Browsing by Subject "hydrology"
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Item An Investigation of Linked Physical And Biogeochemical Processes In Heterogeneous Soils In The Vadose Zone(2012-10-19) Hansen, David JosephChemical dynamics in the vadose zone are poorly understood due to the transient nature of chemical and hydrologic conditions, but are nonetheless critical to understanding contaminant fate and transport. This work explored the effects of soil structure (i.e. layers, lenses) on linked geochemical, hydrological, and microbiological processes under changing hydrologic conditions (e.g. rainfall, introduction of groundwater, and fluctuating water table heights). A homogenized medium-grained sand, homogenized organic-rich loam and a sand-over-loam layered column were constructed for the first series of experiments. The second series of experiments employed two soil columns with lenses that were packed identically with sterilized and untreated sediments. Each consisted of two lenses of organic-rich loam in a medium-grained sand matrix. Lenses were located at different vertical depths and were horizontally offset. In-situ collocated probes collected soil hydrologic and chemical data. In the layered column, enhanced biogeochemical cycling was observed over the texturally homogeneous soil columns. Enumerations of Fe(III) and SO42- reducing microorganisms also show 1-2 orders of magnitude greater community numbers in the layered column. The greatest concentrations of aqueous FeS clusters (FeSaq) were observed in close proximity to the soil interface. To our knowledge, this was the first documentation of FeSaq in partially saturated sediments. Mineral and soil aggregate composite layers were also most abundant near the soil layer interface; the presence of which, likely contributed to an order of magnitude decrease of hydraulic conductivity. In the live lens column, Fe-oxide bands formed at the fringes of the lenses that retarded water flow rates by an order of magnitude compared to the sterilized column. Microbial activity also produced insoluble gases and that led to the creation of a separate gas phase that reduced hydraulic conductivity. This limited the interaction between groundwater with soil-pore waters that led to the formation of geochemically distinct water masses in relatively close proximity to one another. No such changes were observed in the sterilized column. When compared to homogenous columns, the presence of soil heterogeneities altered biogeochemical and hydrologic processes considerably which highlights the need to consider soil heterogeneity in contaminant fate and transport models. These findings suggest that quantifying coupled hydrologic-biogeochemical processes occurring at small scale soil interfaces is critical to accurately describing and predicting chemical changes at the larger system scale.Item Characterizing and Modeling Arctic Shrub Expansion on the North Slope of Alaska, USA(2014-07-30) Naito, Adam TakashiShrub expansion is one of the most recognized components of terrestrial Arctic change and has been documented in studies involving fine-scale experimental manipulations or broad-scale satellite remote sensing. The characteristics and drivers of this phenomenon at the landscape scale, however, are understudied. The motivation of this dissertation was to develop an improved understanding of the historic spatial characteristics of shrub expansion on the North Slope of Alaska and its environmental drivers at this landscape scale. This work has three objectives, which include: 1) mapping and quantifying historic shrub expansion patterns; 2) examining a relationship between shrub expansion and its hydrological controls; and 3) designing and implementing a spatially-explicit simulation model to develop hypotheses regarding the landscape-scale drivers of shrub expansion (i.e., modes of reproduction, hydrological constraints, and their interactions). Shrubs maps were generated from semi-automated classification of historic vertical aerial photographs and contemporary high-resolution satellite imagery within a GIS. The spatial patterns of historic shrub expansion were quantified using FRAGSTATS and the multi-scale information fractal dimension. Relationships between shrub expansion and local hydrology was determined statistically through associations between areas that gained shrub cover and topographic wetness index values derived from a digital elevation model. The contribution of shrub reproductive characteristics was determined by developing a C#-based spatially-explicit simulation model that simulates clonal and sexual reproduction of shrubs. The reproductive mode(s) producing spatial patterns most similar to the observed patterns was determined through principal components analyses. Results from this work suggest that: 1) the shrub-tundra ecotone within river valleys on the North Slope is has either initiated or completed a phase transition from tundra to shrubland; 2) shrub development is promoted in areas where the potential for water accumulation or throughflow is higher; and 3) vegetative reproduction appears to have been dominant mode of reproduction . Considering our current understanding of the fine-scale relationships between shrub expansion and hydrology, surface energy balances, and C and nutrient cycling, continued expansion may have considerable implications for circumpolar tundra ecosystems. These findings will facilitate the development of improved projections of the structure and function of these ecosystems and their feedbacks to climate change.Item Hillslope Hydrological Processes in a Costa Rican Rainforest: Water Supply Partitioning Using Isotope Tracers(2014-04-24) DuMont, Andrea LynCosta Rican tropical premontane rainforests are among the world's most valuable ecosystems in terms of diversity of animals, plants, and natural resources. These environments are dependent on water resources which fluctuate in quantity during the dry and wet seasons and which are significantly influenced by vegetation feedbacks. Currently, tropical premontane forest watersheds are insufficiently characterized in terms of groundwater and stream water interactions due to their limited accessibility and complex geological conditions. However, water produced from these watersheds is a critical renewable resource in Costa Rica. It plays a significant role in the production of downstream hydropower and acts as a supply for water distribution systems in many rural areas. In this study, stable isotope tracing of ?