Scholarly Works, Virginia Water Resources Research Center

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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.
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.
Comparing Sediment Trap Data With Erosion Models for Evaluation of Forest Haul Road Stream Crossing Approaches
Lang, A. J.; Aust, W. Michael; Bolding, M. Chad; McGuire, Kevin J.; Schilling, Erik B. (American Society of Agricultural & Biological Engineers, 2017-01-01)
Soil erosion and sediment delivery models have been developed to estimate the inherent complexities of soil erosion, but most models are not specifically modified for forest operation applications. Three erosion models, the Universal Soil Loss Equation for forestry (USLE-Forest), Revised Universal Soil Loss Equation Version 2 (RUSLE2), and Water Erosion Prediction Project (WEPP), were compared to one year of trapped sediment data for 37 forest haul road stream crossings. We assessed model performance from five variations of the three erosion models: USLE-Roadway, USLE-Soil Survey, RUSLE2, WEPP-Default, and WEPP-Modified. Each road approach was categorized into one of four levels of erosion (very low, low, moderate, and high) based on trapped erosion rate data and erosion rates reported in recent peerreviewed literature. Model performance metrics included: (1) summary statistics and nonparametric analysis, (2) linear relationships, (3) percent agreement within erosion categories and tolerable error ranges, and (4) contingency table metrics. Sediment trap data varied from negligible (<0.1) to hundreds of Mg ha-1 year-1. The soil erosion models evaluated could estimate erosion within 5 Mg ha-1year-1 for most approaches having erosion rates less than 11.2 Mg ha-1 year-1, while models estimates varied widely for approaches that eroded at rates above 11.2 Mg ha-1year-1. Kruskal-Wallis nonparametric analyses revealed that only WEPP-Modified estimates were not significantly different from trapped sediment data (p ≥ 0.107). While WEPP-Modified ranked best for most model performance metrics, the time, effort, modeling expertise, and uncertainty associated with model results may discourage the use of WEPP as a forest management tool. WEPP is better suited for researchers and government agencies that have the capability to measure extensive parameter data. Additional sensitivity analysis is needed to expand default parameters for forest roads within the WEPP and USLE models.
Cross-regional prediction of long-term trajectory of stream water DOC response to climate change
Laudon, Hjalmar; Buttle, J.; Carey, S. K.; McDonnell, J.; McGuire, Kevin J.; Seibert, J.; Shanley, James B.; Soulsby, C.; Tetzlaff, D. (American Geophysical Union, 2012-09-22)
There is no scientific consensus about how dissolved organic carbon (DOC) in surface waters is regulated. Here we combine recent literature data from 49 catchments with detailed stream and catchment process information from nine well established research catchments at mid- to high latitudes to examine the question of how climate controls stream water DOC. We show for the first time thatmean annual temperature (MAT) in the range from -3 to +10 degrees C has a strong control over the regional stream water DOC concentration in catchments, with highest concentrations in areas ranging between 0 and +3 degrees C MAT. Although relatively large deviations from thismodel occur for individual streams, catchment topography appears to explain much of this divergence. These findings suggest that the long-term trajectory of stream water DOC response to climate change may be more predictable than previously thought.
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.
The evolution of root-zone moisture capacities after deforestation: a step towards hydrological predictions under change?
