VTechWorks Repository :: Browsing by Author "Scott, Durelle T."

Browsing by Author "Scott, Durelle T."

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Advancing Water Security and Environmental Sustainability Through Evaluation of Water Use From the Field to State-Wide Scale
Sangha, Laljeet Singh (Virginia Tech, 2023-01-17)
The United States (US) has experienced a surge in water shortages and droughts in recent times. Water shortages can result from population growth, climate change, inadequate water management policies, and the improper use of available technologies. The existing data and research on water use associated with water management policy structures are limited. Many states in the US follow strict regulations on water discharge into streams to enforce water quality standards; however, water withdrawal restrictions from streams are limited and inadequate in terms of water management at times of low flow. In states such as Virginia (VA), the Virginia Department of Environmental Quality (VDEQ) requires a Virginia Water Protection (VWP) permit for all water withdrawals from VA's surface waters. However, under certain provisions of VWP regulations, users are exempted from having a permit. Such permit exemptions exist in many states and present a severe challenge to water supply management. Chapter 2 compares the impact of permit exemptions on surface water availability and drought flows and compares these impacts to the relatively well-studied risks presented by dry climate change and demand growth in Virginia (VA). It was observed that in some regions, the impacts under the exempt user scenario were higher than those under the dry climate change scenario. In addition, water supply managers and government agencies use user-reported water withdrawal data to develop water management programs. Irrigated agriculture is the largest source of water consumption in the US. However, water-reporting regulations exempt users from withdrawing water for irrigation under a certain threshold. Moreover, as water is not metered, users often do not report their irrigation water use, resulting in considerable uncertainty about the impacts of irrigation withdrawals, which could potentially impact other water users, lead to water shortages or conflicts, and negatively impact stream ecology. Chapter 3 focuses on developing a novel methodology for quantifying unreported irrigation water withdrawals using publicly available USDA-Census and USDA-IWMS datasets. This method was used to evaluate the unreported water withdrawals in the VA. Finally, water use practices at the field level intersect with other environmental issues at a larger scale. For example, irrigation practices can influence nutrient uptake and transport at the field level. Insufficient water for irrigation, especially during critical growth stages, results in yield and economic losses and reduces agricultural productivity. However, excessive irrigation can lead to wasted water and energy as well as runoff and leaching of nutrients and agricultural chemicals. Therefore, the adoption of technological advancements at the field scale can reduce the amount of water needed to fulfill the needs while mitigating any nutrient impacts on the soil due to the excessive use of water. This is highly important when fertilizer prices are always high. Chapter 4 focuses on quantifying the impact of the use of short-term weather forecast data in irrigation scheduling on nutrient and water use efficiency in humid climates: experimental results for corn and cotton. It was found that irrigation scheduling using short-term weather forecast data is helpful for improving the nutrient and water use efficiency of corn. For cotton, nutrient and water use efficiency are highly influenced by irrigation and precipitation with respect to the growth stage.
Applicability of Stormwater Best Management Practices in the Virginia Coastal Plain
Johnson, Rachael Diane (Virginia Tech, 2016-06-06)
The Virginia Runoff Reduction Method (RRM) was adopted in 2014 as a compliance tool for evaluation of stormwater volume and quality, and necessitates use of urban stormwater best management practices (BMPs) to meet regulatory standards. Coastal Virginia is characterized by flat terrain, shallow water tables, and low permeable soils that may limit the application of BMPs as recommended by state regulations. Soil morphological features are often used to estimate the seasonal high water table (SHWT) for initial feasibility, but existing soil data misrepresented expected SHWT depths in the Virginia Beach, VA, study area. A GIS-based methodology relying on perennial surface water elevations and USGS groundwater monitoring data was developed to estimate the SHWT depth in Virginia Beach. The SHWT map was shown to be consistently more reliable than available predictions based on soil morphology, and was used as input to a BMP siting tool. The tool, known as BMP Checker, was developed to explore how flat terrain, shallow water tables, and poor soils influence BMP siting in coastal Virginia. The BMP Checker algorithm was validated on 11 Virginia Beach sites before application on 10,000 ft2 (929 m2) area sections across the city. Citywide application showed that the most widely applicable BMPs in the study area include wet ponds that intercept groundwater and constructed wetlands. Conversely, sheet flow to conservation area and infiltration practices are the least applicable. Because the RRM assigns more credit to infiltration-based practices, sites in Virginia Beach may find it difficult to meet regulatory standards.
