Browsing by Author "McGuire, Kevin J."
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- Analysis of Human Influence on Drought Conditions in the Upper Colorado River Basin (Texas)Whittemore, Aaron Maitland (Virginia Tech, 2020-06-19)Globally, it is expected that arid and semi-arid areas will face increasing frequency of drought through the 21st century. Drought is normally attributed to climatic factors. However, humans constantly alter hydrologic systems through manipulating and consuming water, which can also cause drought. However, human influence on drought, outside of influences on warming-driven climate change, is rarely studied. Here, the upper Colorado River Basin (Texas) is studied to assess the human influence on drought conditions in a semi-arid basin. An observation-modeling framework is used to simulate naturalized runoff conditions which are compared to observed data in an undisturbed (little human influence) and disturbed (much human influence) period to elucidate human influences on drought. Further, public water storage and supply data are incorporated to analyze how human water management may be specifically affecting downstream hydrologic drought in the upper Colorado River Basin. Results show that according to observed data, drought occurred more often, persisted longer on average, and had a higher maximum duration during the disturbed period. Naturalized model output did not predict such increases, indicating that human influence is responsible. Water deliveries in the study area were found to significantly affect downstream flow and are connected to instances of human-influenced drought. Results suggest that in order to reduce downstream drought conditions, deliveries will likely have to be reduced and that reducing deliveries during periods of low rainfall, or during months in which deliveries constitute a large portion of human influenced drought severity could be especially helpful in alleviating downstream drought.
- Assessing the Performance of HSPF When Using the High Water Table Subroutine to Simulate Hydrology in a Low-Gradient WatershedForrester, Michael Scott (Virginia Tech, 2012-04-17)Modeling ground-water hydrology is critical in low-gradient, high water table watersheds where ground-water is the dominant contribution to streamflow. The Hydrological Simulation Program-FORTRAN (HSPF) model has two different subroutines available to simulate ground water, the traditional ground-water (TGW) subroutine and the high water table (HWT) subroutine. The HWT subroutine has more parameters and requires more data but was created to enhance model performance in low-gradient, high water table watershed applications. The objective of this study was to compare the performance and uncertainty of the TGW and HWT subroutines when applying HSPF to a low-gradient watershed in the Coastal Plain of northeast North Carolina. One hundred thousand Monte Carlo simulations were performed to generate data needed for model performance comparison. The HWT model generated considerably higher Nash-Sutcliffe efficiency (NSE) values while performing slightly worse when simulating the 50% lowest and 10% highest flows. Model uncertainty was assessed using the Average Relative Interval Length (ARIL) metric. The HWT model operated with more average uncertainty throughout all flow regimes. Based on the results, the HWT subroutine is preferable when applying HSPF to a low-gradient watershed and the accuracy of simulated stream discharge is important. In situations where a balance between performance and uncertainty is called for, the choice of which subroutine to employ is less clear cut.
- Basin-scale spatiotemporal analysis of hydrologic floodplain connectivityMcCann, David Michael (Virginia Tech, 2014-05-30)Floodplain inundation often provides water quality benefits by trapping sediment and biogeochemically transforming other pollutants. Hydrologic floodplain connectivity is a measure of water exchanges and interactions between the main channel and the floodplain via surface (inundation) and subsurface (groundwater) connections. Using an automated model combining GIS and numerical analysis software, this study examined floodplain inundation patterns and measured floodplain connectivity for the Mahantango Creek watershed (Pennsylvania, USA). Connectivity was quantified by developing a metric that included inundation area and duration. Long-term hydrographs at each reach in the watershed were developed via QPPQ (Flow-Percentile-Percentile-Flow) methodology using regional regression analysis to calculate the ungauged flow duration curves (FDC). Inundation area (normalized to stream length) was found to increase with drainage area, suggesting larger streams have more area available for biogeochemical activity. Annual connectivity increased with drainage area, suggesting larger streams, having higher connectivity, should be the focus of individual reach restoration projects due to higher potential for water quality benefits. Across the watershed as a whole, however, the total annual connectivity across first order streams was greater than higher order streams, suggesting the collection of small streams in a watershed may have a stronger effect on outlet water quality. Connectivity was consistently higher during the non-growing season, which was attributed to higher flows. Despite higher connectivity during the non-growing season, increased floodplain biological activity may be negated by low temperatures, reducing microbial activity. Correlations between land use and connectivity were also found, emphasizing dynamics between flow, channel morphology, and floodplain inundation.
