The importance of incorporating diurnally fluctuating stream discharge in stream temperature energy balance models

Although stream temperature energy balance models are useful to predict temperature through time and space, a major unresolved question is whether fluctuations in stream discharge reduce model accuracy when not exactly represented. However, high‐frequency (e.g., subdaily) discharge observations are often unavailable for such simulations, and therefore, diurnal streamflow fluctuations are not typically represented in energy balance models. These fluctuations are common due to evapotranspiration, snow pack or glacial melt, tidal influences within estuaries, and regulated river flows. In this work, we show when to account for diurnally fluctuating streamflow. To investigate how diurnal streamflow fluctuations affect predicted stream temperatures, we used a deterministic stream temperature model to simulate stream temperature along a reach in the Quilcayhuanca Valley, Peru, where discharge varies diurnally due to glacial melt. Diurnally fluctuating streamflow was varied alongside groundwater contributions via a series of computational experiments to assess how uncertainty in reach hydrology may impact simulated stream temperature. Results indicated that stream temperatures were more sensitive to the rate of groundwater inflow to the reach compared with the timing and amplitude of diurnal fluctuations in streamflow. Although incorporating observed diurnal fluctuations in discharge resulted in a small improvement in model RMSE, we also assessed other diurnal discharge signals and found that high amplitude signals were more influential on modelled stream temperatures when the discharge peaked at specific times. Results also showed that regardless of the diurnal discharge signal, the estimated groundwater flux to the reach only varied from 1.7% to 11.7% of the upstream discharge. However, diurnal discharge fluctuations likely have a stronger influence over longer reaches and in streams where the daily range in discharge is larger, indicating that diurnal fluctuations in stream discharge should be considered in certain settings.     

Structure,morphology and water flux of a subglacial drainage system,Midtdalsbreen, Norway

Digital elevation models of the surface and bed of Midtdalsbreen, Norway are used to calculate subglacial hydraulic potential and infer drainage system structure for a series of subglacial water pressure assumptions ranging from atmospheric to ice overburden. A distributed degree‐day model is used to calculate the spatial distribution of melt on the glacier surface throughout a typical summer, which is accumulated along the various drainage system structures to calculate water fluxes beneath the glacier and exiting the portals for the different water pressure assumptions. In addition, 78 dye‐tracing tests were performed from 33 injection sites and numerous measurements of water discharge were made on the main proglacial streams over several summer melt seasons. Comparison of the calculated drainage system structures and water fluxes with dye tracing results and measured proglacial stream discharges suggests that the temporally and spatially averaged steady‐state water pressures beneath the glacier are ~70% of ice overburden. Analysis of the dye return curves, together with the calculated subglacial water fluxes shows that the main drainage network on the eastern half of the glacier consists of a hydraulically efficient system of broad, low channels (average width/height ratio ≈ 75). The smaller drainage network on the west consists of a hydraulically inefficient distributed system, dominated by channels that are exceptionally broad and very low (average width/height ratio ≈ 350). The even smaller central drainage network also consists of a hydraulically inefficient distributed system, dominated by channels that are very broad and exceptionally low (average width/height ratio ≈ 450). The channels beneath the western and central glacier must be so broad and low that they can essentially be thought of as a linked cavity system. Copyright © 2011 John Wiley & Sons, Ltd.     

Proglacial sediment dynamics from daily to seasonal scales in a glaciated Alpine catchment (Bossons glacier,Mont Blanc massif,France)