Nijzink, R.; Hutton, C.; Pechlivanidis, I.; Capell, R.; Arheimer, B.; Freer, J.; Han, D.; Wagener, T.; McGuire, Kevin J.; Savenije, H.; Hrachowitz, M. (2016)
The core component of many hydrological systems, the moisture storage capacity available to vegetation, is impossible to observe directly at the catchment scale and is typically treated as a calibration parameter or obtained from a priori available soil characteristics combined with estimates of rooting depth. Often this parameter is considered to remain constant in time. Using long-term data (30–40 years) from three experimental catchments that underwent significant land cover change, we tested the hypotheses that: (1) the root-zone storage capacity significantly changes after deforestation, (2) changes in the root-zone storage capacity can to a large extent explain post-treatment changes to the hydrological regimes and that (3) a time-dynamic formulation of the root-zone storage can improve the performance of a hydrological model. A recently introduced method to estimate catchment-scale root-zone storage capacities based on climate data (i.e. observed rainfall and an estimate of transpiration) was used to reproduce the temporal evolution of root-zone storage capacity under change. Briefly, the maximum deficit that arises from the difference between cumulative daily precipitation and transpiration can be considered as a proxy for root-zone storage capacity. This value was compared to the value obtained from four different conceptual hydrological models that were calibrated for consecutive 2-year windows. It was found that water-balance-derived root-zone storage capacities were similar to the values obtained from calibration of the hydrological models. A sharp decline in root-zone storage capacity was observed after deforestation, followed by a gradual recovery, for two of the three catchments. Trend analysis suggested hydrological recovery periods between 5 and 13 years after deforestation. In a proof-of-concept analysis, one of the hydrological models was adapted to allow dynamically changing root-zone storage capacities, following the observed changes due to deforestation. Although the overall performance of the modified model did not considerably change, in 51% of all the evaluated hydrological signatures, considering all three catchments, improvements were observed when adding a time-variant representation of the root-zone storage to the model. In summary, it is shown that root-zone moisture storage capacities can be highly affected by deforestation and climatic influences and that a simple method exclusively based on climate data can not only provide robust, catchment-scale estimates of this critical parameter, but also reflect its timedynamic behaviour after deforestation.
Flushing of distal hillslopes as an alternative source of stream dissolved organic carbon in a headwater catchment
Gannon, John P.; Bailey, Scott W.; McGuire, Kevin J.; Shanley, James B. (American Geophysical Union, 2015-10-01)
We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and stream water samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high-frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near-stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near-stream groundwater were low (mean52.4 mg/L, standard error50.6 mg/L), less than hillslope groundwater farther from the channel (mean55.7 mg/L, standard error50.4 mg/L). Furthermore, water tables in near-stream soils did not rise into the carbon-rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean514.5 mg/L, standard error50.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean520.0 mg/L, standard error54.6 mg/L) measured in the catchment. Correspondingly, stream water in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter-covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads.
Headwater stream length dynamics across four physiographic provinces of the Appalachian Highlands
Jensen, Carrie K.; McGuire, Kevin J.; Prince, Philip S. (Wiley, 2017-09-15)
Understanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in three forested catchments (<75 ha) in each of four physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length seven times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially-derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.
Hydrologic Analysis of Discharge Sustainability from an Abandoned Underground Coal Mine
Burbey, Thomas J.; Younos, Tamim M.; Anderson, Eric T. (American Water Resources Association, 2000-10)
Discharge from flooded abandoned subsurface coal mines is considered a potential source for water sources are not available. The objective of this study was to develop procedures for determining sustainability of mine-water discharge using rain fall and discharge data for a case study site.
Hydrological connectivity of hillslopes and streams: Characteristic time scales and nonlinearities
McGuire, Kevin J.; McDonnell, J. J. (American Geophysical Union, 2010-10-30)
Subsurface flow from hillslopes is widely recognized as an important contributor to streamflow generation; however, processes that control how and when hillslopes connect to streams remain unclear. We investigated stream and hillslope runoff dynamics through a wet‐up period in watershed 10 of the H. J. Andrews Experimental Forest in the western Cascades of Oregon where the riparian zone has been removed by debris flows. We examined the controls on hillslope‐stream connectivity on the basis of observations of hydrometric, stable isotope, and applied tracer responses and computed transit times for multiple runoff components for a series of storms during the wet‐up phase of the 2002–2003 winter rainy season. Hillslope discharge was distinctly threshold‐like with a near linear response and average quick flow ratio of 0.58 when antecedent rainfall was greater than 20 mm. Hillslope and stream stormflow varied temporally and showed strong hysteretic relationships. Event water mean transit times (8–34 h) and rapid breakthrough from applied hillslope tracer additions demonstrated that subsurface contributing areas extend far upslope during events. Despite rapid hillslope transport processes during events, soil water and runoff mean transit times during nonstorm conditions were greater than the time scale of storm events. Soil water mean transit times ranged between 10 and 25 days. Hillslope seepage and catchment base flow mean transit times were between 1 and 2 years. We describe a conceptual model that captures variable physical flow pathways, their synchronicity, threshold activation, hysteresis, and transit times through changing antecedent wetness conditions that illustrate the different stages of hillslope and stream connectivity.