- Benthic macroinvertebrate community structure responses to multiple stressors in mining-influenced streams of central Appalachia USADrover, Damion R. (Virginia Tech, 2018-06-25)Headwaters are crucial linkages between upland ecosystems and navigable waterways, serving as important sources of water, sediment, energy, nutrients and invertebrate prey for downstream ecosystems. Surface coal mining in central Appalachia impacts headwaters by burying streams and introducing pollutants to remaining streams including excessive sediments, trace elements, and salinity. Benthic macroinvertebrates are widely used as indicators of biological conditions of streams and are frequently sampled using semi-quantitative methods that preclude calculations of areal densities. Studies of central Appalachian mining impacts in non-acidic streams often focus on biotic effects of salinity, but other types of pollution and habitat alteration can potentially affect benthic macroinvertebrate community (BMC) structure and perhaps related functions of headwater streams. Objectives were: 1) use quantitative sampling and enumeration to determine how density, richness, and composition of BMCs in non-acidic central Appalachian headwaters respond to elevated salinity caused by coal surface mining, and 2) determine if BMC structural differences among study streams may be attributed to habitat and water-quality effects in addition to elevated salinity. I analyzed BMC structure, specific conductance (SC, surrogate measure of salinity), and habitat-feature data collected from 15 streams, each visited multiple times during 2013-2014. BMC structure changed across seasonal samples. Total benthic macroinvertebrate densities did not appear to be impacted by SC during any months, but reduced densities of SC-sensitive taxa were offset by increased densities of SC-tolerant taxa in high-SC streams. Total richness also declined with increasing SC, whereas BMCs in high-SC streams were simplified and dominated by a few SC-tolerant taxa. Taxonomic replacement was detected in high-SC streams for groups of benthic macroinvertebrates that did not exhibit density or richness response, showing that taxonomic replacement could be a valuable tool for detecting BMC changes that are not evident from analyses using conventional metrics. Specific conductance, water-column selenium concentration, large-cobble-to-fines ratio of stream substrate, and relative bed stability were associated with changes in BMC structure. These results suggest multiple stressors are influencing BMCs in mining-influenced Appalachian streams. These findings can inform future management of headwater streams influenced by mining in central Appalachia.
- Catchment Structure Regulates Hydrodynamic Drivers of Chemical Weathering in Shallow Forest SoilsPennino, Amanda (Virginia Tech, 2023-06-12)Determining where, when, and how subsurface flow affects soil processes and the resulting arrangement of soil development along flow paths is challenging. While hydrologic regime and soil solution acidity are known to influence weathering rates and soil transformation processes, an integrated understanding of these factors together is still lacking. This dissertation explores the effects of subsurface flow on the mobility and distribution of dissolved organic carbon (DOC) and base cations to explain spatial patterns in chemical weathering in a forested headwater catchment. In the first chapter, relationships between hydrologic behavior, fluxes of weathered elements, and the extent of soil elemental loss across landscape positions are established. The second chapter investigates what specific groundwater behavior best explains spatial patterns in solution DOC concentrations during storm events. Lastly, in the third chapter, near surface saturation dynamics are examined to determine when and where DOC mobilization might be enhanced by subsurface flow. Results show that weathering extent was greatest in the upper reaches of the catchment, where O horizon saturation frequency and DOC concentrations are highest. Annual base cation fluxes, which were also greatest in these positions, could indicate where weathering is likely still enhanced. Additionally, while O horizon saturation occurred across the catchment, spatial differences in DOC concentrations suggest there are other sources of acidity to groundwater solutions other than just leaching from O horizons. Shallow organic soils, near bedrock outcrops at the top of the catchment is likely this additional C source, in which drainage water is transported downslope to nearby mineral soils when water tables are high and hydrologic connectivity between soils is increased. Spring and fall storm events were identified as times when groundwater most frequently reached O horizons during the snow-free year, providing insight into the timing of these processes throughout the year. This dissertation highlights how catchment structure mediates DOC flushing events, which in turn, influences the spatial architecture of soil development and chemical weathering processes across the landscape.
- A catchment water balance assessment of an abrupt shift in evapotranspiration at the Hubbard Brook Experimental Forest, New Hampshire, USAGreen, 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.