The sediment yields of Alpine catchments are commonly determined from streamload measurements made some distance downstream from glaciers. However, this approach indiscriminately integrates erosion processes occurring in both the glacial and proglacial areas. A specific method is required to ascertain the respective inputs from (i) subglacial and supraglacial sediments, (ii) proglacial hillslopes and (iii) proglacial alluvial areas or sandurs. This issue is addressed here by combining high‐resolution monitoring (2 min) of suspended sediment concentrations at different locations within a catchment with discharge gauging and precipitation data. This methodological framework is applied to two proglacial streams draining the Bossons glacier (Mont Blanc massif, France): the Bossons and Crosette streams. For the Bossons stream, discharge and suspended load data were acquired from June to October 2013 at 1.15 and 1.5 km from the glacial terminus, respectively upstream and downstream from a small valley sandur. These hydro‐sedimentary data are compared with the Crosette stream dataset acquired at the outlet of the Bossons glacier subglacial drainage system. A fourfold analysis focusing on seasonal changes in streamload and discharge, multilinear regression modelling, evaluation of the sandur flux balance and probabilistic uncertainty assessment is used to determine the catchment sediment budget and to explain the proglacial sediment dynamics. The seasonal fluctuation of the sediment signal observed is related to the gradual closing of the subglacial drainage network and to the role of the proglacial area in the sediment cascade: the proglacial hillslopes appear to be disconnected from the main channel and the valley sandur acts as a hydrodynamic sediment buffer both daily and seasonally. Our findings show that an understanding of proglacial sediment dynamics can help in evaluating paraglacial adjustment and subglacial erosion processes. Copyright © 2017 John Wiley & Sons, Ltd.     

A comparison of fibre‐optic distributed temperature sensing to traditional methods of evaluating groundwater inflow to streams

There are several methods for determining the spatial distribution and magnitude of groundwater inputs to streams. We compared the results of conventional methods [dye dilution gauging, acoustic Doppler velocimeter (ADV) differential gauging, and geochemical end‐member mixing] to distributed temperature sensing (DTS) using a fibre‐optic cable installed along 900 m of Ninemile Creek in Syracuse, New York, USA, during low‐flow conditions (discharge of 1·4 m³ s^?1). With the exception of differential gauging, all methods identified a focused, contaminated groundwater inflow and produced similar groundwater discharge estimates for that point, with a mean of 66·8 l s^?1 between all methods although the precision of these estimates varied. ADV discharge measurement accuracy was reduced by non‐ideal conditions and failed to identify, much less quantify, the modest groundwater input, which was only 5% of total stream flow. These results indicate ambient tracers, such as heat and geochemical mixing, can yield spatially and quantitatively refined estimates of relatively modest groundwater inflow even in large rivers. DTS heat tracing, in particular, provided the finest spatial characterization of groundwater inflow, and may be more universally applicable than geochemical methods, for which a distinct and consistent groundwater end member may be more difficult to identify. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of a rock glacier spring on the stream energy budget and cold‐water refuge in an alpine stream

The thermal regimes of alpine streams remain understudied and have important implications for cold‐water fish habitat, which is expected to decline due to climatic warming. Previous research has focused on the effects of distributed energy fluxes and meltwater from snowpacks and glaciers on the temperature of mountain streams. This study presents the effects of the groundwater spring discharge from an inactive rock glacier containing little ground ice on the temperature of an alpine stream. Rock glaciers are coarse blocky landforms that are ubiquitous in alpine environments and typically exhibit low groundwater discharge temperatures and resilience to climatic warming. Water temperature data indicate that the rock glacier spring cools the stream by an average of 3 °C during July and August and reduces maximum daily temperatures by an average of 5 °C during the peak temperature period of the first two weeks in August, producing a cold‐water refuge downstream of the spring. The distributed stream surface and streambed energy fluxes are calculated for the reach along the toe of the rock glacier, and solar radiation dominates the distributed stream energy budget. The lateral advective heat flux generated by the rock glacier spring is compared to the distributed energy fluxes over the study reach, and the spring advective heat flux is the dominant control on stream temperature at the reach scale. This study highlights the potential for coarse blocky landforms to generate climatically resilient cold‐water refuges in alpine streams.     