Application of a New Species-Richness Based Flow Ecology Framework for Assessing Flow Reduction Effects on Aquatic Communities
Rapp, Jennifer L.; Burgholzer, Robert; Kleiner, Joseph; Scott, Durelle T.; Passero, Elaina M. (2020)
Water-resources managers are challenged with maintaining a balance among beneficial uses throughout river networks and need robust means of assessing potential risks to aquatic life resulting from flow alterations. This study generated ecological limit functions from species-streamflow relations to quantify potential fish richness response to flow alteration and compared results to currently accepted streamflow management guidelines. Modeled responses of absolute richness change were watershed specific and varied among sample sets derived from hydrologic unit classifications of different sizes (large HUC 6 basins to regional scale HUC 8). With a 20% flow reduction, 10% of HUC 8 predicted a richness decrease in one or more taxa. While absolute richness change was consistent across streams within a HUC, percent richness change was stream size dependent. Comparisons with Instream Flow Incremental Methodology habitat models predicted habitat loss greater than percent richness change; however, predictions for habitat and richness decreased similarly as stream size decreased. Watershed-specific responses from flow reductions could allow water-resources management decisions to be made locally based on the predicted richness change for certain sized streams. Quantitative results highlight the utility of a richness-based framework for generating watershed-specific risk assessments that validate and inform currently employed water-resources management practices.
Basin-scale spatiotemporal analysis of hydrologic floodplain connectivity
McCann, 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.
Characterizing and modeling wet stream length dynamics in Appalachian headwaters
Jensen, 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 Reach Scale Hyporheic Exchange and Denitrification Induced by Instream Restoration Structures and Natural Streambed Morphology
Brooks, Kristen Elise (Virginia Tech, 2017-07-10)
A common water quality issue is an excess of nutrients which can lead to problems such as eutrophication. Stream restoration is one method by which improvements in water quality may be attempted. One strategy is increasing hyporheic zone flow at baseflow by addition of instream structures. The hyporheic zone can be an area of increased biogeochemical activity, with potential enhancement of reactions such as denitrification. However, the comparative effects of various instream restoration techniques, as well as the role of watershed setting and corresponding environmental characteristics in which restoration occurs (e.g., hydraulic conductivity, stream slope), are still poorly understood. In this study we numerically modeled groundwater and surface water interaction in a 200 m second order stream reach in southwestern Virginia using MIKE SHE. We calibrated the model to hydrologic and tracer data available during field tests of restoration techniques. We then simulated different types of instream restoration techniques (e.g., fully and partially channel-spanning weirs and buried structures), and varied hydrologic and biogeochemical controlling factors driven by watershed setting. The measured effects for this sensitivity analysis were direction and magnitude of surface water-groundwater exchange and amount of denitrification. We found that factors related to watershed setting had the greatest effect on surface water-groundwater exchange and on denitrification, including streambed hydraulic conductivity, natural or background stream topography and slope, and groundwater levels. Type and number of instream structures also influenced surface water-groundwater exchange and denitrification, but to a lesser degree, and the effect of structures was in turn controlled by watershed setting. Watershed setting was thus the largest control, both on exchange overall, and the effectiveness of structures. Human effects on watersheds such as agriculture and urbanization therefore likely play a role in whether reach-scale restoration practices succeed in achieving water quality goals. More broadly, restoration efforts at the watershed scale itself, such as reducing fertilizer use or improving stormwater management, may be necessary to achieve ambitious water quality goals. Nevertheless, reach-scale restoration efforts such as in-stream structures may play a useful role in certain watershed settings. Furthermore, other reach-scale restoration techniques that affect streambed topography, such as addition of pool-riffle sequences, may be more effective, and bear investigation.
