Catchment Modelling ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 2
    Language: English
    In: Journal of Hydrology, 28 June 2013, Vol.494, pp.72-82
    Description: River discharge is a commonly measured hydrologic variable; however, estimate uncertainty is often higher than acceptable limits. To quantify method limitations and spatiotemporal variability, a multi-year hydrologic flow partitioning investigation was completed under monsoonal conditions in the ungauged complex terrain of the Haean Catchment, South Korea. Our results indicate that sediment transport from a single annual monsoonal event can significantly modify the channel cross-sectional area resulting in inaccurate stage-discharge rating curves. We compare six discharge measurement methods at 13 locations that vary in slope from 1% to 80%, with discharge ranging up to four orders in magnitude, which enabled us to weight the accuracy of each method over a specific range in discharge. The most accurate discharge estimation methods are the weir, the acoustic Doppler current profiler, and the in-stream velocity area method; however, under certain conditions each of these methods is less desirable than other methods. The uncertainty in the three methods is on average 0.4%, 4.7%, and 6.1% of the total discharge, respectively. The accuracy of the discharge estimates has a direct influence on the characterization of basin-wide hydrologic partitioning, which can lead to significant variability in sediment erosion rates and nutrient fate and transport.
    Keywords: Terreco ; Korea ; Discharge ; Baseflow ; Topography ; River ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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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: Journal of Geophysical Research: Biogeosciences, May 2014, Vol.119(5), pp.910-928
    Description: At the interface between stream water, groundwater, and the hyporheic zone (HZ), important biogeochemical processes that play a crucial role in fluvial ecology occur. Solutes that infiltrate into the HZ can react with each other and possibly also with upwelling solutes from the groundwater. In this study, we systematically evaluate how variations of gaining and losing conditions, stream discharge, and pool‐riffle morphology affect aerobic respiration (AR) and denitrification (DN) in the HZ. For this purpose, a computational fluid dynamics model of stream water flow is coupled to a reactive transport model. Scenarios of variations of the solute concentration in the upwelling groundwater were conducted. Our results show that solute influx, residence time, and the size of reactive zones strongly depend on presence, magnitude, and direction of ambient groundwater flow. High magnitudes of ambient groundwater flow lower AR efficiency by up to 4 times and DN by up to 3 orders of magnitude, compared to neutral conditions. The influence of stream discharge and morphology on the efficiency of AR and DN are minor, in comparison to that of ambient groundwater flow. Different scenarios of O and NO concentrations in the upwelling groundwater reveal that DN efficiency of the HZ is highest under low upwelling magnitudes accompanied with low concentrations of O and NO. Our results demonstrate how ambient groundwater flow influences solute transport, AR, and DN in the HZ. Neglecting groundwater flow in stream‐groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems. Coupling of CFD model to reactive transport model Losing and gaining conditions reduce aerobic respiration and denitrification HZ has higher potential for aerobic respiration compared to denitrification
    Keywords: Hyporheic Zone ; Pool‐Riffle ; Streambed ; Reactive Transport Modeling ; Denitrification
    ISSN: 2169-8953
    E-ISSN: 2169-8961
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  • 5
    Language: English
    In: Advances in Water Resources, October 2016, Vol.96, pp.95-107
    Description: Solute concentration variability is of fundamental importance for the chemical and ecological state of streams. It is often closely related to discharge variability and can be characterized in terms of a solute export regime. Previous studies, especially in lowland catchments, report that nitrate is often exported with an accretion pattern of increasing concentrations with increasing discharge. Several modeling approaches exist to predict the export regime of solutes from the spatial relationship of discharge generating zones with solute availability in the catchment. For a small agriculturally managed lowland catchment in central Germany, we show that this relationship is controlled by the depth to groundwater table and its temporal dynamics. Principal component analysis of groundwater level time series from wells distributed throughout the catchment allowed derivation of a representative groundwater level time series that explained most of the discharge variability. Groundwater sampling revealed consistently decreasing nitrate concentrations with an increasing thickness of the unsaturated zone. The relationships of depth to groundwater table to discharge and to nitrate concentration were parameterized and integrated to successfully model catchment discharge and nitrate export on the basis of groundwater level variations alone. This study shows that intensive and uniform agricultural land use likely results in a clear and consistent concentration-depth relationship of nitrate, which can be utilized in simple approaches to predict stream nitrate export dynamics at the catchment scale.