Diel stream geochemistry,Taylor Valley,Antarctica

Unlike temperate and polythermal proglacial streams, the proglacial streams in Taylor Valley (TV), Antarctica, are derived primarily from glacier surface melt with no subglacial or groundwater additions. Solute responses to flow reflect only the interaction of glacial meltwater with the valley floor surrounding the stream channel. We have investigated the major, minor and trace element 24‐h variations of two proglacial melt streams, Andersen Creek and Canada Stream, originating from the Canada Glacier in TV, Antarctica. Both streams exhibited diel mid‐austral summer diurnal flow variation, with maximum flow being more than 50 times the minimum flow. Dissolved (< 0.4 µm) major, minor and trace solute behaviors through diel periods were strongly controlled by the availability of readily solubilized material on the valley floor and hyporheic‐biological exchanges. Anderson Creek had generally greater solute concentrations than Canada Stream because of its greater receipt of eolian sediment. Andersen Creek also acquired greater solute concentrations in the rising limb of the hydrograph than the falling limb because of dissolution of eolian material at the surface of the stream channel coupled with minimal hyporheic‐biological exchange. Conversely, Canada Stream had less available eolian sediment, but a greater hyporheic‐biological exchange, which preferentially removed trace and major solutes in the rising limb and released them in the falling limb. Given the dynamic nature of discharge, eolian, and hyporheic‐biological processes, solute loads in TV streams are difficult to predict. Copyright © 2012 John Wiley & Sons, Ltd.     

Inter‐annual variability in the effects of riparian woodland on micro‐climate,energy exchanges and water temperature of an upland Scottish stream

The influence of riparian woodland on stream temperature, micro‐climate and energy exchange was investigated over seven calendar years. Continuous data were collected from two reaches of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) with contrasting land use characteristics: (1) semi‐natural riparian forest and (2) open moorland. In the moorland reach, wind speed and energy fluxes (especially net radiation, latent heat and sensible heat) varied considerably between years because of variable riparian micro‐climate coupled strongly to prevailing meteorological conditions. In the forested reach, riparian vegetation sheltered the stream from meteorological conditions that produced a moderated micro‐climate and thus energy exchange conditions, which were relatively stable between years. Net energy gains (losses) in spring and summer (autumn and winter) were typically greater in the moorland than the forest. However, when particularly high latent heat loss or low net radiation gain occurred in the moorland, net energy gain (loss) was less than that in the forest during the spring and summer (autumn and winter) months. Spring and summer water temperature was typically cooler in the forest and characterised by less inter‐annual variability due to reduced, more inter‐annually stable energy gain in the forested reach. The effect of riparian vegetation on autumn and winter water temperature dynamics was less clear because of the confounding effects of reach‐scale inflows of thermally stable groundwater in the moorland reach, which strongly influenced the local heat budget. These findings provide new insights as to the hydrometeorological conditions under which semi‐natural riparian forest may be effective in mitigating river thermal variability, notably peaks, under present and future climates. © 2014 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

Floodplain connection buffers seasonal changes in urban stream water quality

Urban streams in the Northeastern United States have large road salt inputs during the winter, increased nonpoint sources of inorganic nitrogen and decreased short‐term and permanent storage of nutrients. Restoration activities that re‐establish connection between streams and riparian environments may be effective for improving urban stream water quality. Meadowbrook Creek, a first‐order stream in Syracuse, NY, provides a unique setting to explore impacts of stream–floodplain connection because it flows along a negative urbanization gradient, from channelized and armoured headwaters to a broad, vegetated floodplain with a riparian aquifer. In this study, we investigated how reconnection to groundwater and introduction of riparian vegetation impacted urban surface water chemistry by making biweekly longitudinal surveys of stream water chemistry in the creek from May 2012 until June 2013. We used multiple methods to measure groundwater discharge rates along the creek. Chloride concentrations in the upstream, disconnected reach were influenced by discharge of road salt during snow melt events and ranged from 161.2 to 1440 mg/l. Chloride concentrations in the downstream, connected reach had less temporal variation, ranging from 252.0 to 1049 mg/l, because of buffering by groundwater discharge, as groundwater chloride concentrations ranged from 84.0 to 655.4 mg/l. In the summer, there was little to no nitrate in the disconnected reach because of limited sources and high primary productivity, but concentrations reached over 1 mg N/l in the connected reach because of the presence of riparian vegetation. During the winter, when temperatures fell below freezing, nitrate concentrations in the disconnected reach increased to 0.58 mg N/l but were still lower than the connected reach, which averaged 0.88 mg N/l. Urban stream restoration projects that restore floodplain connection may impact water quality by storing high salinity road run‐off during winter overbank events and discharging that water year‐round, thereby attenuating seasonal fluctuations in chloride. Contrary to prior findings, we observed that floodplain connection and riparian vegetation may alter nitrate sources and sinks such that nitrate concentrations increase longitudinally in connected urban streams. Copyright © 2014 John Wiley & Sons, Ltd.     