A comparison of runoff quantity and quality among three cattle stocking treatments
Williams, Emily Diane (Virginia Tech, 2014-03-11)
Measurements of runoff quantity and quality from three cattle stocking treatments applied to pastureland in southwestern Virginia indicate the need for further research to determine treatment effects. Three cattle stocking treatments (1) Continuous, 2) Rotational, and 3) Mob) were applied to three pastures at the Virginia Tech Prices Fork Research Farm. Rainfall simulations were performed over replicated plots in each treatment to induce runoff for collection of runoff quantity and quality data during the 2012 grazing season. Additionally, rainfall simulations were performed prior to applying the grazing treatments to establish initial conditions. Monitored runoff quantity and quality response variables included runoff depth, mean nutrient concentrations, and nutrient mass loss. Response variables were compared among the three pastures for initial conditions and among treatments for post-treatment conditions. Additionally, the trends in response variables within the 2012 season were compared among treatments. Plot and rainfall conditions that were expected to influence responses were also collected and analyzed in relation to response variables. Analyses of the response variables suggested that the variability within treatments likely muted any treatment effect on the response variables. Therefore, we concluded that further research is needed to determine treatment effects on runoff quantity and quality.
Comparison of Two PARAFAC Models of Dissolved Organic Matter Fluorescence for a Mid-Atlantic Forested Watershed in the USA
Singh, Shatrughan; Inamdar, Shreeram; Scott, Durelle T. (Hindawi, 2013-07-11)
The composition of dissolved organic matter (DOM) in a mid-Atlantic forested watershed was evaluated using two fluorescence models—one based on previously validated model (Cory and McKnight, 2005) and the other developed specifically for our study site. DOM samples for the models were collected from multiple watershed sources over a two-year period. The previously validated parallel factor analysis (PARAFAC) model had 13 DOM components whereas our site-specific model yielded six distinct components including two terrestrial humic-like, two microbial-derived humic-like, and two protein-like components. The humic-like components were highest in surficial watershed sources and decreased from soil water to groundwater whereas the protein-like components were highest for groundwater sources. Discriminant analyses indicated that our site-specific model was more sensitive to subtle differences in DOM and the sum of the humic- and protein-like constituents yielded more pronounced differences among watershed sources as opposed to the prevalidated model. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations and selected DOM metrics were also more strongly correlated with the site-specific model components. These results suggest that while the pre-validated model may capture broader trends in DOM composition and allow comparisons with other study sites, a site-specific model will be more sensitive for characterizing within-site differences in DOM.
Complementary Effects of In-Stream Structures and Inset Floodplains on Solute Retention
Azinheira, David Lee (Virginia Tech, 2013-06-14)
The pollution of streams and rivers is a growing concern, and environmental guidance increasingly suggests stream restoration to improve water quality. ï¿½Solute retention in off channel storage zones such as hyporheic zones and floodplains is typically necessary for significant reaction to occur. ï¿½Yet the effects of two common restoration techniques, in stream structures and inset floodplains, on solute retention have not been rigorously compared. ï¿½We used MIKE SHE to model hydraulics and solute transport in the channel, inset floodplain, and hyporheic zone of a 2nd order stream. ï¿½We varied hydraulic conditions (winter baseflow, summer baseflow, and storm flow), geology (hydraulic conductivity), and stream restoration design parameters (inset floodplain length, and presence of in stream structures). ï¿½In stream structures induced hyporheic exchange during summer baseflow with a low groundwater table (~20% of the year), while floodplains only retained solutes during storm flow conditions (~1% of the year). ï¿½Flow through the hyporheic zone increased linearly with hydraulic conductivity, while residence times decreased linearly. ï¿½Flow through inset floodplains and residence times in both the channel and floodplains increased non linearly with the fraction of bank with floodplains installed. ï¿½The fraction of stream flow that entered inset floodplains was one to three orders of magnitude higher than that through the hyporheic zone, while the residence time and mass storage in the hyporheic zone was one to five orders of magnitude larger than that in floodplain segments. ï¿½Our model results suggest that in stream structures and inset floodplains are complementary practices.
Consumptive Water Use: Refining State Water Supply Estimates with Discharge and Withdrawal Data
McCarthy, Mary Morgan (Virginia Tech, 2019-06-11)
Water scarcity has quickly become one of the most pressing issues in the 21st century. Knowledge of the stress consumption places on water supply is therefore necessary for improved resource management. This research leverages monthly facility level withdrawal and discharge data from two different sources to provide important observations of consumptive water use across several spatial scales and water use sectors in Virginia between 2010-2016. Consumptive water is defined as water which is withdrawn and not returned to a water resource system. Consumption was estimated on statewide, watershed, county, and facility levels. The agriculture/irrigation, aquaculture, commercial, industrial, energy, and municipal sectors were considered for analysis. Facilities were matched between the two data sources by narrowing potential matches by distance and then by facility name using an approximate string distance mechanism. This analysis revealed that inconsistent discharge reporting affects estimates of consumption through time and any errors at finer spatial scales are ultimately masked at coarser levels. Statewide energy consumption in Virginia was found to be between 4-20% considering all available data and 0.4-4% across matched facilities. Non-energy consumption was an estimated 37-51% considering all available data and only 28-33% across matched facilities. Inconsistent reporting of discharge made it difficult to determine if consumption trends truly exist in Virginia, but monthly consumption appears to be persistent through time and slightly increasing in non-energy sectors. Industrial consumption in Virginia was also found to be higher than literature values. Results from this study are beneficial for water supply modeling and planning by providing more refined estimates of the actual stress withdrawals place on water supply.