Integrating Climate Forecasts with the Soil and Water Assessment Tool (SWAT) for High-Resolution Hydrologic Simulations and Forecasts in the Southeastern U.S.
Sehgal, Vinit; Sridhar, Venkataramana; Juran, Luke; Ogejo, Jactone Arogo (MDPI, 2018-08-29)
This study provides high-resolution modeling of daily water budget components at Hydrologic Unit Code (HUC)-12 resolution for 50 watersheds of the South Atlantic Gulf (SAG) region in the southeastern U.S. (SEUS) by implementing the Soil and Water Assessment Tool (SWAT) model in the form of a near real-time, semi-automated framework. A near real-time hydrologic simulation framework is implemented with a lead time of nine months (March–December 2017) by integrating the calibrated SWAT model with National Centers for Environmental Prediction coupled forecast system model version 2 (CFSv2) weather data to forecast daily water balance components. The modeling exercise is conducted as a precursor for various future hydrologic studies (retrospective or forecasting) for the region by providing a calibrated hydrological dataset at high spatial (HUC-12) and temporal (1-day) resolution. The models are calibrated (January 2003–December 2010) and validated (January 2011–December 2013) for each watershed using the observed streamflow data from 50 United States Geological Survey (USGS) gauging stations. The water balance analysis for the region shows that the implemented models satisfactorily represent the hydrology of the region across different sub-regions (Appalachian highlands, plains, and coastal wetlands) and seasons. While CFSv2-driven SWAT models are able to provide reasonable performance in near real-time and can be used for decision making in the region, caution is advised for using model outputs as the streamflow forecasts display significant deviation from observed streamflow for all watersheds for lead times greater than a month.
Integrating Service-Learning into Watershed Management Programs: Opportunities and Challenges
Younos, Tamim M.; Léon, Raymond de; Lewicki, Christine (American Water Resources Association, 2003-02)
The objective of this article is to open a dialogue on integrating service-learning into community based watershed management programs and to discuss opportunities and challenges that a service-learning program presents to universities and communities.
Isotopic signals of summer denitrification in a northern hardwood forested catchment
Wexler, S. K.; Goodale, Christine L.; McGuire, Kevin J.; Bailey, Scott W.; Groffman, Peter M. (National Academy of Sciences, 2014-11-18)
Despite decades of measurements, the nitrogen balance of temperate forest catchments remains poorly understood. Atmospheric nitrogen deposition often greatly exceeds streamwater nitrogen losses; the fate of the remaining nitrogen is highly uncertain. Gaseous losses of nitrogen to denitrification are especially poorly documented and are often ignored. Here, we provide isotopic evidence (δ15NNO3 and δ18ONO3) from shallow groundwater at the Hubbard Brook Experimental Forest indicating extensive denitrification during midsummer, when transient, perched patches of saturation developed in hillslopes, with poor hydrological connectivity to the stream, while streamwater showed no isotopic evidence of denitrification. During small rain events, precipitation directly contributed up to 34% of streamwater nitrate, which was otherwise produced by nitrification. Together, these measurements reveal the importance of denitrification in hydrologically disconnected patches of shallow groundwater during midsummer as largely overlooked control points for nitrogen loss from temperate forest catchments.
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.
