Hydraulic characteristics of a small Coastal Plain stream of the southeastern United States: effects of hydrology and season

Various physical and biological properties affect solute transport patterns in streams. We measured hydraulic characteristics of Payne Creek, a low‐gradient upper Coastal Plain stream, using tracer experiments and parameter estimation with OTIS‐P (one‐dimensional transport with inflow and storage with parameter optimization). The primary objective of this study was to estimate the effects of varying discharge, season, and litter accumulation on hydraulic parameters. Channel area A ranged from 0·081 to 0·371 m² and transient storage area A_s ranged from 0·027 to 0·111 m². Dispersion D ranged from 1·5 to 11·1 m² min⁻¹ and exchange coefficient α ranged from 0·009 to 0·038 min⁻¹. Channel area and dispersion were positively correlated to discharge Q, whereas storage area and exchange coefficient were not. Relative storage size A_s/A ranged from 0·17 to 0·59, and was higher during fall than other seasons under a similar Q. The fraction of median travel time due to transient storage ranged from 8·8 to 34·5% and was significantly correlated with Q through a negative power function. Both metrics indicated that transient storage was a significant component affecting solute transport in Payne Creek, especially during the fall. Comparison between the measured channel area A_c and A suggested that surface storage was dominant in Payne Creek. During fall, accumulation of leaf litter resulted in larger A and A_s and lower velocity and D than during other seasons with similar discharge. Seasonal changes in discharge and organic matter accumulation, and dynamic channel morphology affected the magnitude of transient storage and overall hydraulic characteristics of Payne Creek. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of redox and transport processes in a riparian wetland on stream water quality in the Fichtelgebirge region,southern Germany

Biologically mediated redox processes in the riparian zone, like denitrification, can have substantially beneficial impacts on stream water quality. The extent of these effects, however, depends greatly on the hydrological boundary conditions. The impact of hydrological processes on a wetland's nitrogen sink capacity was investigated in a forested riparian fen which is drained by a first‐order perennial stream. Here, we analysed the frequency distributions and time‐series of pH and nitrogen, silica, organic carbon and oxygen concentrations in throughfall, soil solution, groundwater and stream water, and the groundwater levels and stream discharges from a 3‐year period. During baseflow conditions, the stream was fed by discharging shallow, anoxic groundwater and by deep, oxic groundwater. Whereas the latter delivered considerable amounts of nitrogen (～0·37 mg l^?1) to the stream, the former was almost entirely depleted of nitrogen. During stormflow, near‐surface runoff in the upper 30 cm soil layer bypassed the denitrifying zone and added significant amounts to the nitrogen load of the stream. Nitrate‐nitrogen was close to 100% of deep groundwater and stream‐water nitrogen concentration. Stream‐water baseflow concentrations of nitrate, dissolved carbon and silica were about 1·6 mg l^?1, 4 mg l^?1 and 7·5 mg l^?1 respectively, and >3 mg l^?1, >10 mg l^?1 and <4 mg l^?1 respectively during discharge peaks. In addition to that macroscale bypassing effect, there was evidence for a corresponding microscale effect: Shallow groundwater sampled by soil suction cups indicated complete denitrification and lacked any seasonal signal of solute concentration, which was in contrast to piezometer samples from the same depth. Moreover, mean solute concentration in the piezometer samples resembled more that of suction‐cup samples from shallower depth than that of the same depth. We conclude that the soil solution cups sampled to a large extent the immobile soil‐water fraction. In contrast, the mobile fraction that was sampled by the piezometers exhibited substantially shorter residence time, thus being less exposed to denitrification, but predominating discharge of that layer to the stream. Consequently, assessing the nitrogen budget based on suction‐cup data tended to overestimate the nitrogen consumption in the riparian wetland. These effects are likely to become more important with the increased frequency and intensity of rainstorms that are expected due to climate change. Copyright © 2006 John Wiley & Sons, Ltd.     

