Scholarly Works, Virginia Water Resources Research Center

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Development of a lateral topographic weathering gradient in temperate forested podzols
Bower, Jennifer A.; Ross, Donald S.; Bailey, Scott W.; Pennino, Amanda M.; Jercinovic, Michael J.; McGuire, Kevin J.; Strahm, Brian D.; Schreiber, Madeline E. (Elsevier, 2023-11)
Mineral weathering is an important soil-forming process driven by the interplay of water, organisms, solution chemistry, and mineralogy. The influence of hillslope-scale patterns of water flux on mineral weathering in soils is still not well understood, particularly in humid postglacial soils, which commonly harbor abundant weatherable primary minerals. Previous work in these settings showed the importance of lateral hydrologic patterns to hillslope-scale pedogenesis. In this study, we hypothesized that there is a corresponding relationship between hydrologically driven pedogenesis and chemical weathering in podzols in the White Mountains of New Hampshire, USA. We tested this hypothesis by quantifying the depletion of plagioclase in the fine fraction (≤2 mm) of closely spaced, similar-age podzols along a gradient in topography and depth to bedrock that controls lateral water flow. Along this gradient, laterally developed podzols formed through frequent, episodic flushing by upslope groundwater, and vertically developed podzols formed through characteristic vertical infiltration. We estimated the depletion of plagioclase-bound elements within the upper mineral horizons of podzols using mass transfer coefficients (τ) and quantified plagioclase losses directly through electron microscopy and microprobe analysis. Elemental depletion was significantly more pronounced in the upslope lateral eluvial (E horizon-dominant) podzols relative to lateral illuvial (B horizon-dominant) and vertical (containing both E and B horizons) podzols downslope, with median Na losses of ∼74 %, ∼56 %, and ∼40 %, respectively. When comparing genetic E horizons, Na and Al were significantly more depleted in laterally developed podzols relative to vertically developed podzols. Microprobe analysis revealed that ∼74 % of the plagioclase was weathered from the mineral pool of lateral eluvial podzols, compared to ∼39 % and ∼23 % for lateral illuvial podzols and vertically developed podzols, respectively. Despite this intense weathering, plagioclase remains the second most abundant mineral in soil thin sections. These findings confirm that the concept of soil development as occurring vertically does not accurately characterize soils in topographically complex regions. Our work improves the current understanding of pedogenesis by identifying distinct, short-scale gradients in mineral weathering shaped by local patterns of hydrology and topography.
Importance of tree diameter and species for explaining the temporal and spatial variations of xylem water delta O-18 and delta H-2 in a multi-species forest
Fresne, Maelle; Chun, Kwok P.; Hrachowitz, Markus; McGuire, Kevin J.; Schoppach, Remy; Klaus, Julian (Wiley, 2023-05)
Identifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species-specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal component regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes, while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size- and species-specific as well as water availability-dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes-based ecohydrological and catchments studies.
Transit Time Estimation in Catchments: Recent Developments and Future Directions
Benettin, Paolo; Rodriguez, Nicolas B.; Sprenger, Matthias; Kim, Minseok; Klaus, Julian; Harman, Ciaran J.; Velde, Ype; Hrachowitz, Markus; Botter, Gianluca; McGuire, Kevin J.; Kirchner, James W.; Rinaldo, Andrea; McDonnell, Jeffrey J. (American Geophysical Union, 2022-11-14)
Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water-age-balance approaches and data-based approaches. We discuss and compare methods including StorAge-Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice.