Linking water age and solute dynamics in streamflow at the Hubbard Brook Experimental Forest, NH, USA
Benettin, Paolo; Bailey, Scott W.; Campbell, John L.; Green, M. B.; Rinaldo, Andrea; Likens, Gene E.; McGuire, Kevin J.; Botter, Gianluca (American Geophysical Union, 2015-11-01)
We combine experimental and modeling results from a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, to explore the link between stream solute dynamics and water age. A theoretical framework based on water age dynamics, which represents a general basis for characterizing solute transport at the catchment scale, is here applied to conservative and weatheringderived solutes. Based on the available information about the hydrology of the site, an integrated transport model was developed and used to compute hydrochemical fluxes. The model was designed to reproduce the deuterium content of streamflow and allowed for the estimate of catchment water storage and dynamic travel time distributions (TTDs). The innovative contribution of this paper is the simulation of dissolved silicon and sodium concentration in streamflow, achieved by implementing first-order chemical kinetics based explicitly on dynamic TTD, thus upscaling local geochemical processes to catchment scale. Our results highlight the key role of water stored within the subsoil glacial material in both the short-term and long-term solute circulation. The travel time analysis provided an estimate of streamflow age distributions and their evolution in time related to catchment wetness conditions. The use of age information to reproduce a 14 year data set of silicon and sodium stream concentration shows that, at catchment scales, the dynamics of such geogenic solutes are mostly controlled by hydrologic drivers, which determine the contact times between the water and mineral interfaces. Justifications and limitations toward a general theory of reactive solute circulation at catchment scales are discussed.
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.
Mineral weathering and podzolization control acid neutralization and streamwater chemistry gradients in upland glaciated catchments, Northeastern United States
Bailey, Scott W.; McGuire, Kevin J.; Ross, Donald S.; Green, Mark B.; Fraser, Olivia L. (2019-02-26)
Headwater streams in the White Mountains, NH, United States have been shown to have downstream gradients of increasing pH and concentrations of base cations coupled with decreasing concentrations of aluminum. A two-stage acid neutralization model involving shallow soil exchange processes in headwaters coupled with deeper mineral weathering downstream had been proposed to explain these gradients. We conducted synoptic sampling of three headwater catchments in this region that showed variations in this longitudinal pattern, ranging from streams that remain acidic throughout their length to streams with circumneutral pH beginning at their source. To explain these differences, we mapped soils using a hydropedologic approach that emphasizes the influence of groundwater saturation frequency and water table regime on soil formation processes. Stream segments with lower pH and base cation concentrations, coupled with higher concentrations of dissolved organic carbon (DOC), aluminum, and in one case iron, were in subcatchments mapped with shallow to bedrock soils where eluvial soil forming processes dominated. In contrast, stream segments with higher pH and base cation concentrations coupled with low concentrations of DOC and aluminum were associated with subcatchments with deeper soils where illuvial processes were more dominant. Concentrations of sodium and silicon were relatively uniform across these gradients. Coupled with the higher concentrations of dissolved aluminum and small pools of exchangeable aluminum in the areas of bedrock outcrops and shallow soils, these data suggest that primary mineral dissolution is an important process influencing upper stream reaches, not just along longer, deeper flowpaths in downslope areas. In contrast, some stream reaches with obvious groundwater springs show a more abrupt transition in pH and base cation concentrations higher along the stream, suggesting that neutralization along deeper flowpaths may play a role in upslope areas as well. These data suggest a new three stage model of stream chemistry evolution. First, organic acids are introduced by frequent flushing of organic soils on shallow bedrock along ridge areas. Second, upper acidic reaches are controlled by mineral dissolution coupled with eluvial soil development. Third, downstream reaches are influenced by illuviation as organometallic complexes precipitate in spodic soil horizons, removing organic acids, and acid-mobile metals from drainage waters. This new model highlights differences in critical zone evolution along hillslopes in glaciated catchments with implications for understanding gradients in water quality, soil fertility, and response and recovery from disturbances.
Modeling wet headwater stream networks across multiple flow conditions in the Appalachian Highlands
Jensen, Carrie K.; McGuire, Kevin J.; Shao, Yang; Dolloff, C. Andrew (Wiley, 2018-05-25)
Despite the advancement of remote sensing and geospatial technology in recent decades, maps of headwater streams continue to have high uncertainty and fail to adequately characterize temporary streams that expand and contract in the wet length. However, watershed management and policy increasingly require information regarding the spatial and temporal variability of flow along streams. We used extensive field data on wet stream length at different flows to create logistic regression models of stream network dynamics for four physiographic provinces of the Appalachian Highlands: New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. The topographic wetness index (TWI) was the most important parameter in all four models, and the topographic position index (TPI) further improved model performance in the Appalachian Plateau, Valley and Ridge, and Blue Ridge. We included stream runoff at the catchment outlet as a model predictor to represent the wetness state of the catchment, but adjustment of the probability threshold defining wet stream presence/absence to high values for low flows was the primary mechanism for approximating network extent at multiple flow conditions. Classification accuracy was high overall (> 0.90), and McFadden's pseudo R2 values ranged from 0.69 for the New England model to 0.79 in the Appalachian Plateau. More notable errors included an overestimation of wet stream length in wide valleys and inaccurate reach locations amid boulder deposits and along headwardly eroding tributaries. Logistic regression was generally successful for modeling headwater streams at high and low flows with only a few simple terrain metrics. Modification and application of this modeling approach to other regions or larger areas would be relatively easy and provide a more accurate portrayal of temporary headwaters than existing datasets.