Wetland nitrogen dynamics in an Adirondack forested watershed

Wetlands often form the transition zone between upland soils and watershed streams, however, stream–wetland interactions and hydrobiogeochemical processes are poorly understood. We measured changes in stream nitrogen (N) through one riparian wetland and one beaver meadow in the Archer Creek watershed in the Adirondack Mountains of New York State, USA from 1 March to 31 July 1996. In the riparian wetland we also measured changes in groundwater N. Groundwater N changed significantly from tension lysimeters at the edge of the peatland to piezometer nests within the peatland. Mean N concentrations at the peatland perimeter were 1·5, 0·5 and 18·6 µmol L^?1 for NH₄⁺, NO₃^? and DON (dissolved organic nitrogen), respectively, whereas peatland groundwater N concentration was 56·9, 1·5 and 31·6 µmol L^?1 for NH₄⁺, NO₃^? and DON, respectively. The mean concentrations of stream water N species at the inlet to the wetlands were 1·5, 10·1 and 16·9 µmol L^?1 for NH₄⁺, NO₃^? and DON, respectively and 1·6, 28·1 and 8·4 µmol L^?1 at the wetland outlet. Although groundwater total dissolved N (TDN) concentrations changed more than stream water TDN through the wetlands, hydrological cross‐sections for the peatland showed that wetland groundwater contributed minimally to stream flow during the study period. Therefore, surface water N chemistry was affected more by in‐stream N transformations than by groundwater N transformations because the in‐stream changes, although small, affected a much greater volume of water. Stream water N input–output budgets indicated that the riparian peatland retained 0·16 mol N ha^?1 day^?1 of total dissolved N and the beaver meadow retained 0·26 mol N ha^?1 day^?1 during the study period. Nitrate dominated surface water TDN flux from the wetlands during the spring whereas DON dominated during the summer. This study demonstrates that although groundwater N changed significantly in the riparian peatland, those changes were not reflected in the stream. Consequently, although in‐stream changes of N concentrations were less marked than those in groundwater, they had a greater effect on stream water chemistry—because wetland groundwater contributed minimally to stream flow. Copyright © 2004 John Wiley & Sons, Ltd.     

A stream tracer technique employing ionic tracers and specific conductance data applied to the Maimai catchment,New Zealand

The stream tracer technique and transient storage models (TSMs) have become common tools in stream solute and hyporheic exchange studies. The expense and logistics associated with water sample collection and analysis often results in limited temporal resolution of stream tracer breakthrough curves (BTCs). Samples are often collected without a priori or real‐time knowledge of BTC information, which can result in poor sample coverage of the critical shoulder (initial rise) and tail (post‐steady state fall) of the BTC. We illustrate the use of specific conductance (SC) measurements as a surrogate for conservative dissolved tracer (Br⁻) samples. The advantages of collecting SC data for use in the TSM are (1) cost, (2) ease of data collection, and (3) well‐defined breakthrough curves, which strengthen TSM parameter optimization. This method is based on developing an ion concentration (IC)–SC relationship from limited discrete tracer solute samples. SC data can be collected on a more frequent basis at no additional analysis cost. TSM simulations can then be run for the conservative tracer data derived from SC breakthrough curves and the IC–SC relationship. This technique was tested in a 120 m reach of stream (2–60 m subreaches) in the Maimai M15 catchment, New Zealand during baseflow recession. Dissolved LiBr was injected for 12·92 h, with Br⁻ as the conservative ion of interest. Four TSM simulations using the OTIS model are optimized using UCODE to fit (1) Br⁻ data derived from the Br⁻–SC relationship (n = 1307 observations at each of two stream sampling sites), (2) all stream Br⁻ data collected (n = 58 in upper reach, n = 60 in lower reach), (3) half of the stream Br⁻ data collected, and (4) 20 stream Br⁻ samples from each site. No two simulations resulted in the same optimal parameter values. Results suggest that the greater the frequency of observations, the greater the confidence in estimated parameter values. Br⁻–SC simulations resulted in the best overall model fits to the data, with the lowest calculated error variance of 6·37, narrowest 95% parameter estimate confidence intervals, and the highest correlation coefficient of 0·99 942, among the four simulations. This is largely due to the improved representation of the shoulder and tail of the BTC with this method. The IC–SC correlation method is robust in situations in which (1) changes in background SC data can be accounted for, and (2) the data used to define the IC–SC relationship are representative of the range of data collected. This method provides more efficient sample analysis, improved data resolution, and improved model results compared to the alternative stream tracer data gathering methods presented. Additionally, we describe a new parameterization of the cross‐sectional area of the stream during flow recession, as a function of discharge, based on a stream hydraulic geometry relationship. This variant of the OTIS model provides a more realistic representation of stream dynamics during unsteady discharge. Copyright © 2005 John Wiley & Sons, Ltd.     