Assessing regional‐scale spatio‐temporal patterns of groundwater–surface water interactions using a coupled SWAT‐MODFLOW model

Interaction between groundwater and surface water in watersheds has significant impacts on water management and water rights, nutrient loading from aquifers to streams, and in‐stream flow requirements for aquatic species. Of particular importance are the spatial patterns of these interactions. This study explores the spatio‐temporal patterns of groundwater discharge to a river system in a semi‐arid region, with methods applied to the Sprague River Watershed (4100 km²) within the Upper Klamath Basin in Oregon, USA. Patterns of groundwater–surface water interaction are explored throughout the watershed during the 1970–2003 time period using a coupled SWAT‐MODFLOW model tested against streamflow, groundwater level and field‐estimated reach‐specific groundwater discharge rates. Daily time steps and coupling are used, with groundwater discharge rates calculated for each model computational point along the stream. Model results also are averaged by month and by year to determine seasonal and decadal trends in groundwater discharge rates. Results show high spatial variability in groundwater discharge, with several locations showing no groundwater/surface water interaction. Average annual groundwater discharge is 20.5 m³/s, with maximum and minimum rates occurring in September–October and March–April, respectively. Annual average rates increase by approximately 0.02 m³/s per year over the 34‐year period, negligible compared with the average annual rate, although 70% of the stream network experiences an increase in groundwater discharge rate between 1970 and 2003. Results can assist with water management, identifying potential locations of heavy nutrient mass loading from the aquifer to streams and ecological assessment and planning focused on locations of high groundwater discharge. Copyright © 2016 John Wiley & Sons, Ltd.     

Deducing the spatial variability of exchange within a longitudinal channel water balance

Developing an appropriate data collection scheme to infer stream–subsurface interactions is not trivial due to the spatial and temporal variability of exchange flowpaths. Within the context of a case study, this paper presents the results from a number of common data collection techniques ranging from point to reach scales used in combination to better understand the spatial complexity of subsurface exchanges, infer the hydrologic conditions where individual influences of hyporheic and groundwater exchange components on stream water can be characterized, and determine where gaps in information arise. We start with a tracer‐based, longitudinal channel water balance to quantify hydrologic gains and losses at a sub‐reach scale nested within two consecutive reaches. Next, we look at groundwater and stream water surface levels, shallow streambed vertical head gradients, streambed and aquifer hydraulic conductivities, water chemistry, and vertical flux rates estimated from streambed temperatures to provide more spatially explicit information. As a result, a clearer spatial understanding of gains and losses was provided, but some limitations in interpreting results were identified even when combining information collected over various scales. Due to spatial variability of exchanges and areas of mixing, each technique frequently captured a combination of groundwater and hyporheic exchange components. Ultimately, this study provides information regarding technique selection, emphasizes that care must be taken when interpreting results, and identifies the need to apply or develop more advanced methods for understanding subsurface exchanges. Copyright © 2013 John Wiley & Sons, Ltd.     

Riparian influence on hyporheic‐zone formation downstream of a small dam in the Blackland Prairie region of Texas

Small‐order streams have highly variable flows that can result in large temporal and spatial variation of the hyporheic zone. Dam construction along these intermittent headwater streams alters downstream flow and influences the hydrologic balance between stream water and the adjacent riparian zone. A 3‐year site study was conducted along an impounded second‐order stream to determine the water balance between stream, unsaturated zone, groundwater and riparian vegetation. The presence of the upstream impoundment provided near‐perennial water flow in the stream channel. The observed woody plant transpiration accounted for 71% of average annual water loss in the site. The overall contribution of stream water via the hyporheic zone to site water balance was 73 cm, or 44% of total inputs. This exceeded both rainfall and upland subsurface contribution to the site. A highly dynamic hyporheic zone was indicated by high water use from woody plants that fluctuated seasonally with stream water levels. We found leaf area development in the canopy layer to be closely coupled with stream and groundwater fluctuations, indicating its usefulness as a potential indicator of site water balance for small dam systems. The net result of upstream impoundment increased riparian vegetation productivity by influencing movement of stream water to storage in the groundwater system. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonal evolution of meltwater generation,storage and discharge at a non‐temperate glacier in Svalbard