    Keywords: Water Quality ; Nitrate ; Lowland Catchment ; Export Regime ; Concentration-Discharge Relationship ; Engineering
    ISSN: 0309-1708
    E-ISSN: 1872-9657
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  • 6
    In: Journal of Geophysical Research: Biogeosciences, March 2017, Vol.122(3), pp.628-644
    Description: Transient storage zones for water represent potential hot spots for metabolic activity in streams. In lowland rivers, the high abundance of submerged vegetation can increase water transient storage, bioreactive surface areas, and, ultimately, in‐stream metabolic activity. Changes in flow resulting from climatic and anthropogenic factors that influence the presence of aquatic vegetation can also, thereby, impact in‐stream metabolism and nutrient cycling. We investigated the effects of water column depth on aquatic vegetation cover and its implications on water transient storage and associated metabolic activity in stream mesocosms ( = 8) that represent typical conditions of lowland streams. Continuous injections of metabolically reactive (resazurin‐resorufin) tracers were conducted and used to quantify hydraulic transport and whole‐mesocosm aerobic respiration. Acetate, a labile carbon source, was added during a second stage of the tracer injection to investigate metabolic responses. We observed both higher vegetation coverage and resazurin uptake velocity, used as a proxy of mesocosm respiration, with increasing water column depth. The acetate injection had a slight, positive effect on metabolic activity. A hydrodynamic model estimated the water transport and retention characteristics and first‐order reactivity for three mesocosms. These results suggest that both the vegetated surface water and sediments contribute to metabolically active transient storage within the mesocosms, with vegetation having a greater influence on ecosystem respiration. Our findings suggest that climate and external factors that affect flow and submerged vegetation of lowland rivers will result in changes in stream respiration dynamics and that submerged vegetation is a particularly important and sensitive location for stream respiration. Ecosystem respiration is positively correlated with water depth, discharge, and vegetation coverage In‐stream vegetation beds are significant sites of metabolically active transient storage Declining stream flows may negatively impact aquatic vegetation and ecosystem function in lowland rivers
    Keywords: Resazurin ; Metabolism ; Ecosystem Respiration ; Transient Storage ; Lowland Rivers ; Vegetation
    ISSN: 2169-8953
    E-ISSN: 2169-8961
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  • 7
    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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  • 8
    Language: English
    In: Hydrological Processes, 15 July 2009, Vol.23(15), pp.2138-2149
    Description: Subsurface flow in streambeds can vary at different scales in time and space. Recognizing this variability is critical for understanding biogeochemical and ecological processes associated with the hyporheic zone. The aim of this study was to examine the variability of hydraulic conductivity (), vertical hydraulic gradients (VHGs), and subsurface fluxes, over a riffle–step–pool sequence and at a high spatio‐temporal resolution. A 20 m reach was equipped with a network of piezometers in order to determine the distribution of VHGs and . During a summer month, temporal variations of VHGs were regularly surveyed and, for a subset of piezometers, the water level was automatically recorded at 15 min intervals by logging pressure transducers. Additionally, point‐dilution tests were carried out on the same subset of piezometers. Whereas the distribution of vertical fluxes can be derived from and VHG values, point‐dilution tests allow for the estimation of horizontal fluxes where no VHG is detectable. Results indicate that, spatially, VHGs switched from upwelling to downwelling across lateral as well as longitudinal sections of the channel. Vertical fluxes appeared spatially more homogeneous than VHGs, suggesting that the latter can be a poor indicator of the intensity of flow. Finally, during flow events, some VHGs showed little or no fluctuations; this was interpreted as the result of a pressure wave propagating from upstream through highly diffusive alluvial sediments. Copyright © 2009 John Wiley & Sons, Ltd.
    Keywords: Hyporheic Exchange Flow ; Heterogeneity ; Hydraulic Conductivity ; Vertical Hydraulic Gradient ; Groundwater–Surface Water Interactions
    ISSN: 0885-6087
    E-ISSN: 1099-1085
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  • 9
    Language: English
    In: Hydrological Processes, 15 July 2009, Vol.23(15), pp.2195-2211
    Description: For an experimental field site at the River Leith, United Kingdom, the spatial and temporal distribution of nitrate was observed along the upwelling flow path from groundwater to surface water. The study was carried out during baseflow conditions for two successive years. For two contrasting stream reaches, the physical and chemical characteristics of streambed sediment cores were analysed together with observations of hydraulic head, dissolved oxygen, redox and nitrogen speciation using an array of nested streambed piezometers. Pressure head gradients in the streambed piezometers showed that upwelling flows dominated the exchange between groundwater and surface water throughout the observation period. Infiltration of surface water into the streambed was not evident at depths below 10 cm. Pore water collected from sediment cores and streambed piezometers showed spatially variable redox conditions and nitrogen speciation within up to 100 cm depth in the streambed. In particular, nitrate concentrations along upwelling flow paths appeared to follow two opposite trends, with both decreasing and increasing nitrate concentrations being observed at different points in the experimental reach. The observed changes of nitrate concentrations in the upwelling groundwater are restricted to the loose superficial sediments that overlay the sandstone bedrock and do not appear to coincide with surface water–groundwater mixing in the streambed. The magnitude of variation in nitrate concentration along the upwelling flow path to the streambed appears to be governed by the sediment structure and characteristics in the two contrasting field sites. The results suggest that changes in redox status and pore water nitrate concentrations in the hyporheic may occur at depths greater than surface water infiltration into the streambed and may call for new conceptual understanding of hyporheic nutrient transformations. Copyright © 2009 John Wiley & Sons, Ltd.
    Keywords: Hyporheic Zone ; Nitrate ; Groundwater–Surface Water Interface ; Riparian
    ISSN: 0885-6087
    E-ISSN: 1099-1085
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  • 10
    Language: English
    In: Hydrological Processes, 15 July 2009, Vol.23(15), pp.2108-2116
    Description: The exchange of oxygen and nutrients between the well‐aerated stream water and the subsurface water is crucial for the biochemical conditions of the hyporheic zone. The metabolic activity of the hyporheic microorganisms controls the fate of nitrogen and phosphorus in the pore water, and influences the fate of these nutrients at the catchment scale. Unfortunately, the incomplete knowledge of the complex hydrodynamics of the coupled surface‐subsurface flow field often hinders the understanding of the ecological relevance of the hyporheic processes. Here, we analyse the influence of groundwater discharge through the streambed on bedform‐induced hyporheic exchange. A simple mathematical model of a coupled stream‐aquifer system is developed in order to describe the essential feature of the surface‐subsurface exchange. The most representative characteristics of the hyporheic exchange, e.g. the depth of the hyporheic zone ‐ are parametrized in terms of a small number of easily measurable quantities. This information on the hyporheic flow field provides the fundamental basis for the study of the ecological function of the hyporheic zone. Copyright © 2009 John Wiley & Sons, Ltd.
    Keywords: Exchange Modelling ; Upwelling ; Hyporheic Exchange ; Large Scale ; Small Scale ; Ground Water Discharge
    ISSN: 0885-6087
    E-ISSN: 1099-1085
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