- Characterization of a hydraulically induced bedrock fractureBrandon, Ryan (Virginia Tech, 2014-09-17)Hydraulic fracturing is a controversial practice because of concerns about environmental impacts due to its widespread use in recovering unconventional petroleum and natural gas deposits. However, water-only hydraulic fracturing has been used safely and successfully for many years to increase the permeability of aquifers used for drinking and irrigation water supply. This process extends and widens existing bedrock fractures, allowing groundwater storage to increase. Researchers have studied the behavior of fractured-rock aquifers for decades, but little has been published on the hydraulic and mechanical properties of hydraulically enhanced fractures. In this study, a multi-faceted approach consisting of aquifer and tracer testing is used to estimate the transmissivity and storage coefficient of a hydraulically induced fracture and observe its behavior as a contaminant flow pathway. The results of the aquifer tests indicated a decrease in both the transmissivity and storage coefficient of the fracture of three orders of magnitude after cessation of pumping. The aquifer temporarily experienced incomplete recovery following pumping tests, likely due to slow recharge. After complete recovery occurred, subsequent tests showed that these hydraulic properties returned to their original values, indicating elastic compression of the fracture during periods of applied stress. The results of the tracer test indicated rapid, uniform, one-dimensional flow through the fracture, with average fluid velocity approaching 1 km/day in an induced steady flow field of 6 x 10-5 m3/s (1 gal/min) and a fracture volume of 0.238 m3 (63 gal). The complex heterogeneity of fractured-rock aquifers necessitates the use of multiple lines of testing in order to arrive at a detailed description of the behavior of these systems. This study demonstrates one effective method of investigating a single fracture that can uncover information about the behavior of a hydraulically enhanced aquifer that is otherwise difficult to obtain.
- Characterizing and modeling wet stream length dynamics in Appalachian headwatersJensen, Carrie Killeen (Virginia Tech, 2018-05-03)Headwater streams change in wet length in response to storm events and seasonal moisture conditions. These low-order channels with temporary flow are pervasive across arid and humid environments yet receive little attention in comparison to perennial waterways. This dissertation examines headwater stream length dynamics at multiple spatial and temporal scales across the Appalachians. I mapped wet stream length in four Appalachian physiographic provinces--the Appalachian Plateau, Blue Ridge, New England, and Valley and Ridge--to characterize seasonal expansion and contraction of the wet network at a broad, regional scale. Conversely, most existing field studies of stream length in headwaters are limited to a single study area or geographic setting. Field mappings showed that wet stream length varies widely within the Appalachians; network dynamics correlated with regional geology as well as local site lithology, geologic structure, and the depth, size, and spatial distribution of surficial sediment deposits. I used the field data to create logistic regression models of the wet network in each physiographic province at high and low runoffs. Topographic metrics derived from elevation data were able to explain the discontinuous pattern of headwater streams at different flow conditions with high classification accuracy. Finally, I used flow intermittency sensors in a single Valley and Ridge catchment to record channel wetting and drying at a high temporal resolution. The sensors indicated stream length hysteresis during storms with low antecedent moisture, with a higher wet network proportion on the rising limb than on the falling limb of events. As a result, maximum network extension can precede peak runoff by minutes to hours. Accurate maps of headwater streams and an understanding of wet network dynamics through time are invaluable for applications surrounding watershed management and environmental policy. These findings will contribute to the burgeoning research on temporary streams and are additionally relevant for studies of runoff generation, biogeochemical cycling, and mass fluxes of material from headwaters.
- Comparing Sediment Trap Data With Erosion Models for Evaluation of Forest Haul Road Stream Crossing ApproachesLang, 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 changeLaudon, 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.
- Development of a lateral topographic weathering gradient in temperate forested podzolsBower, 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.
- Differing Levels of Forestry Best Management Practices at Stream Crossing Structures Affect Sediment Delivery and Installation CostsMorris, Brian C.; Bolding, M. Chad; Aust, W. Michael; McGuire, Kevin J.; Schilling, Erik B.; Sullivan, Jay (MDPI, 2016-03-10)Forestry best management practices (BMPs) are used to reduce sedimentation from forest streamcrossings. Three BMP treatments (BMP, BMP-std, and BMP+) were applied to three forest road stream crossings (bridge, culvert, and ford). BMP did not meet existing BMP guidelines, BMP-std met standard recommendations, and BMP+ treatments exceeded recommendations. Following BMP applications, three simulated rainfall intensities (low, medium, and high) were applied in order to evaluate sediment delivery from crossing type and BMP level. During rainfall simulation, sediment concentrations (mg/L) were collected with automated samplers and discharge (L/s) was estimated to calculate total sediment loading. Costs of stream crossings and BMP levels were also quantified. Mean sediment associated with the three stream crossings were 3.38, 1.87, and 0.64 Mg for the BMP, BMP-std, and BMP+ levels, respectively. Ford, culvert, and bridge crossings produced 13.04, 12.95, and 0.17 Mg of sediment during construction, respectively. BMP enhancement was more critical for sediment control at the culvert and ford crossings than at the bridge. Respective costs for BMP , BMP-std, and BMP+ levels were $5,368, $5,658, and $5,858 for the bridge; $3,568, $4,166 and $4,595 for the culvert; and $180, $420 and $1,903 for the ford. Costs and sediment values suggest that current standard BMP levels effectively reduce stream sediment while minimizing costs.