In glacierized catchments, meteorological inputs driving surface melting are translated into runoff outputs mediated by the glacier hydrological system: analysis of the relationship between meteorology and diurnal and seasonal patterns of runoff should reflect the functioning of that system, with the role of meltwater storage likely to be of particular importance. Daily meltwater storage is determined for a glacier at 78 °N in the Svalbard archipelago, by comparing inputs calculated from a surface energy balance model with measured outputs (proglacial discharge). Solar radiation, air temperature, wind speed and proglacial discharge are then analysed by regression and time‐series methods, in order to assess the meteorology–discharge relationship and its variation at diurnal and seasonal time‐scales. The recorded discharge time‐series can be divided into two contrasting intervals: up to early August, proglacial discharge was high and variable, mean hydrographs showed little indication of diurnal cycling, ARIMA models of discharge indicated a non‐seasonal, moving‐average generating process, and there was a net loss of meltwater from storage; from early August, proglacial discharge was low and relatively invariable, but with clearer diurnal cycles, regression models of discharge showed substantially improved correlations with air temperature and solar radiation, ARIMA models indicated a non‐seasonal, autoregressive generating process, and eventually a seasonal component, and there was a net gain in meltwater storage. The transition between the two periods is brief compared with the duration of the melt season. The runoff response to meteorology therefore lacks the strongly progressive element previously identified in mid‐latitude glacierized catchments. In particular, the glacier hydrological system only appears responsive to diurnal forcing following the depletion of the seasonal snowpack meltwater store. Copyright © 2001 John Wiley & Sons, Ltd.     

Assessing the importance of seepage and springs to nitrate flux in a stream network in the Wisconsin sand plains

Evaluating the flow paths that contribute to solute flux in stream networks can lead to greater understanding of the linkages between biogeochemistry and hydrology. We compared the contributions of groundwater in spring brooks and in seepage through the streambed to nitrate flux in the Emmons Creek network in the Wisconsin sand plains. We predicted that spring brooks would contribute disproportionately to nitrate flux due to the presumed higher advection rates in springs and less opportunity for nitrate removal relative to seeps. Nitrate flux was measured in 15 spring brooks that entered Emmons Creek. Nitrate flux from seepage was measured at the locations of 30 piezometers, based on Darcy's Law, and by a reach‐scale injection of Rhodamine water tracing (RWT). When seepage discharge was estimated from the RWT release, groundwater inputs from seepage and springs accounted for the discharge gain in the Emmons Creek channel. Springs brooks and seepage (based on the RWT release) contributed 37% and 63%, respectively, to nitrate flux inputs in the study reach. Contrary to our prediction, seeps contributed disproportionately to nitrate flux relative to their discharge. Relatively high rates of seepage discharge and higher than anticipated nitrate concentrations in the shallow pore water at seepage locations contributed to the unanticipated result.     

Soil water dye tracing,with special reference to the use of rhodamine WT,lissamine FF and amino G acid

Three fluorescent dyes (Rhodamine WT, Lissamine FF and Amino G Acid) are compared for use in soil water tracing. Severe limitations are evident, but practical applications are possible. Background fluorescence, adsorption, desorption, pH and other non-adsorptive effects are reviewed in the contexts of soil column work and field tracing of soil water. Lissamine FF and Amino G Acid are to be preferred for soil column work because of their lower adsorption; Rhodamine WT exhibits higher adsorption but is useful in field situations where organic fluorescence backgrounds are high. Semi-quantitative work may be undertaken in soil columns once a priming and flushing procedure has been adopted.     