Subsurface permeability contrasts control shallow groundwater flow dynamics in the critical zone of a glaciated, headwater catchment
Benton, Joshua R.; McGuire, Kevin J.; Schreiber, Madeline E. (Wiley, 2022-09-01)
Groundwater flow direction within the critical zone of headwater catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced by subsurface permeability contrasts, which can result in variability in flow direction and magnitude. In this study, we investigated the relationship between shallow groundwater flow direction, surface topography, and the subsurface topography of low permeability units in a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), NH. We continuously monitored shallow groundwater levels in the solum throughout several seasons in a well network (20 wells of 0.18–1.1 m depth) within the upper hillslopes of Watershed 3 of the HBEF. Water levels were also monitored in four deeper wells, screened from 2.4 to 6.9 m depth within glacial drift of the C horizon. We conducted slug tests across the well network to determine the saturated hydraulic conductivity (Ksat) of the materials surrounding each well. Results showed that under higher water table regimes, groundwater flow direction mimics surface topography, but under lower water table regimes, flow direction can deviate as much as 56 degrees from surface topography. Under these lower water table conditions, groundwater flow direction instead followed the topography of the top of the C horizon. The interquartile range of Ksat within the C horizon was two orders of magnitude lower than within the solum. Overall, our results suggest that the land surface topography and the top of the C horizon acted as end members defining the upper and lower bounds of flow direction variability. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone and runoff generation studies of headwater catchments that are underlain by glacial drift.
Comparison of benthic macroinvertebrate assessment methods along a salinity gradient in headwater streams
Pence, Rachel A.; Cianciolo, Thomas R.; Drover, Damion R.; McLaughlin, Daniel L.; Soucek, David J.; Timpano, Anthony J.; Zipper, Carl E.; Schoenholtz, Stephen H. (Springer, 2021-12-01)
Benthic macroinvertebrate community assessments are used commonly to characterize aquatic systems and increasingly for identifying their impairment caused by myriad stressors. Yet sampling and enumeration methods vary, and research is needed to compare their abilities to detect macroinvertebrate community responses to specific water quality variables. A common assessment method, rapid bioassessment, uses subsampling procedures to identify a fixed number of individual organisms regardless of total sample abundance. In contrast, full-enumeration assessments typically allow for expanded community characterization resulting from higher numbers of identified organisms within a collected sample. Here, we compared these two sampling and enumeration methods and their abilities to detect benthic macroinvertebrate response to freshwater salinization, a common stressor of streams worldwide. We applied both methods in headwater streams along a salinity gradient within the coal-mining region of central Appalachia USA. Metrics of taxonomic richness, community composition, and trophic function differed between the methods, yet most metrics exhibiting significant response to SC for full-enumeration samples also did for rapid bioassessment samples. However, full-enumeration yielded taxonomic-based metrics consistently more responsive to the salinization gradient. Full-enumeration assessments may potentially provide more complete characterization of macroinvertebrate communities and their response to increased salinization, whereas the more cost-effective and widely employed rapid bioassessment method can detect community alterations along the full salinity gradient. These findings can inform decisions regarding such tradeoffs for assessments of freshwater salinization in headwater streams and highlight the need for similar research of sampling and enumeration methodology in other aquatic systems and for other stressors.
Taxon and trait-based sampling curves can be used as a tool for assessing impairment in salinized headwater streams
James, Aryanna; Pence, Rachel A.; Pond, Gregory; Schoenholtz, Stephen H.; Timpano, Anthony J.; Zipper, Carl E.; Entrekin, Sally A. (Elsevier, 2022-06-01)
Many ecosystems are losing biodiversity, raising concern for the services they provide. However, the extent of loss is uncertain, especially for diverse insects, because of incomplete sampling. Modeling techniques for estimating upper bounds on diversity are needed to assess benthic freshwater insect abundances, taxa richness, and diversity because some species are declining at alarming rates while others are increasing. In central Appalachian streams of the U.S.A., aquatic insect communities have lost diversity following salinization caused by mining activities. However, the number of taxa observed are dependent upon sampling effort. Incomplete sampling could misrepresent biodiversity and functional losses. Our goal was to use taxon sampling curves to estimate sampling effort required to maximize the probability of accurate benthic macroinvertebrate characterization in dominant riffle habitats of headwater streams. We collected 