Cumulative Impacts of Stream Restoration on Watershed-Scale Flood Attenuation, Floodplain Inundation, and Nitrate Removal
Goodman, Lucas M. (Virginia Tech, 2024-01)
Severe flooding and excess nutrient pollution, exacerbated by heightened anthropogenic pressures (e.g., climate change, urbanization, land use change, unsustainable agricultural practices), have been detrimental to riverine systems and their estuaries. The degradation of riverine systems can negatively impact human and environmental health, as well as local, regional, and even global economies. Floods provide beneficial ecosystem services (e.g., processing pollutants, transferring nutrients and sediment, supporting biodiversity), but they can also damage infrastructure and result in the loss of human life. Meanwhile, eutrophication can cause anoxic dead zones, harming aquatic ecosystems and public health. To address the issues facing riverine systems, focus has shifted to watershed-scale management plans. However, it can prove challenging to quantify the cumulative impacts of multiple stream restoration projects within a single watershed on flooding and nutrient removal. Previous studies have quantified the effects of stream restoration on flood attenuation. However, our first study fills a substantial knowledge gap by evaluating the impacts of different floodplain restoration practices, varied by location and length, on flood attenuation and floodplain inundation dynamics at the watershed scale during more frequent storm recurrence intervals (i.e., 2-year, 1-year, 0.5-year, and monthly). We created a 1D HEC-RAS model to simulate the effects of Stage 0 restoration within a 4th-order generic watershed based on the Chesapeake Bay watershed. By varying the percent river length restored and location, we found that Stage 0 restoration, especially in 2nd-order rivers, can be particularly effective at enhancing flood attenuation and floodplain inundation locally and farther downstream. We addressed the water quality component by using a random forest machine learning approach coupled with artificial neural networks to find trends and predict nitrate removal rates associated with spatial, temporal, hydrologic, and restoration features. Our results showed that hydrologic conditions were the most important variable for predicting actual nitrate removal rates. Overall, both studies demonstrate the importance of hydrologic connectivity for flood attenuation, channel-floodplain exchange, and nutrient processing.
Cumulative Impacts of Watershed-Scale Hyporheic Stream Restoration on Nitrate Loading to Downstream Waterbodies
Calfe, Michael Louis (Virginia Tech, 2020-01-23)
Excess nutrient pollution and eutrophication are widespread problems that must be solved at watershed scales, and stream restoration is increasingly implemented as a solution. Yet few studies evaluate the cumulative effects of multiple individual restoration projects on watershed-scale nutrient loading. We constructed a HEC-RAS model of stream restoration implemented throughout a generic 4th order watershed typical of the Piedmont physiographic province of the eastern USA. We simulated restoration of hyporheic exchange as one increasingly popular technique that receives dissolved nitrate-nitrogen (NO3--N) mitigation credit under the Chesapeake Bay TMDL. We populated the model with hyporheic exchange (0.3% of surface flow per hyporheic-exchange inducing in-stream restoration structure) and NO3--N removal (supply-limited denitrification removes all NO3--N that enters the hyporheic zone) values from prior literature on in-stream structures and related restoration techniques. We then varied the percentage of stream channels in the watershed in which restoration occurred. For watersheds with less than 100% of stream channels restored, we also varied where in the watershed (i.e. stream order) that restoration occurred. We found that hyporheic restoration in our 4th order watersheds has the potential to reduce NO3--N loading to downstream waterbodies by up to 83%, but that a maximum of <100% reduction exists given certain watershed characteristics. Model results revealed a nonlinear relationship between percent of stream channels restored and percent NO3--N loading reduction that occurred at the watershed outlet. This indicates that the effects of individual projects are not linearly additive, and must be evaluated in the context of how much of the watershed has already been restored. We also found that restoration was more effective at reducing NO3--N loading when it occurred in higher order streams (e.g., 3rd and 4th order), yielding load reductions upward of 30% compared to < 10% in lower order streams (e.g., 1st and 2nd order). Thus, the location of an individual restoration project within a watershed is important in determining its effect on NO3--N. Overall, our results indicate that hyporheic restoration can have significant effects on watershed NO3--N loading to downstream waterbodies, yet the watershed must be viewed as a whole to understand the potential impacts of any particular project under consideration.