The effect of transient storage on nitrate uptake lengths in streams: an inter‐site comparison

Surface water–groundwater interaction in the hyporheic zone may enhance biogeochemical cycling in streams, and it has been hypothesized that streams exchanging more water with the hyporheic zone should have more rapid nitrate utilization. We used simultaneous conservative solute and nitrate addition tracer tests to measure transient storage (which includes hyporheic exchange and in‐stream storage) and the rate of nitrate uptake along three reaches within the Red Canyon Creek watershed, Wyoming. We calibrated a one‐dimensional transport model, incorporating transient storage (OTIS‐P), to the conservative solute breakthrough curves and used the results to determine the degree of transient storage in each reach. The nitrate uptake length was quantified from the exponential decrease in nitrate concentration with distance during the tracer tests. Nitrate uptake along the most downstream reach of Red Canyon Creek was rapid (turnover time K^?1_c = 32 min), compared with nitrate uptake reported in other studies (K^?1_c = 12 to 551 min), but other sites within the watershed showed little nitrate retention or loss. The uptake length S_w‐NO^?₃ for the most downstream reach was 500 m and the mass transfer coefficient V_f‐NO^?₃ was 6·3 m min^?1. Results from 15 other nitrate‐addition tracer tests were used to create a regression model relating transient storage and measures of stream flow to nitrate uptake length. The model, which includes specific discharge and transient storage area, explains almost half the variability in nitrate uptake length (adjusted R² = 0·44) and is most effective for comparing sites with very different stream characteristics. Although large differences in specific discharge and storage zone area explain inter‐site differences in nitrate uptake, other unmeasured variables, such as available organic carbon and microbial community composition, are likely important for predicting differences in nitrate uptake between sites with similar specific discharge rates and storage zone areas, such as when making intra‐site comparisons. Copyright © 2007 John Wiley & Sons, Ltd.     

The role of perched aquifers in hydrological connectivity and biogeochemical processes in vernal pool landscapes,Central Valley,California

Relatively little is known about the role of perched aquifers in hydrological, biogeochemical, and biological processes of vernal pool landscapes. The objectives of this study are to introduce a perched aquifer concept for vernal pool formation and maintenance and to examine the resulting hydrological and biogeochemical phenomena in a representative catchment with three vernal pools connected to one another and to a seasonal stream by swales. A combined hydrometric and geochemical approach was used. Annual rainfall infiltrated but perched on a claypan/duripan, and this perched groundwater flowed downgradient toward the seasonal stream. The upper layer of soil above the claypan/duripan is ～0·6 m in thickness in the uplands and ～0·1 m in thickness in the vernal pools. Some groundwater flowed through the vernal pools when heads in the perched aquifer exceeded ～0·1 m above the claypan/duripan. Perched groundwater discharge accounted for 30–60% of the inflow to the vernal pools during and immediately following storm events. However, most perched groundwater flowed under or around the vernal pools or was recharged by annual rainfall downgradient of the vernal pools. Most of the perched groundwater was discharged to the outlet swale immediately upgradient of the seasonal stream, and most water discharging from the outlet swale to the seasonal stream was perched groundwater that had not flowed through the vernal pools. Therefore, nitrate‐nitrogen concentrations were lower (e.g. 0·17 to 0·39 mg l^?1) and dissolved organic carbon concentrations were higher (e.g. 5·97 to 3·24 mg l^?1) in vernal pool water than in outlet swale water discharging to the seasonal stream. Though the uplands, vernal pools, and seasonal stream are part of a single surface‐water and perched groundwater system, the vernal pools apparently play a limited role in controlling landscape‐scale water quality. Copyright © 2005 John Wiley & Sons, Ltd.     

Influence of bedrock groundwater on streamflow characteristics in a volcanic catchment

Groundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km²) and the Hosotani river (SR) catchment (4.0 km²) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 10³ years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km² and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km² at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small‐scale hydrological conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

On the Panday and Huyakorn surface–subsurface hydrology test case: analysis of internal flow dynamics