Hillslope groundwater discharges provide localized stream ecosystem buffers from regional per- and polyfluoroalkyl substances contamination

Emerging groundwater contaminants such as per- and polyfluoroalkyl substances (PFAS) may impact surface-water quality and groundwater-dependent ecosystems of gaining streams. Although complex near-surface hydrogeology of stream corridors challenges sampling efforts, recent advances in heat tracing of discharge zones enable efficient and informed data collection. For this study, we used a combination of streambed temperature push-probe and thermal infrared methods to guide a discharge-zone-oriented sample collection along approximately 6 km of a coastal trout stream on Cape Cod, MA. Eight surface-water locations and discharging groundwater from 24 streambed and bank seepages were analysed for dissolved oxygen (DO), specific conductance, stable water isotopes, and a range of PFAS compounds, which are contaminants of emerging concern in aquatic environments. The results indicate a complex system of groundwater discharge source flowpaths, where the sum of concentrations of six PFAS compounds (corresponding to the U.S. Environmental Protection Agency third Unregulated Contaminant Monitoring Rule “UCMR 3”) showed a median concentration of 52 ± 331 (SD) ng/L with two higher outliers and three discharges with PFAS concentrations below the quantification limit. Higher PFAS concentration was related (− 0.66 Spearman rank, p < .001) to discharging groundwater that showed an evaporative signature (deuterium excess), indicating flow through at least one upgradient kettle lake. Therefore, more regional groundwater flowpaths originating from outside the local river corridor tended to show higher PFAS concentrations as evaluated at their respective discharge zones. Conversely, PFAS concentrations were typically low at discharges that did not indicate evaporation and were adjacent to steep hillslopes and, therefore, were classified as locally recharged groundwater. Previous research at this stream found that the native brook trout spawn at discharge points of groundwater recharged on local hillslopes, likely in response to generally higher levels of DO. Our study shows that by targeting high oxygen discharges the trout may thereby be avoiding emerging contaminants such as PFAS in groundwater recharged farther from the stream.     

Bidirectional stream–groundwater flow in response to ephemeral and intermittent streamflow and groundwater seasonality

It is often assumed that the net groundwater flow direction is towards the channel in headwater streams in humid climates, with magnitudes dependent on flow state. However, studies that characterize stream–groundwater interactions in ephemeral and intermittent streams in humid landscapes remain sparse. Here, we examined seasonally driven stream–groundwater interactions in response to temporary streamflow on the basis of field observations of streamflow and groundwater on an adjacent hillslope. The direction of hydraulic head gradients between the stream and groundwater shifted seasonally. The stream gained water (head gradients were towards the stream) when storage state was high. During this period, streamflow was persistent. The stream lost water to the groundwater system (head gradients were away from the stream) when storage state was low. During this period, streamflow only occurred in response to precipitation events, and head gradients remained predominantly away from the stream during events. This suggested that mechanisms other than deep groundwater contributions produced run‐off when storage was low, such as surface and perched subsurface flowpaths above the water table. Analysis of the annual water balance for the study period showed that the residual between precipitation inputs and streamflow and evapotranspiration outputs, which were attributed to the loss of water to the deeper, regional groundwater system, was similar in magnitude to streamflow. This, coupled with results that showed bidirectionality in stream–groundwater head gradients, indicated that headwaters composed of temporary (e.g., ephemeral and intermittent) streams can be important focal areas for regional groundwater recharge, and both contribute to and receive water, solutes, and materials from the groundwater system.     

Initial results from a distributed,physically based model of glacier hydrology

This paper describes the development and testing of a distributed, physically based model of glacier hydrology. The model is used to investigate the behaviour of the hydrological system of Haut Glacier d'Arolla, Valais, Switzerland. The model has an hourly time-step and three main components: a surface energy balance submodel, a surface flow routing submodel and a subglacial hydrology submodel. The energy balance submodel is used to calculate meltwater production over the entire glacier surface. The surface routing submodel routes meltwater over the glacier surface from where it is produced to where it either enters the subglacial hydrological system via moulins or runs off the glacier surface. The subglacial hydrology submodel calculates water flow in a network of conduits, which can evolve over the course of a melt season simulation in response to changing meltwater inputs. The main model inputs are a digital elevation model of the glacier surface and its surrounding topography, start-of-season snow depth distribution data and meteorological data. Model performance is evaluated by comparing predictions with field measurements of proglacial stream discharge, subglacial water pressure (measured in a borehole drilled to the glacier bed) and water velocities inferred from dye tracer tests. The model performs best in comparison with the measured proglacial stream discharges, but some of the substantial features of the other two records are also reproduced. In particular, the model results show the high amplitude water pressure cycles observed in the borehole in the mid-melt season and the complex velocity/discharge hysteresis cycles observed in dye tracer tests. The results show that to model outflow hydrographs from glacierized catchments effectively, it is necessary to simulate spatial and temporal variations in surface melt rates, the delaying effect of the surface snowpack and the configuration of the subglacial drainage system itself. The model's ability to predict detailed spatial and temporal patterns of subglacial water pressures and velocities should make it a valuable tool for aiding the understanding of glacier dynamics and hydrochemistry. © 1998 John Wiley & Sons, Ltd.     