- Digital terrain analysis to predict soil spatial patterns at the Hubbard Brook Experimental ForestGillin, Cody Palmer (Virginia Tech, 2013-05-15)Topographic analysis using digital elevation models (DEMs) has become commonplace in soil and hydrologic modeling and analysis and there has been considerable assessment of the effects of grid resolution on topographic metrics using DEMs of 10 m resolution or coarser. However, examining fine-scale (i.e., 1-10 m) soil and hydrological variability of headwater catchments may require higher-resolution data that has only recently become available, and both DEM accuracy and the effects of different high-resolution DEMs on topographic metrics are relatively unknown. This study has two principle research components. First, an error analysis of two high-resolution DEMs derived from light detection and ranging (LiDAR) data covering the same headwater catchment was conducted to assess the applicability of such DEMs for modeling fine-scale environmental phenomena. Second, one LiDAR-derived DEM was selected for computing topographic metrics to predict fine-scale functional soil units termed hydropedological units (HPUs). HPU development is related to topographic and surface/subsurface heterogeneity resulting in distinct hydrologic flowpaths leading to variation of soil morphological expression. Although the two LiDAR datasets differed with respect to data collection methods and nominal post-spacing of ground returns, DEMs interpolated from each LiDAR dataset exhibited similar error. Grid resolution affected DEM-delineated catchment boundaries and the value of computed topographic metrics. The best topographic metrics for predicting HPUs were the topographic wetness index, bedrock-weighted upslope accumulated area, and Euclidean distance from bedrock. Predicting the spatial distribution of HPUs may provide a more comprehensive understanding of hydrological and biogeochemical functionality of headwater systems.
- Effectiveness, cost, and implications of forest haul road stream crossing structures and best management practices in VirginiaMorris, Brian C. (Virginia Tech, 2015-07-06)Forest roads and stream crossings have the potential to be sources of sediment from forest operations. Recent litigation has renewed interest in furthering research related to forest road Best Management Practices (BMPs). Three legacy (100 year old) forest road stream crossings were monitored for suspended sediment for nine months before and six months after upgrading three unimproved ford crossings with one bridge, one culvert, and one improved ford. During construction, rainfall simulation was utilized to estimate the sediment contribution of each crossing with minimal BMPs (BMP-), BMPs equal to state recommendations (BMP), and BMPs beyond state recommendations (BMP+). Construction costs were recorded to quantify the change in cost with a change in BMP level. Three levels of rainfall simulation were used on each BMP treatment for each crossing resulting in 27 rainfall simulations. Water samples collected by an automatic sampler downstream of the crossings were analyzed for suspended sediment. Pre - and post- construction time periods were compared to assess how the improved crossings altered total suspended sediment concentrations downstream of the crossings. The number of stream crossings constructed per year in Virginia was also estimated using satellite imagery on 400 harvest tracts. Site visits were conducted on 240 harvest tracts where data were collected on the presence of crossings, the types of crossings, and the level of BMP implementation. Rainfall simulation experiments showed decreased sediment with increased BMP level and daily total suspended sediment concentrations measured over 15 months showed a decrease in mean daily sediment concentration after construction of the bridge and culvert crossings. There was no decrease in sediment concentration for the ford crossing. Statewide crossing construction and BMP implementation rates were estimated. Approximately 67% of the audited stream crossings were characterized as having BMPs that were equal to or beyond state recommendations. Increased BMPs and upgrading of stream crossings resulted in decreased total suspended sediment. However, increased BMP implementation also increased stream crossing construction costs. Effectiveness of increased levels of BMPs and the pre and post construction analysis suggests the improvement of a legacy stream crossing may reduce total suspended sediment concentrations.