A number of previous studies using models of integrated surface‐subsurface hydrology have adopted the Panday and Huyakorn (P&H) tilted V‐catchment test case (Panday S, Huyakorn PS. 2004. A fully coupled spatially distributed model for evaluating surface/subsurface flow. Advances in Water Resources 27: 361–382) to show inter‐code comparability. The P&H test case is used to evaluate models that simulate a broad range of hydrological processes, and yet only the catchment outflow hydrograph has been presented as verification of the consistency between codes. Therefore, a more comprehensive evaluation of the surface‐subsurface hydrology of the P&H case is needed. This study explores the internal catchment functioning of the P&H case, using the popular catchment simulator MODHMS. The processes leading to streamflow generation in the model are illustrated, including separation of overland flow (OLF) and groundwater discharge to the stream. The results identify non‐physical flow processes due to the problem set‐up, and modifications to the P&H case are suggested that include changes to stream roughness and incision of the stream channel to overcome these shortcomings. A modified P&H case produced more plausible transfers between OLF and the stream, and an increased groundwater discharge to the stream (6·5% of streamflow in the modified case compared to 0·5% in the original case). Despite changes to internal flow processes, near‐identical outflow hydrographs were obtained, showing the importance of considering and comparing internal flow processes when using surface‐subsurface hydrology test cases to evaluate integrated hydrological simulators. Copyright © 2011 John Wiley & Sons, Ltd.     

Stable isotope tracers as diagnostic tools in upscaling flow path understanding and residence time estimates in a mountainous mesoscale catchment

δ¹⁸O measurements of precipitation and stream waters were used as a natural tracer to investigate hydrological pathways and residence times in the River Feshie, a complex mesoscale (231 km²) catchment in the Cairngorm Mountains of Scotland. Precipitation δ¹⁸O exhibited strong seasonal variation over the 2001–02 hydrological year, ranging from −6·9‰ in the summer, to −12·0‰ during winter snowfalls (mean δ¹⁸O −9·59‰). Although damped, this seasonality was reflected in stream water outputs at seven sampling sites in the catchment, allowing δ¹⁸O variations to be used to infer hydrological source areas. Thus, stream water δ¹⁸O was generally controlled by a seasonally variable storm flow end member, mixing with groundwater of more constant isotopic composition. Periodic regression analysis allowed the differences in this mixing process between monitoring subcatchments to be assessed more quantitatively to provide a preliminary estimate of mean stream water residence time. This demonstrated the importance of responsive hydrological pathways associated with peat and shallow alpine soils in the headwater subcatchments in producing seasonally variable runoff with short mean residence times (33–113 days). In contrast, other tributaries with more freely draining soils and larger groundwater storage in shallow aquifers provided more effective mixing of variable precipitation inputs, resulting in longer residence time estimates (178–445 days). The mean residence time of runoff leaving the Feshie catchment reflected an integration of these contrasting influences (110–200 days). These insights from δ¹⁸O measurements extend the hydrological understanding of the Feshie catchment gained from other hydrochemical tracers, and demonstrate the utility of isotope tracers in investigating hydrological processes at the mesoscale. Copyright © 2005 John Wiley & Sons, Ltd.     

Suspended sediment dynamics in a steep,glacier‐fed mountain stream,Place Creek,Canada

This study examined suspended sediment concentration (SSC) during the ablation seasons of 2000 and 2001 in Place Creek, Canada, a steep, glacier‐fed mountain stream. Comparison of stream flow in Place Creek with that in an adjacent, almost unglacierized catchment provided a rational basis for separating the ablation seasons into nival, nival–glacial, glacial and autumn recession subseasons. Distinct groupings of points in plots of electrical conductivity against discharge supported the validity of the subseasonal divisions in terms of varying hydrological conditions. Relationships between SSC and discharge (Q) varied between the two study seasons, and between subseasons. Hysteresis in the SSC–Q relationship was evident at both event and weekly time‐scales. Some suspended sediment released from pro‐glacial Place Lake (the source of Place Creek) appeared to be lost to channel storage at low flows, especially early in the ablation season, with re‐entrainment at higher flows. Multiple regression models were derived for the subseasons using predictor variables including Q, Q², the change in Q over the previous 3 h, cumulative discharge over the ablation season, total precipitation over the previous 24 h and SSC measured at 1500 hours as an index value for each day. The models produced adjusted R² values ranging from 0·71 to 0·91, and provided tentative insights into the differences in SSC dynamics amongst subseasons. Introduction of the index value of SSC significantly improved the model fit during the nival–glacial and glacial subseasons for both years, as it adjusts the model to the current condition of sediment supply. Copyright © 2003 John Wiley & Sons, Ltd.     