Hydrogeologic controls on summer stream temperatures in the McKenzie River basin,Oregon

Stream temperature is a complex function of energy inputs including solar radiation and latent and sensible heat transfer. In streams where groundwater inputs are significant, energy input through advection can also be an important control on stream temperature. For an individual stream reach, models of stream temperature can take advantage of direct measurement or estimation of these energy inputs for a given river channel environment. Understanding spatial patterns of stream temperature at a landscape scale requires predicting how this environment varies through space, and under different atmospheric conditions. At the landscape scale, air temperature is often used as a surrogate for the dominant controls on stream temperature. In this study we show that, in regions where groundwater inputs are key controls and the degree of groundwater input varies in space, air temperature alone is unlikely to explain within-landscape stream temperature patterns. We illustrate how a geologic template can offer insight into landscape-scale patterns of stream temperature and its predictability from air temperature relationships. We focus on variation in stream temperature within headwater streams within the McKenzie River basin in western Oregon. In this region, as in other areas of the Pacific Northwest, fish sensitivity to summer stream temperatures continues to be a pressing environmental issue. We show that, within the McKenzie, streams which are sourced from deeper groundwater reservoirs versus shallow subsurface flow systems have distinct summer temperature regimes. Groundwater streams are colder, less variable and less sensitive to air temperature variation. We use these results from the western Oregon Cascade hydroclimatic regime to illustrate a conceptual framework for developing regional-scale indicators of stream temperature variation that considers the underlying geologic controls on spatial variation, and the relative roles played by energy and water inputs. Copyright © 2007 John Wiley & Sons, Ltd.     

Groundwater contributions of flow and nitrogen in a headwater agricultural watershed

Nonpoint sources of nitrogen (N) and other nutrients are a major source of water pollution within the Chesapeake Bay watershed and other basins around the world. Human activities associated with agricultural practices can account for a large percentage of N loadings delivered to streams and rivers. This work aims to improve understanding of N transport from groundwater to surface waters, quantifying the principal hydrological processes driving water and N fluxes into and out of a headwater agricultural stream reach. The study site is a 175-m stream reach in a heavily cultivated 40-ha watershed in east-central Pennsylvania. This subwatershed is underlain by fractured shale bedrock, and receives most of its baseflow from groundwater, either by diffuse matrix discharge through the streambed or by localized discharge through riparian seeps. Samples of stream, seep, and shallow groundwater were collected approximately monthly under steady hydrologic conditions in 2017. Calculated matrix flow from hydraulic head and conductivity measurements paired with differential stream gauging was used to solve for the riparian seep flux using a mass balance approach. Riparian seep fluxes ranged from 45 to 217 m³/d, transporting 0.6–4.2 kg N d⁻¹ of nitrate-N from the fractured bedrock aquifer to the stream. Hydrochemical data suggest that the stream is mainly disconnected from the underlying aquifer and that seeps supply essentially all water and N to the system. Seeps are likely sourced with N in nearby agricultural fields and accelerated through the system with shorter residence times than shallow groundwater. Water isotope data reinforced this notion. This study underscores the importance of agriculture as a source of N to ground and surface waters. Identifying source areas that are causing groundwater enrichment of N and seep areas where N discharges to streams is beneficial for developing N pollution mitigation strategies and implementing management practices that aim to reduce nutrient loads to the Chesapeake Bay.