A Dam Conundrum: The Role of Impoundments in Stream Flow Alteration
Brogan, Connor O'Beirne (Virginia Tech, 2018-09-12)
Over the past century, the world's rivers have become increasingly impounded to combat water scarcity and fossil-fuel reliance. Large dams have faded from popularity due to their adverse environmental effects, but small ponds and reservoirs continue to be constructed at high rates. Due to limited data regarding their size and flow, it has been difficult to assess how these smaller impoundments impact rivers. This study combined rainfall runoff data from the Chesapeake Bay Model with the unique routing framework of VA Hydro to create a simplistic hydrologic model capable of analyzing impoundment-induced flow alteration. Using standard design techniques and satellite imagery, a methodology was developed to build realistic stage-storage-discharge relationships for small and large impoundments. Eleven impoundments of the Difficult Run watershed were modeled within VA Hydro to assess their cumulative impact on downstream flow. Multiple models were created with different active impoundments and run for the full model period, 1984 - 2005. Flow alteration increased significantly with additional impoundments. Peak flows were attenuated as water was stored behind outlets, but median flows were increased as this water was slowly released. Average storm duration increased due to extended rising and falling limbs caused by impoundment outlets. Headwater channels increasingly ran dry, decreasing extreme low flows due to impoundment evaporation. Large reservoirs had a greater impact on median flows, but smaller ponds dominated low flow alteration. These results suggest that traditional hydrologic assumptions and metrics may be incapable of analyzing a changing flow regime without explicitly considering small and large impoundments upstream.
Development of a Design-Based Computational Model of Bioretention Systems
Liu, Jia (Virginia Tech, 2013-12-03)
Multiple problems caused by urban runoff have emerged as a consequence to the continuing development of urban areas in recent decades. The increase of impervious land areas can significantly alter watershed hydrology and water quality. Typical impacts to downstream hydrologic regimes include higher peak flows and runoff volumes, shorter concentration times, and reduced infiltration. Urban runoff increases the transport of pollutants and nutrients and thus degrades water bodies adjacent to urban areas. One of the most frequently used practices to restore the hydrology and water quality of urban watersheds is bioretention (also known as a rain garden). Despite its wide applicability, an understanding of its multiple physiochemical and biological treatment processes remains an active research area. To provide a wide ability to evaluate the hydrologic input to bioretention systems, spatial and temporal distribution of storm events in Virginia were studied. Results generated from long-term frequency analysis of 60-year precipitation data demonstrate that the 90 percentile, or 10-year return period rainfall depth and dry duration in Virginia are between 22.9 – 35.6 mm and 15.3 – 25.8 days, respectively. Monte-Carlo simulations demonstrated that sampling programs applied in different regions would likely encounter more than 30% of precipitation events less than 2.54 mm, and 10% over 25.4 mm. Further experimental research was conducted to evaluate bioretention recipes for retaining stormwater nitrogen (N) and phosphorus (P). A mesocosm experiment was performed to simulate bioretention facilities with 3 different bioretention blends as media layers with underdrain pipes for leachate collection. A control group with 3 duplicates for each media was compared with a replicated vegetated group. Field measurement of dissolved oxygen (DO), oxidation-reduction potential (ORP), pH, and total dissolved solids (TDS) was combined with laboratory analyses of total suspended solids (TSS), nitrate (NO3), ammonium (NH4), phosphate (PO4), total Kjeldahl nitrogen (TKN) and total phosphorus (TP) to evaluate the nutrient removal efficacies of these blends. Physicochemical measurements for property parameters were performed to determine characteristics of blends. Isotherm experiments to examine P adsorption were also conducted to provide supplementary data for evaluation of bioretention media blends. The results show that the blend with water treatment residuals (WTR) removed >90% P from influent, and its effluent had