Significance of processes in the near‐stream zone on stream water acidity in a small acidified forested catchment

The near‐stream zone has received increasing attention owing to its influence on stream water chemistry in general and acidity in particular. Possible processes in this zone include cation exchange, leaching of organic matter and redox reactions of sulphur compounds. In this study the influences of processes in the near‐stream zone on the acidity in runoff from a small, acidified catchment in central southern Sweden were investigated. The study included sampling of groundwater, soil water and stream water along with hydrological measurements. An input–output budget for the catchment was established based on data from the International Co‐operative Programme on Integrated Monitoring at this site. The catchment was heavily acidified by deposition of anthropogenic sulphur, with pH in stream water between 4·4 and 4·6. There was also no relationship between stream flow and pH, which is indicative of chronic acidification. Indications of microbial reduction of sulphate were found in some places near the stream, but the near‐stream zone did not have a general impact on the sulphate concentration in discharging groundwater. The near‐stream zone was a source of dissolved organic carbon (DOC) in the stream, which had a median DOC of 6·8 mg L¹. The influence on stream acidity from organic anions was overshadowed by the effect of sulphate, however, except during a spring flow episode, when additional organic matter was flushed out and the sulphate‐rich ground water was mixed with more diluted event water. Ion exchange was not an important process in the near‐stream zone of the Kindla catchment. Different functions of the near‐stream zone relating to discharge acidity are reported in the literature. In this study there was even a variation within the site. There is therefore a need for more case studies to provide a more detailed understanding of the net effects that the near‐stream zone can have on stream chemistry under different circumstances. Copyright © 2001 John Wiley & Sons, Ltd.     

Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Land‐use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and ¹⁸O/¹⁶O (δ¹⁸O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ¹⁸O scatter plots indicated that for both land‐use types, two run‐off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B‐horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run‐off. A 2‐component isotopic hydrograph separation indicated a larger contribution of rainfall water to run‐off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run‐off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns.     

Stage‐dependent transient storage of phosphorus in alluvial floodplains

Models for contaminant transport in streams commonly idealize transient storage as a well mixed but immobile system. These transient storage models capture rapid (near‐stream) hyporheic storage and transport, but do not account for large‐scale, stage‐dependent interaction with the alluvial aquifer. The objective of this research was to document transient storage of phosphorus (P) in coarse gravel alluvium potentially influenced by large‐scale, stage‐dependent preferential flow pathways (PFPs). Long‐term monitoring was performed at floodplain sites adjacent to the Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping which was correlated to hydraulic conductivity data, observation wells were installed both in higher hydraulic conductivity and lower hydraulic conductivity subsoils. Water levels in the wells were monitored over time, and water samples were obtained from the observation wells and the stream to document P concentrations at multiple times during high flow events. Contour plots indicating direction of flow were developed using water table elevation data. Contour plots of total P concentrations showed the alluvial aquifer acting as a transient storage zone, with P‐laden stream water heterogeneously entering the aquifer during the passage of a storm pulse, and subsequently re‐entering the stream during baseflow conditions. Some groundwater in the alluvial floodplains had total P concentrations that mirrored the streams' total P concentrations. A detailed analysis of P forms indicated that particulate P (i.e. P attached to particulates greater than 0·45 µm) was a significant portion of the P transport. This research suggests the need for more controlled studies on stage‐dependent transient storage in alluvial systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantifying groundwater discharge to a small perennial stream in southern Ontario, Canada

A study of the interaction between groundwater and surface water was undertaken within a small agricultural watershed in southern Ontario, Canada. Groundwater contributions to streamflow were measured along a section of stream during baseflow conditions and during rainfall events. Four techniques were used to estimate the contribution of groundwater to the stream along a 450 m reach (three during baseflow and one during stormflow conditions). Under baseflow conditions, streamflow measurements using the velocity–area technique indicated that the net groundwater flux to the stream during the summer months was 10 ml s⁻¹ m⁻¹. Hydrometric measurements (i.e. hydraulic gradient and hydraulic conductivity) taken using mini-piezometers installed in the sediments beneath the stream resulted in net groundwater flux estimates that were four to five times lower. Seepage meters failed to provide any measurements of water flux into or out of the stream. Therefore, based on these results, the velocity–area technique gives the best estimate of groundwater discharge. Hydrograph separations were conducted using isotopic ratios and electrical conductivity on two large rainfall events with different antecedent moisture conditions in the catchment. Both events showed that pre-event water (generally considered groundwater) dominated streamflow and tile drain flow with 64%–80% of the total discharge contributed by pre-event water. High water table conditions within the catchment resulted in greater stream discharge and a greater contribution of event water in the streamflow than that observed under low water table conditions for similar intensity storm events. The results also showed that differences in riparian zone width, vegetation and surface saturation conditions between the upper and lower catchment can influence the relative magnitude of streamflow response from the two catchment areas.     