5 to 10 quantitative benthic macroinvertebrate samples in each of six, first-order streams in the central Appalachian region. For our single-habitat, mesoscale approach, we predicted: (1) macroinvertebrate taxa richness would be the most robust indicator of salinization response compared to diversity, evenness, and density, (2) less sampling effort would be needed to capture taxonomic richness in salinized streams compared to reference streams, and (3) response diversity would also be lower in salinized streams because select trait states would be represented by fewer taxa. Results suggested nominally lower taxa richness, evenness, and diversity in some salinized streams but not in all despite greater or more variable within-stream densities. Nonetheless, sampling effort required to characterize macroinvertebrate communities did not differ between reference and salinized streams, though uneven within-stream taxa distributions corresponded with greater sampling effort requirements for complete characterization. Benthic macroinvertebrate community characteristics were different where higher densities and richness of small-bodied and fast-developing taxa were more common in salinized streams. Response diversity depended on trait states. For example, only about five shredder taxa were represented in salinized streams compared to eight taxa in reference streams. Despite some indication of lower response diversity across some functional feeding groups, more than five samples were needed for robust comparisons. Taxon and trait-based sampling curves suggest that greater overall sampling effort is needed and equal samples per stream was critical for complete diversity assessments regardless of the level of mining-induced salinization. Model-based sampling curves can serve as a tool to assess upper bounds on diversity metrics and sample-effort rigor.
An Interactive Web Application Helps Students Explore Water Balance Concepts
Gannon, John P.; McGuire, Kevin J. (Frontiers, 2022-04-12)
The concept of a water balance is a foundational topic in hydrology classrooms. While understanding and applying this concept is crucial to the introduction of more advanced topics, students often struggle to develop a thorough understanding of the relationships between components, assumptions, and limitations of a water balance. To aid students in developing a working understanding of a water balance, we developed a web application that runs a one dimensional Thornthwaite-type water balance at any of thousands of NOAA climate stations across the continental United States using the local soil-water storage capacity at the station location. Within the app, students can manipulate the soil-water storage capacity, latitude, temperature, and precipitation to better understand how it works and explore scenarios of land use, extreme weather, and climate change. The application is free and will run on any device that can open an internet browser window (laptops, chromebooks, smartphones, etc). Here we present the details of the model, functionality of the application, and link to several ready-made classroom activities. Finally, results from student surveys in two hydrology classrooms show that students may learn water balance concepts more effectively than traditional methods such as spreadsheet computations.
Identifying Controls on Nitrate Sources and Flowpaths in a Forested Catchment Using a Hydropedological Framework
Pardo, Linda H.; Green, M. B.; Bailey, S. W.; McGuire, Kevin J.; McDowell, W. H. (American Geophysical Union, 2022-02-01)
Catchment-scale assessments of nitrogen retention and loss rarely account for soil and landscape heterogeneity and are, thus, unable to account for the suite of nitrogen cycling processes that ultimately affect the export of nitrate via stream water. Long-term study at the Hubbard Brook Experimental Forest, NH has generated a unique data set that facilitates spatially explicit examination of interactions among hydrology, soil development, and nitrogen cycling processes. Using high-frequency streamwater chemistry data with intensive subsurface hydrology and solute monitoring, we tracked areas of the catchment that are hydrologically active under different flow conditions to identify the source area of streamwater nitrate. We hypothesize that as the drainage network expands, increasing hydrologic connection to bedrock outcrop-associated soils, streamwater nitrate concentration, and flux at the catchment outlet increase. Most nitrate export (>80%) occurred during high flows when high nitrate, bedrock-controlled areas of the catchment were most connected hydrologically to the drainage network (∼15% of the time). End-member mixing analysis demonstrated that the bedrock-controlled upper part of the catchment influences nitrate concentration at the outlet and contributes most to catchment nitrate export compared to the near-stream soil units and seeps. Most of the time, nitrate at the catchment outlet comes from seeps and the near-stream zone; under high flow conditions, bedrock-controlled hotspots for nitrate production contribute more to export of nitrate. This analysis demonstrates how the source area of streamwater nitrate varies under different flow conditions, suggesting that long-term nitrate dynamics may be driven primarily by a relatively small part of the catchment.