the least TDS / TSS. Another blend with mulch-free compost retained the most (50 – 75%) total nitrogen (TN), and had the smallest DO / ORP values, which appears to promote denitrification under anaerobic conditions. Increase of hydraulic retention time (HRT) to 6 h could influence DO, ORP, TKN, and TN positively. Plant health should also be considered as part of a compromise mix that sustains vegetation. Two-way analysis of variance (ANOVA) found that single and interaction effects of HRT and plants existed, and could affect water quality parameters of mesocosm leachate. Based upon the understanding of the physiochemical and hydrologic conditions mentioned previously, a design model of a bioretention system became the next logical step. The computational model was developed within the Matlab® programming environment to describe the hydraulic performance and nutrient removal of a bioretention system. The model comprises a main function and multiple subroutines for hydraulics and treatment computations. Evapotranspiration (ET), inflow, infiltration, and outflow were calculated for hydrologic quantitation. Biomass accumulation, nitrogen cycle and phosphorus fate within bioretention systems were also computed on basis of the hydrologic outputs. The model was calibrated with the observed flow and water quality data from a field-scale bioretention in Blacksburg, VA. The calibrated model is capable of providing quantitative estimates on flow pattern and nutrient removal that agree with the observed data. Sensitivity analyses determined the major factors affecting discharge were: watershed width and roughness for inflow; pipe head and diameter for outflow. Nutrient concentrations in inflow are very influential to outflow quality. A long-term simulation demonstrates that the model can be used to estimate bioretention performance and evaluate its impact on the surrounding environment. This research advances the current understanding of bioretention systems in a systematic way, from hydrologic behavior, monitoring, design criteria, physiochemical performance, and computational modeling. The computational model, combined with the results from precipitation frequency analysis and evaluation of bioretention blends, can be used to improve the operation, maintenance, and design of bioretention facilities in practical applications.
Dissolved Organic Matter Sources from Soil Horizons with Varying Hydrology and Distance from Wetland Edge
Wardinski, Katherine Mary (Virginia Tech, 2021-09-03)
Understanding hydrologic controls on carbon accumulation and export within geographically isolated wetlands (GIW) has implications for the success of wetland restoration efforts intended to produce carbon sinks. However, little is known about how hydrologic connectivity along the aquatic-terrestrial interface in GIW catchments influences carbon dynamics, particularly regarding dissolved organic matter (DOM) transport and transformation. The organic matter (carbon) that accumulates in wetland soils may be released into water, generating DOM. DOM is mobile and reactive, making it influential to aquatic metabolism and water quality. To understand the role of different soil horizons as potential sources of DOM, extractable soil organic matter (ESOM) was measured in soil horizons collected from upland to wetland transects at four Delmarva Bay GIWs on the Delmarva Peninsula in the eastern United States. ESOM quantity and quality were analyzed to provide insights to organic matter sources and chemical characteristics. Findings demonstrated that ESOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant-like signatures. ESOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and protein-like signatures. Organic soil horizons had the largest quantities of ESOM, and ESOM decreased with increasing soil depth. ESOM quantities also generally decreased from the upland to the wetland, suggesting that continuous soil saturation leads to a decreased quantity of ESOM. Despite wetland soils having lower ESOM, these horizons are thicker and continuously hydrologically connected to wetland surface water, leading to wetland soils representing the largest potential source of DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in Delmarva and other GIW systems will become increasingly important as climate change is expected to alter the hydrologic connectivity of wetland soils to the surface water-groundwater continuum and as wetlands are more frequently designed for carbon sequestration.