- Estimating Costs and Effectiveness of Upgrades in Forestry Best Management Practices for Stream CrossingsNolan, Lindsay; Aust, W. Michael; Barrett, Scott M.; Bolding, M. Chad; Brown, Kristopher; McGuire, Kevin J. (MDPI, 2015-12-08)Forestry Best Management Practices (BMPs) are used for protection of water quality at forest stream crossings, yet effects and costs for gradients of BMPs are not well documented. We evaluated forty-two truck road and skid trail stream crossings using three surrogates of BMP adequacy: (1) potential erosion rates for stream crossing approaches; (2) adequacy of stream crossing BMPs; and (3) overall BMP rating (BMP−, BMP-standard, and BMP+). Subsequently, BMP upgrades were recommended for enhancing BMP− or BMP-standard stream crossings. Costs for BMP upgrades were estimated using an existing road and skid trail cost method. The majority of truck road stream crossings were culverts, while skid trail stream crossings were primarily portable bridges. Potential erosion estimates, BMP audit scores, and BMP ratings all indicated that skid crossings have lower BMP implementation than truck road crossings. BMP improvements commonly identified for skid trail and truck crossings included addition of cover and water control structures. Improved BMPs at skid trail crossings were less expensive than those at truck road crossings. Current BMP guidelines provide economical and effective techniques for reducing erosion, and BMP upgrades have the potential to reduce erosion rates to similar levels found in undisturbed forests.
- 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 catchmentGannon, 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.
- Forest catchment structure mediates shallow subsurface flow and soil base cation fluxesPennino, Amanda; Strahm, Brian D.; McGuire, Kevin J.; Bower, Jennifer A.; Bailey, Scott W.; Schreiber, Madeline E.; Ross, Donald S.; Duston, Stephanie A.; Benton, Joshua R. (Elsevier, 2024-10)Hydrologic behavior and soil properties across forested landscapes with complex topography exhibit high variability. The interaction of groundwater with spatially distinct soils produces and transports solutes across catchments, however, the spatiotemporal relationships between groundwater dynamics and soil solute fluxes are difficult to directly evaluate. While whole-catchment export of solutes by shallow subsurface flow represents an integration of soil environments and conditions but many studies compartmentalize soil solute fluxes as hillslope vs. riparian, deep vs. shallow, or as individual soil horizon contributions. This potentially obscures and underestimates the hillslope variation and magnitude of solute fluxes and soil development across the landscape. This study determined the spatial variation and of shallow soil base cation fluxes associated with weathering reactions (Ca, Mg, and Na), soil elemental depletion, and soil saturation dynamics in upland soils within a small, forested watershed at the Hubbard Brook Experimental Forest, NH. Base cation fluxes were calculated using a combination of ion-exchange resins placed in shallow groundwater wells (0.3 – 1 m depth) located across hillslope transects (ridges to lower backslopes) and measurements of groundwater levels. Groundwater levels were also used to create metrics of annual soil saturation. Base cation fluxes were positively correlated with soil saturation frequency and were greatest in soil profiles where primary minerals were most depleted of base cations (i.e., highly weathered). Spatial differences in soil saturation across the catchment were strongly related to topographic properties of the upslope drainage area and are interpreted to result from spatial variations in transient groundwater dynamics. Results from this work suggest that the structure of a catchment defines the spatial architecture of base cation fluxes, likely reflecting the mediation of subsurface stormflow dynamics on soil development. Furthermore, this work highlights the importance of further compartmentalizing solute fluxes along hillslopes, where certain areas may disproportionately contribute solutes to the whole catchment. Refining catchment controls on base cation generation and transport could be an important tool for opening the black box of catchment elemental cycling.
- Geospatial Analysis of Forest Fragmentation and Connectivity in VirginiaFynn, Iris Ekua Mensimah (Virginia Tech, 2019-06-20)This research evaluated the extent to which forests in Virginia have either become fragmented (disconnected) and/or connected over a ten year time period. The study analyzed the accuracy of forest fragmentation analysis depending on the spatial resolution of the satellite imagery used. This analysis highlights the importance of using appropriate satellite images for forest fragmentation analysis. Secondly, this research focused on building a model to identify the significance of factors such as slope, physiographic region and forest types on Virginia's populations of Wood Thrush and Ovenbird. This assessment identified the difference in effects of variables on bird populations depending on the scale at which the analysis is carried out. Third and final analysis combined the first two assessments to determine how management policies can be used to mitigate negative effects of forest fragmentation and protect biodiversity. The research results highlight increasing forest fragmentation trends in Virginia between 2001 and 2011 and the negative impacts of this trend on Wood Thrush and Ovenbird species. The results also demonstrate the effectiveness of riparian buffers as corridors.
- Headwater stream length dynamics across four physiographic provinces of the Appalachian HighlandsJensen, 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.