Isotope hydrology of the Allt a' Mharcaidh catchment,Cairngorms, Scotland: implications for hydrological pathways and residence times

The hydrology of oxygen‐18 (¹⁸O) isotopes was monitored between 1995 and 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ¹⁸O=−7·69‰) exhibited strong seasonal variation in δ¹⁸O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ¹⁸O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen‐18 proved a useful tracer and streamwater δ¹⁸O variations could be explained in terms of a two‐component mixing model, involving a seasonally variable δ¹⁸O signature in storm runoff, mixing with groundwater characterized by relatively stable δ¹⁸O levels. Variations in soil water δ¹⁸O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near‐surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ¹⁸O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near‐surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These ¹⁸O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright © 2000 John Wiley & Sons, Ltd.     

Conceptualization of runoff processes using a geographical information system and tracers in a nested mesoscale catchment

Tracer investigations were combined with a geographical information system (GIS) analysis of the 31 km² Girnock catchment (Cairngorm Mountains, Scotland) in order to understand hydrological functioning by identifying dominant runoff sources and estimating mean residence times. The catchment has a complex geology, soil cover and topography. Gran alkalinity was used to demonstrate that catchment geology has a dominant influence on baseflow chemistry, but flow paths originating in acidic horizons in the upper soil profiles controlled stormflow alkalinity. Chemically based hydrograph separations at the catchment scale indicated that ～30% of annual runoff was derived from groundwater sources. Similar contributions (23–36%) were estimated for virtually all major sub‐basins. δ¹⁸O of precipitation (mean: ? 9·4‰; range: ? 16·1 to ? 5·0‰) and stream waters (mean: ? 9·1‰; range: ? 11·6 to ? 7·4‰) were used to assess mean catchment and sub‐basin residence times, which were in the order ～4–6 months. GIS analysis showed that these tracer‐based diagnostic features of catchment functioning were consistent with the landscape organization of the catchment. Soil and HOST (Hydrology of Soil Type) maps indicated that the catchment and individual sub‐basins were dominated by hydrologically responsive soils, such as peats (Histosol), peaty gleys (Histic Gleysols) and rankers (Umbric Leptosols and Histosols). Soil cover (in combination with a topographic index) predicted extensive areas of saturation that probably expand during hydrological events, thus providing a high degree of hydrological connectivity between catchment hillslopes and stream channel network. This was validated by aerial photographic interpretation and groundtruthing. These characteristics of hydrological functioning (i.e. dominance of responsive hydrological pathways and short residence times) dictate that the catchment is sensitive to land use change impacts on the quality and quantity of streamflows. It is suggested that such conceptualization of hydrological functioning using tracer‐validated GIS analysis can play an important role in the sustainable management of river basins. Copyright © 2006 John Wiley & Sons, Ltd.     

Dynamics of river–aquifer interactions along a chalk stream: the River Lambourn,UK

This paper describes the spatial and temporal pattern of groundwater flow accretion to the River Lambourn, a 234 km² chalk catchment of the West Berkshire Downs, UK, which has been largely unaffected by groundwater abstraction. Variations in the discharge measured at four fixed gauges in the catchment, coupled with information on the length of flowing channel over the period 1983–2001, are used to describe regional patterns in flow accretion. Mean catchment accretion generally exceeds 0·15 m³ s⁻¹ km⁻¹, but there are significant differences between perennial reaches indicating how the combination of local structural controls and seasonal changes in the drainage net affect flow accretion. Data from current meter surveys were used to determine the spatial variability in flow accretion: 505 paired observations along 12 reaches between 1 and 2·95 km in length indicated a consistent spatial trend in accretion. Accretion was high in upstream and downstream channel reaches, and in middle reaches where dry valleys intersected the main valley. A flow accretion index was developed to describe the relationship of flow accretion in each of the 12 study reaches to catchment discharge. The relationship varied from a strong positive correlation with catchment discharge (two reaches), a weak positive correlation (three reaches), a strong negative correlation (two reaches), to no relationship to catchment discharge (four reaches). The results highlight the need to reconsider the usual assumption of uniform, or uniformly increasing, flow accretion in chalk catchments. Moreover, they emphasize the importance of catchment topography, and illustrate how flow accretion in individual reaches may vary between high and low groundwater levels. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon,nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km² forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.     

Dynamic river–groundwater exchange in the presence of a saline,semi‐confined aquifer

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m³/m/day compared with 26 and 3 m³/m/day for the same periods via mass balance using Radon (²²²Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.