Drone-based water sampling and characterization of three freshwater harmful algal blooms in the United States
Hanlon, Regina; Jacquemin, Stephen J.; Birbeck, Johnna A.; Westrick, Judy A.; Harb, Charbel; Gruszewski, Hope; Ault, Andrew P.; Scott, Durelle T.; Foroutan, Hosein; Ross, Shane D.; González-Rocha, Javier; Powers, Craig; Pratt, Lowell; Looney, Harry; Baker, Greg; Schmale, David G. III (Frontiers, 2022-08-24)
Freshwater harmful algal blooms (HABs), caused mostly by toxic cyanobacteria, produce a range of cyanotoxins that threaten the health of humans and domestic animals. Climate conditions and anthropogenic influences such as agricultural run-off can alter the onset and intensity of HABs. Little is known about the distribution and spread of freshwater HABs. Current sampling protocols in some lakes involve teams of researchers that collect samples by hand from a boat and/or from the shoreline. Water samples can be collected from the surface, from discrete-depth collections, and/or from depth-integrated intervals. These collections are often restricted to certain months of the year, and generally are only performed at a limited number of collection sites. In lakes with active HABs, surface samples are generally sufficient for HAB water quality assessments. We used a unique DrOne Water Sampling SystEm (DOWSE) to collect water samples from the surface of three different HABs in Ohio (Grand Lake St Marys, GLSM and Lake Erie) and Virginia (Lake Anna), United States in 2019. The DOWSE consisted of a 3D-printed sampling device tethered to a drone (uncrewed aerial system, or UAS), and was used to collect surface water samples at different distances (10–100 m) from the shore or from an anchored boat. One hundred and eighty water samples (40 at GLSM, 20 at Lake Erie, and 120 at Lake Anna) were collected and analyzed from 18 drone flights. Our methods included testing for cyanotoxins, phycocyanin, and nutrients from surface water samples. Mean concentrations of microcystins (MCs) in drone water samples were 15.00, 1.92, and 0.02 ppb for GLSM, Lake Erie, and Lake Anna, respectively. Lake Anna had low levels of anatoxin in nearly all (111/120) of the drone water samples. Mean concentrations of phycocyanin in drone water samples were 687, 38, and 62 ppb for GLSM, Lake Erie, and Lake Anna, respectively. High levels of total phosphorus were observed in the drone water samples from GLSM (mean of 0.34 mg/L) and Lake Erie (mean of 0.12 mg/L). Lake Anna had the highest variability of total phosphorus with concentrations that ranged from 0.01 mg/L to 0.21 mg/L, with a mean of 0.06 mg/L. Nitrate levels varied greatly across sites, inverse with bloom biomass, ranging from below detection to 3.64 mg/L, with highest mean values in Lake Erie followed by GLSM and Lake Anna, respectively. Drones offer a rapid, targeted collection of water samples from virtually anywhere on a lake with an active HAB without the need for a boat which can disturb the surrounding water. Drones are, however, limited in their ability to operate during inclement weather such as rain and heavy winds. Collectively, our results highlight numerous opportunities for drone-based water sampling technologies to track, predict, and respond to HABs in the future.
Ecohydrologic Indicators of Low-flow Habitat Availability in Eleven Virginia Rivers
Hoffman, Kinsey H. (Virginia Tech, 2015-10-26)
Increasing demand and competition for freshwater is threatening instream uses including ecosystem services and aquatic habitat. A standard method of evaluating impacts of alternative water management scenarios on instream habitat is Instream Flow Incremental Methodology (IFIM). The primary outputs of IFIM studies are: 1) habitat rating curves that relate habitat availability to streamflow for every species, lifestage, or recreational use modelled; and 2) habitat time series under alternative water management scenarios. We compiled 428 habitat rating curves from previous IFIM studies across 11 rivers in Virginia and tested the ability to reduce this number based on similarities in flow preferences and responses to flow alteration. Individual site-species combinations were reduced from 428 objects to four groups with similar seasonal habitat availability patterns using a hierarchical, agglomerative cluster analysis. A seasonal habitat availability (SHA) ratio was proposed as a future indicator of seasonal flow preferences. Four parameters calculated from the magnitude and shape of habitat rating curves were proposed as response metrics that indicate how a lifestage responds to flow alteration. Univariate and multivariate analyses of variance and post-hoc tests identified significantly different means for the SHA ratio, QP (F=63.2, p<2e-16) and SK (F=65.6, p<2e-16). A reduced number of instream flow users can simplify the incorporation of aquatic habitat assessment in statewide water resources management.