A catchment water balance assessment of an abrupt shift in evapotranspiration at the Hubbard Brook Experimental Forest, New Hampshire, USA
Green, Mark; Bailey, Scott W.; Campbell, John L.; McGuire, Kevin J.; Bailey, Amey; Fahey, Timothy; Lany, Nina; Zietlow, David (Wiley, 2021-07-01)
Small catchments have served as sentinels of forest ecosystem responses to changes in air quality and climate. The Hubbard Brook Experimental Forest in New Hampshire has been tracking catchment water budgets and their controls - meteorology and vegetation - since 1956. Water budgets in four reference catchments indicated an approximately 30% increase in the evapotranspiration (ET) as estimated by the difference between precipitation (P) and runoff (RO) starting in 2010 and continuing through 2019. We analyzed the annual water budgets, cumulative deviations of the daily P, RO, and water budget residual (WBR = P - RO), potential ET, and indicators of subsurface storage to gain greater insight into this shift in the water budgets. The potential ET and the subsurface storage indicators suggest that this change in WBR was primarily due to increasing ET. While multiple long-term hydrological and micrometeorological data sets were used to detect and investigate this change in ET, additional measurements of groundwater storage and soil moisture would enable better estimation of ET within the catchment water balance. Increasing the breadth of long-term measurements across small gauged catchments allows them to serve as more effective sentinels of substantial hydrologic changes like the ET increase that we observed.
Predicting Solute Transport Through Green Stormwater Infrastructure With Unsteady Transit Time Distribution Theory
Parker, E. A.; Grant, Stanley B.; Cao, Y.; Rippy, Megan A.; McGuire, Kevin J.; Holden, P. A.; Feraud, M.; Avasarala, S.; Liu, H.; Hung, W. C.; Rugh, M.; Jay, J.; Peng, J.; Shao, S.; Li, D. (2021-02)
In this study, we explore the use of unsteady transit time distribution (TTD) theory to model solute transport in biofilters, a popular form of nature-based or "green" storm water infrastructure (GSI). TTD theory has the potential to address many unresolved challenges associated with predicting pollutant fate and transport through these systems, including unsteadiness in the water balance (time-varying inflows, outflows, and storage), unsteadiness in pollutant loading, time-dependent reactions, and scale-up to GSI networks and urban catchments. From a solution to the unsteady age conservation equation under uniform sampling, we derive an explicit expression for solute breakthrough during and after one or more storm events. The solution is calibrated and validated with breakthrough data from 17 simulated storms at a field-scale biofilter test facility in Southern California, using bromide as a conservative tracer. TTD theory closely reproduces bromide breakthrough concentrations, provided that lateral exchange with the surrounding soil is accounted for. At any given time, according to theory, more than half of the water in storage is from the most recent storm, while the rest is a mixture of penultimate and earlier storms. Thus, key management endpoints, such as the pollutant treatment credit attributable to GSI, are likely to depend on the evolving age distribution of water stored and released by these systems.