Network analysis reveals multiscale controls on streamwater chemistry
McGuire, Kevin J.; Torgersen, C. E.; Likens, Gene E.; Buso, D. C.; Lowe, W. H.; Bailey, Scott W. (National Academy of Sciences, 2014-05-13)
By coupling synoptic data from a basin-wide assessment of streamwater chemistry with network-based geostatistical analysis, we show that spatial processes differentially affect biogeochemical condition and pattern across a headwater stream network. We analyzed a high-resolution dataset consisting of 664 water samples collected every 100 m throughout 32 tributaries in an entire fifth-order stream network. These samples were analyzed for an exhaustive suite of chemical constituents. The fine grain and broad extent of this study design allowed us to quantify spatial patterns over a range of scales by using empirical semivariograms that explicitly incorporated network topology. Here, we show that spatial structure, as determined by the characteristic shape of the semivariograms, differed both among chemical constituents and by spatial relationship (flow-connected, flowunconnected, or Euclidean). Spatial structure was apparent at either a single scale or at multiple nested scales, suggesting separate processes operating simultaneously within the stream network and surrounding terrestrial landscape. Expected patterns of spatial dependence for flow-connected relationships (e.g., increasing homogeneity with downstream distance) occurred for some chemical constituents (e.g., dissolved organic carbon, sulfate, and aluminum) but not for others (e.g., nitrate, sodium). By comparing semivariograms for the different chemical constituents and spatial relationships, we were able to separate effects on streamwater chemistry of (i ) fine-scale versus broad-scale processes and (ii ) in-stream processes versus landscape controls. These findings provide insight on the hierarchical scaling of local, longitudinal, and landscape processes that drive biogeochemical patterns in stream networks.
Network analysis reveals multiscale controls on streamwater chemistry
McGuire, Kevin J.; Torgersen, C. E.; Likens, Gene E.; Buso, D. C.; Lowe, W. H.; Bailey, Scott W. (National Academy of Sciences, 2014-05-13)
By coupling synoptic data from a basin-wide assessment of streamwater chemistry with network-based geostatistical analysis, we show that spatial processes differentially affect biogeochemical condition and pattern across a headwater stream network. We analyzed a high-resolution dataset consisting of 664 water samples collected every 100 m throughout 32 tributaries in an entire fifth-order stream network. These samples were analyzed for an exhaustive suite of chemical constituents. The fine grain and broad extent of this study design allowed us to quantify spatial patterns over a range of scales by using empirical semivariograms that explicitly incorporated network topology. Here, we show that spatial structure, as determined by the characteristic shape of the semivariograms, differed both among chemical constituents and by spatial relationship (flow-connected, flowunconnected, or Euclidean). Spatial structure was apparent at either a single scale or at multiple nested scales, suggesting separate processes operating simultaneously within the stream network and surrounding terrestrial landscape. Expected patterns of spatial dependence for flow-connected relationships (e.g., increasing homogeneity with downstream distance) occurred for some chemical constituents (e.g., dissolved organic carbon, sulfate, and aluminum) but not for others (e.g., nitrate, sodium). By comparing semivariograms for the different chemical constituents and spatial relationships, we were able to separate effects on streamwater chemistry of (i ) fine-scale versus broad-scale processes and (ii ) in-stream processes versus landscape controls. These findings provide insight on the hierarchical scaling of local, longitudinal, and landscape processes that drive biogeochemical patterns in stream networks.

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