Effect of Urbanization on the Hyporheic Zone: Lessons from the Virginia Piedmont
Cranmer, Elizabeth Nadine (Virginia Tech, 2011-06-28)
As the world's population shifts toward living in cities, urbanization and its deleterious effects on the environment are a cause of increasing concern. The hyporheic zone is an important part of stream ecosystems, and here we focus on the effect of urbanization on the hyporheic zone from ten first-to-second-order streams within the Virginia Piedmont. We use sediment hydraulic conductivity and stream geomorphic complexity (vertical undulation of thalweg, channel sinuosity) as metrics of the potential for hyporheic exchange (hyporheic potential). Our results include bivariate plots that relate urbanization (e.g., total percent impervious) with hyporheic potential at several spatial scales. For example, at the watershed level, we observed a decrease in horizontal hydraulic conductivity with urbanization and an increase in vertical hydraulic conductivity, which ultimately results in a negligible trend from conflicting processes. Vertical geomorphic complexity increased with total percent impervious cover. This trend was somewhat unexpected and may be due to erosion of legacy sediment in stream banks. At the reach level, hydraulic conductivity increased and sinuosity decreased as the riparian buffer width increased; these trends are weak and are essentially negligible. The hydraulic conductivity results conform to expected trends and are a product of aforementioned concomitant processes. Our results emphasize the complexity of hydrologic and geomorphic processes occurring in urban stream systems at multiple scales. Overall, the watershed level effects enhancing hyporheic exchange, which is contrary to expectations. Given the importance of hyporheic exchange to stream function, further study is warranted to better understand the effects of urbanization.
The Effects of Basin Slope and Boundary Friction on the Character and Plunge Location of Hyperpycnal Flows Entering a Laterally Unbounded Basin
Bhide, Shantanu Vidyadhar (Virginia Tech, 2019-06-19)
This thesis focuses on the behaviour of hyperpycnal plumes in river mouth discharges. The plunging of high density flows in two dimensional channels has been extensively studied before. A fundamental assumption in these studies is that the flow is laterally confined. These studies allow the flow to plunge only in two directions, the horizontal x-direction and the vertical z-direction. The goal of this study is to determine if there is observable plunging of hyperpycnal flows in the lateral y-direction, i.e. lateral spreading, in a three dimensional domain and to find out the parameters influencing the lateral spread. Previous studies conducted in laterally confined channels suggest that hyperpycnal flows plunge when the flow reaches a densimetric Froude number of unity. This study attempts to find the densimetric Froude number at hyperpycnal plunging in a three dimensional domain and if it is influenced by the factors that also influence the spread. This study also analyzes whether the cross-shore location for plunging changes when lateral spreading is accounted for, relative to a two dimensional analysis and if the plunging is limited to flow reaching a certain depth. This was accomplished through a series of experimental simulations on a hypothetical river mouth domain using Delft-3D, a hydrodynamic modeling software. Three parameters viz. the bottom slope of the receiving basin, the bottom friction and the density difference between inflow and ambient liquid were varied to test their influence on the plume spread rate.
Effects of experimentally-altered hydrology on ecosystem function in headwater streams
Northington, Robert M. (Virginia Tech, 2013-05-03)
Forested headwater stream ecosystems are important integrators of terrestrial and aquatic systems and their function depends greatly on water availability. In the southern Appalachians, models of future climate change predict alterations to the timing and intensity of storms such that most precipitation may be relegated to winter and spring. During the summer and fall, relatively less precipitation will translate to lower stream flows in systems that rarely experience such a lack of water. Given these predicted changes to the hydrologic cycle, I experimentally reduced flow to downstream sections of three streams at the Coweeta Hydrologic Laboratory in NC to assess changes to function in perennial ecosystems. The questions that I addressed included: 1) How is organic matter decomposition regulated by changes to the availability of water? and 2) How does the relationship between nutrient uptake and metabolism change under conditions of varying water availability? The availability of water (as discharge) was shown to be a major control of ecosystem function throughout these studies. Rates of leaf decomposition varied between red maple (Acer rubrum L.) and white oak (Quercus alba L.) with lower discharge in the early autumn regulating the breakdown trajectories of leaves through facilitation of colonization by microbes and macroinvertebrates. The return of water during the winter accelerated decomposition rates in the diverted sites such that mass of leaves remaining were similar to those in upstream sections. Colonization of decomposing organic matter by heterotrophic microbes (especially fungi) increased N immobilization leading to an increase in respiration per unit leaf standing stocks during the fall. Nitrification was detectable during summer low flows when leaf standing stocks were low. Changes in the timing and intensity of precipitation and thus discharge may in turn alter the temporal dynamics of ecosystem function. Leaves may remain in the stream unprocessed which will change the availability of food for macroinvertebrates, the production of which provides nutrition to higher trophic levels. Local-scale differences in organic matter processing and nutrient immobilization may translate to regional differences in food availability over both time and space. Hydrology not only acts as a local control of endogenous processes but acts also regionally through the transport of resources and nutrients to downstream reaches.

Browsing by Author "Scott, Durelle T."

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