Logging-debris and vegetation-control treatments influence competitive relationships to limit 15-year productivity of coast Douglas-fir in western Washington and Oregon
Harrington, Timothy B.; Slesak, Robert A.; Dollins, James P.; Schoenholtz, Stephen H.; Peter, David H. (2020-10-01)
At two affiliate sites of the North American Long-Term Soil Productivity Study (Matlock, WA and Molalla, OR, USA), soil chemical properties and stand characteristics of planted coast Douglas-fir (Pseudotsuga menziesii var. menziesii) were compared 15 years after forest harvesting and application of three logging-debris configurations (dispersed, piled, or removed) combined with either initial vegetation control (IVC; year 0) or annual vegetation control (AVC; years 0 to 5). At Matlock, soil carbon (C) and nitrogen (N) concentrations each were 17% greater after IVC than after AVC; at Molalla, soil N was 13% greater where debris was removed than where it was dispersed. At Matlock, cover of nonnative Scotch broom (Cytisus scoparius) after IVC was greater where debris was removed (61%) than where it was piled (27%) or dispersed (7%), despite a control treatment in year 4. Conversely, covers of the native shrubs, trailing blackberry (Rubus ursinus) and salal (Gaultheria shallon) were 20% to 30% greater where debris was dispersed than where it was piled or removed. With AVC versus IVC, Douglas-fir stand volume was 34% to 159% greater at Matlock depending on the logging-debris treatment, and it was 30% greater at Molalla independent of debris treatments. However, Douglas-fir survival and growth after AVC did not differ among logging-debris treatments at either site. Survival of Douglas-fir growing <= 1 m from the edge of debris piles at Matlock averaged 16% greater than that of trees > 1 m from debris piles. Debris dispersal or piling at Matlock strongly mitigated Scotch broom impacts to forest productivity compared to debris removal. Our findings demonstrate how disturbance characteristics associated with forest harvesting and regeneration practices can influence vegetation recruitment and competitive relationships to place limits on longer-term forest productivity.
Predictive modeling of bedrock outcrops and associated shallow soil in upland glaciated landscapes
Fraser, Olivia L.; Bailey, Scott W.; Ducey, Mark J.; McGuire, Kevin J. (2020-10-15)
Identifying the areal extent of bedrock outcrops and shallow soils has important implications for understanding spatial patterns in vegetation composition and productivity, stream chemistry gradients, and hydrologic and soil properties of landscapes. Manual methods of delineating bedrock outcrops and associated shallow soils are still commonly employed, but they are expensive to implement over broad areas and often limited by representation of polygon units. Few studies have automated the delineation of bedrock outcrops. These focused on delineation approaches in landscapes with rapidly eroding hillslopes and sparse vegetation. The objectives of this study were to assess the accuracy of visually interpreting high-resolution relief maps for locating bedrock outcrops and associated shallow soil (BOSS) < 50 cm deep in a heavily forested landscape, to use visually interpreted point locations to train predictive models, and to compare predictions with manually delineated polygons in upland glaciated landscapes. Visual interpretation of Lidar-derived 1 m shaded relief maps at Hubbard Brook Experimental Forest (HBEF), USA resulted in a 79% accuracy of interpreting deep soil locations and 84% accuracy in distinguishing BOSS. We explored four probabilistic classifications of BOSS using multiple Lidar-derived topographic metrics as predictive variables. All four methods identified similar predictors for BOSS, including slope and topographic position indices with a 15, 100 and 200 m circular analysis window, respectively. Although all classifiers yielded similar results with little difference in interpretation, a generalized additive model had slightly higher accuracy predicting BOSS presence, yielding 85% overall accuracy using independent validation data across the primary study area, and 86% overall accuracy in a second validation area.
Watershed studies at the Hubbard Brook Experimental Forest: Building on a long legacy of research with new approaches and sources of data
Campbell, John L.; Rustad, Lindsey E.; Bailey, Scott W.; Bernhardt, Emily S.; Driscoll, Charles T.; Green, Mark B.; Groffman, Peter M.; Lovett, Gary M.; McDowell, William H.; McGuire, Kevin J.; Rosi, Emma J. (2021-01)
The Hubbard Brook Experimental Forest (HBEF) was established in 1955 by the U.S. Department of Agriculture, Forest Service out of concerns about the effects of logging increasing flooding and erosion. To address this issue, within the HBEF hydrological and micrometeorological monitoring was initiated in small watersheds designated for harvesting experiments. The Hubbard Brook Ecosystem Study (HBES) originated in 1963, with the idea of using the small watershed approach to study element fluxes and cycling and the response of forest ecosystems to disturbances, such as forest management practices and air pollution. Early evidence of acid rain was documented at the HBEF and research by scientists at the site helped shape acid rain mitigation policies. New lines of investigation at the HBEF have built on the long legacy of watershed research resulting in a shift from comparing inputs and outputs and quantifying pools and fluxes to a more mechanistic understanding of ecosystem processes within watersheds. For example, hydropedological studies have shed light on linkages between hydrologic flow paths and soil development that provide valuable perspective for managing forests and understanding stream water quality. New high frequency in situ stream chemistry sensors are providing insights about extreme events and diurnal patterns that were indiscernible with traditional weekly sampling. Additionally, tools are being developed for visual and auditory data exploration and discovery by a broad audience. Given the unprecedented environmental change that is occurring, data from the small watersheds at the HBEF are more relevant now than ever and will continue to serve as a basis for sound environmental decision-making.
Effects of intensive management practices on 10-year Douglas-fir growth, soil nutrient pools, and vegetation communities in the Pacific Northwest, USA
Slesak, Robert A.; Harrington, Timothy B.; Peter, David H.; DeBruler, Daniel G.; Schoenholtz, Stephen H.; Strahm, Brian D. (2016-04-01)
Intensive management practices are commonly used to increase fiber production from forests, but potential tradeoffs with maintenance of long-term productivity and early successional biodiversity have yet to be quantified. We assessed soil and vegetation responses in replicated manipulations of logging debris (LD; either retained or removed) and competing vegetation control (VC; either initial or sustained annually for 5 years) for 10 years at two Douglas-fir sites that contrasted strongly in availability of soil nutrients and water. We evaluated (1) survival and growth of Douglas-fir to determine short-term effectiveness for fiber production, (2) change in soil C and nutrient pools as an indicator of longer-term effects of treatments on soil quality and ecosystem production, and (3) vegetation composition and cover for treatment effects on early successional biodiversity. Annual VC caused large increases in Douglas-fir growth at both sites, but increased survival only at the lower-productivity site. In most instances and regardless of site or treatment, soil C and nutrient pools increased following harvesting, but the increases were generally larger with lower intensity practices (LD retained and initial VC). Effects of LD were small and inconsistent at the higher productivity site, but LD retained increased Douglas-fir survival and growth and soil nutrient pools at the lower productivity site. Species diversity was reduced at both sites with annual VC because of increased Douglas-fir cover, but the magnitude was greater and the timing was earlier at the higher quality site where plant communities in all treatments had converged by year 10. Annual VC can be used to increase growth of planted Douglas-fir while maintaining soil nutrient pools for sustained ecosystem productivity, but a concurrent decrease in early successional diversity will occur with impacts increasing with site quality. Logging debris retention can have positive benefits to Douglas fir growth and soil nutrient pools, particularly at lower quality sites. Our results demonstrate a need for careful consideration of site quality to ensure that objectives are realized with regards to fiber production and maintenance of soil productivity and biodiversity with intensive forest management.
Assessing Strontium and Vulnerability to Strontium in Private Drinking Water Systems in Virginia
Scott, Veronica; Juran, Luke; Ling, Erin; Benham, Brian L.; Spiller, Asa (MDPI, 2020-04-08)
A total of 1.7 million Virginians rely on private drinking water (PDW) systems and 1.3 million of those people do not know their water quality. Because most Virginians who use PDW do not know the quality of that water and since strontium poses a public health risk, this study investigates sources of strontium in PDW in Virginia and identifies the areas and populations most vulnerable. Physical factors such as rock type, rock age, and fertilizer use have been linked to elevated strontium concentrations in drinking water. Social factors such as poverty, poor diet, and adolescence also increase social vulnerability to health impacts of strontium. Using water quality data from the Virginia Household Water Quality Program (VAHWQP) and statistical and spatial analyses, physical vulnerability was found to be highest in the Ridge and Valley province of Virginia where agricultural land use and geologic formations with high strontium concentrations (e.g., limestone, dolomite, sandstone, shale) are the dominant aquifer rocks. In terms of social vulnerability, households with high levels of strontium are more likely than the average VAHWQP participant to live in a food desert. This study provides information to help 1.7 million residents of Virginia, as well as populations in neighboring states, understand their risk of exposure to strontium in PDW.
Young runoff fractions control streamwater age and solute concentration dynamics
Benettin, Paolo; Bailey, Scott W.; Rinaldo, Andrea; Likens, Gene E.; McGuire, Kevin J.; Botter, Gianluca (2017-07-31)
We introduce a new representation of coupled solute and water age dynamics at the catchment scale, which shows how the contributions of young runoff waters can be directly referenced to observed water quality patterns. The methodology stems from recent trends in hydrologic transport that acknowledge the dynamic nature of streamflow age and explores the use of water age fractions as an alternative to the mean age. The approach uses a travel time-based transport model to compute the fractions of streamflow that are younger than some thresholds (e.g., younger than a few weeks) and compares them to observed solute concentration patterns. The method is here validated with data from the Hubbard Brook Experimental Forest during spring 2008, where we show that the presence of water younger than roughly 2 weeks, tracked using a hydrologic transport model and deuterium measurements, mimics the variation in dissolved silicon concentrations. Our approach suggests that an age-discharge relationship can be coupled to classic concentration-discharge relationship, to identify the links between transport timescales and solute concentration. Our results highlight that the younger streamflow components can be crucial for determining water quality variations and for characterizing the dominant hydrologic transport dynamics.
Lateral water flux in the unsaturated zone: A mechanism for the formation of spatial soil heterogeneity in a headwater catchment
Gannon, John P.; McGuire, Kevin J.; Bailey, Scott W.; Bourgault, Rebecca R.; Ross, Donald S. (2017-09-30)
Measurements of soil water potential and water table fluctuations suggest that morphologically distinct soils in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire formed as a result of variations in saturated and unsaturated hydrologic fluxes in the mineral soil. Previous work showed that each group of these soils had distinct water table fluctuations in response to precipitation; however, observed variations in soil morphology also occurred above the maximum height of observed saturation. Variations in unsaturated fluxes have been hypothesized to explain differences in soil horizon thickness and presence/absence of specific horizons but have not been explicitly investigated. We examined tensiometer and shallow groundwater well records to identify differences in unsaturated water fluxes among podzols that show distinct morphological and chemical differences. The lack of vertical hydraulic gradients at the study sites suggests that lateral unsaturated flow occurs in several of the soil units. We propose that the variations in soil horizon thickness and presence/absence observed at the site are due in part to slope-parallel water flux in the unsaturated portion of the solum. In addition, unsaturated flow may be involved in the translocation of spodic material that primes those areas to contribute water with distinct chemistry to the stream network and represents a potential source/sink of organometallic compounds in the landscape.
Redistribution of soil metals and organic carbon via lateral flowpaths at the catchment scale in a glaciated upland setting
Bourgault, Rebecca R.; Ross, Donald S.; Bailey, Scott W.; Bullen, Thomas D.; McGuire, Kevin J.; Gannon, John P. (2017-12-01)
Emerging evidence shows that interactions between soils and subsurface flow paths contribute to spatial variations in stream water chemistry in headwater catchments. However, few have yet attempted to quantify chemical variations in soils at catchment and hillslope scales. Watershed 3 (WS3) at Hubbard Brook Experimental Forest, New Hampshire, USA, was studied in order to better understand pedogenesis and its relationship to subsurface water dynamics. In WS3, 99 soil profiles were described, sampled by horizon, and assigned to a hydropedologic unit (HPU), a functional classification previously developed using landscape and morphological metrics which corresponded with distinct water table regimes. Soil samples were extracted with 1) citrate-dithionite (d) and analyzed for Fe-d and Mn-d; and 2) acid ammonium oxalate (o) and analyzed for Al-o, Fe-o and the rare earth elements La-o, Ce-o, and Pr-o. Total organic C was also measured. These elements were redistributed via vertical and lateral podzolization. Typical (horizontally layered) podzols developed in the majority of the catchment due to predominantly vertical, unsaturated flow. However, lateral flow produced four other podzol types with distinct chemistry; thicker spodic horizons of laterally accumulating soils generally reflected larger pools of trace metals and subsoil organic C. The spatial distribution of positive cerium-anomalies (Ce/Ce*) in soil profiles proved to be a consistent hydropedologic indicator of lateral flow and seasonally high water table in three hillslopes. Despite occasional high water table in some of the HPUs, they were not hydric soils and were distinct from wetter podzols of coastal plains due to their high Fe content. This study suggests that vertical and lateral spatial variation in soil chemical composition, including the complexity of Ce distribution, as it relates to subsurface water dynamics should be considered when studying or predicting catchment scale functions such as stream solute export and biogeochemical processes.
Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US
Lin, Jiajia; Compton, Jana E.; Leibowitz, Scott G.; Mueller-Warrant, George; Matthews, William; Schoenholtz, Stephen H.; Evans, Daniel M.; Coulombe, Rob A. (2018-12-19)
We constructed a seasonal nitrogen (N) budget for the year 2008 in the Calapooia River Watershed (CRW), an agriculturally dominated tributary of the Willamette River (Oregon, U.S.) under Mediterranean climate. Synthetic fertilizer application to agricultural land (dominated by grass seed crops) was the source of 90% of total N input to the CRW. Over 70% of the stream N export occurred during the wet winter, the primary time of fertilization and precipitation, and the lowest export occurred in the dry summer. Averaging across all 58 tributary subwatersheds, 19% of annual N inputs were exported by streams, and 41% by crop harvest. Regression analysis of seasonal stream export showed that winter fertilization was associated with 60% of the spatial variation in winter stream export, and this fertilizer continued to affect N export in later seasons. Annual N inputs were highly correlated with crop harvest N (r(2)=0.98), however, seasonal dynamics in N inputs and losses produced relatively low overall nitrogen use efficiency (41%), suggesting that hydrologic factors may constrain improvements in nutrient management. The peak stream N export during fall and early winter creates challenges to reducing N losses to groundwater and surface waters. Construction of a seasonal N budget illustrated that the period of greatest N loss is disconnected from the period of greatest crop N uptake. Management practices that serve to reduce the N remaining in the system at the end of the growing season and prior to the fall and winter rains should be explored to reduce stream N export.
Soil phosphorus fractions vary with harvest intensity and vegetation control at two contrasting Douglas-fir sites in the Pacific northwest
DeBruler, Daniel G.; Schoenholtz, Stephen H.; Slesak, Robert A.; Strahm, Brian D.; Harrington, Timothy B. (2019-09-15)
Effects of intensive forest management on soil phosphorus (P) are unclear and may impact long-term site productivity. We assessed changes in P availability over 10 years associated with harvest intensity (bole-only vs. whole-tree harvest) and vegetation control treatments (initial vegetation control (IVC) vs. five years of annual vegetation control (AVC)) using a P fractionation procedure. Fractions were characterized at 0-15, 15-30, and 30-60 cm soil depths in two coast Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) plantations with strongly contrasting soil properties near Matlock, WA (young soils formed in glacial outwash) and Molalla, OR (relatively old soils formed in igneous residuum and exhibiting andic properties). Al and Fe concentrations associated with short-range order minerals were greater at Molalla than Matlock and generally decreased with depth at both sites. We observed decreases in most total-P and P-fraction concentrations across the three soil depths at the Molalla site. Effects were less pronounced and generally inconsistent at the Matlock site. Decreases in total P and P fraction concentrations were greatest in the AVC treatments at Matlock, but opposite trends were observed at Molalla where decreases were greatest with IVC. There was no difference between harvest treatments on the change in P fractions in most instances, with the exception of the 30-60 cm depth at Matlock where concentrations of some P fractions were maintained or increased with bole-only harvesting. Ten-year responses indicate harvest intensity has limited effects on long-term productivity associated with soil P because of the large size of the soil P pools and the relatively small changes in soil P that occurred with treatment. Decreases in P concentrations with AVC at Matlock and IVC at Molalla were larger than the other treatments and highlight the important role of vegetation in P dynamics following harvesting at these sites.

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