^(18)O and ?D was used to determine the source of water in a stream, and the relative contributions of water budget components (e.g., groundwater, soil water). Samples were collected beginning in the dry season and continuing through the wet season from 2013-2014 as the soil became progressively wetter. The ?^(18)O and ?D samples represent precipitation in the tropical forest, as well as groundwater, soil water, and stream water at several locations. This data is important to understanding the influence of vegetation and hydrogeological properties on groundwater and stream water in tropical headwater catchments. Streamflow averaged 0.06 m^(3)/min in baseflow and greater than 0.10 m^(3)/min during storms. Groundwater was seen to contribute to 80% of streamflow and was the main stream component even during storm events. A small proportion of the total amount of streamflow came from interflow and soil water (1%). Additional findings indicated that precipitation, about 4200 mm/yr, in the rainforest can be recycled source water. Storm tracks alternate from distribution starting in the Pacific Ocean to the Caribbean Sea over the course of the wet season. Overall precipitation was seen to be dominated by deep convection and enhanced during the wet season due to the North American Monsoon and the Intertropical Convergence Zone.Item Hydrology of a Forested Wetland Complex in an Urbanizing Area of the Texas Gulf Coast and Clean Water Act Implications(2014-12-15) Dean, Dex DanielSwales in wetland complexes can provide evidence of hydrologic connectivity for wetlands on the Texas Gulf Coast, supporting the idea that many coastal wetlands in Texas are vitally connected to navigable waters covered by the Clean Water Act. In this study, runoff that accounted for more than 18% of rainfall was observed from a representative ?isolated? wetland complex?wetland depressions and upland areas interconnected by shallow erosional features?southeast of Houston, Texas between March 2005 and April 2010. Annual runoff ranged from 0% in 2005 to 27% in 2007. This result was surprising, given the presumably isolated nature of the wetlands. The wetland complex was predominantly forested, with emergent vegetation dominating some of the depressions. Measured hydrologic fluxes included: (1) rainfall, using a tipping-bucket rain gauge supplemented with official weather station data; (2) surface runoff, using a v-notch weir to measure discharge from a wetland swale; (3) transpiration of Quercus nigra (18.0 cm diameter) and Quercus pagodafolia (15.9 cm diameter) using the heat-dissipation sap flux method; (4) groundwater level changes, using piezometers, and (5) soil moisture changes, using soil moisture probes. Watershedscale evapotranspiration was estimated using the Hargreaves model. Surface runoff, although intermittent, occurred during 25 of 57 months. Monthly runoff ranged from 0% of rainfall to 57% of rainfall. Soil moisture loss trended with increased transpiration rates, where the Q. nigra specimen transpired 11.6 to 35.8 L d-1 and Q. pagodafolia specimen transpired 2.43 to 13.8 L d^-1. Moisture was depleted rapidly in the upper soil layer, emphasizing the importance of considering local weather patterns when identifying wetlands and making jurisdictional decisions. The results of this study call into question regulatory presumptions about coastal plains wetlands (at least 400,000 ha in Texas alone), of which roughly 50% are considered geographically isolated. One way to improve implementation of federal rules for wetlands similar to those in this study, which are reasonably close to both navigable and non-navigable streams, is for regulatory agencies to determine whether the wetlands are adjacent to a navigable water before making other decisions that would lead to a presumption that significant nexus does not exist.Item Infiltration under two contrasting hydrologic scenarios in Texas(Texas A&M University, 2005-08-29) Dasgupta, SurajitInvestigation of infiltration provides insights about the flow of water and transport of contaminants through the vadose zone. Infiltration is governed by prevailing environmental conditions like soil characteristics, plant cover and geologic settings. The main objective was to study preferential flow dominated infiltration at two contrasting hydrologic settings in Texas. For the first study, artificial rainfall was simulated within a plot covered with juniper trees at a karst region of the Edwards Plateau and sub-surface flow was monitored using TDR probes. Sub-surface flow was simulated using HYDRUS-2D. Results demonstrated that sub-surface flow occurred in a tri-modal manner, consisting of flow in karst conduits, planar fractures in the limestone, and soil matrix. Both fracture and matrix flow responses increased with increase in rainfall intensity. During large rainfall events, water exchange was observed between the fractures and matrix. Dye studies indicated that fractures and juniper roots were primary pathways for preferential flow occurring within the plot. The model simulated flow characteristics like exchange processes and differentiated between preferential and conduit flow besides determining approximate van Genuchten parameters for each geologic unit.For the second study, tension infiltrometers were used to conduct infiltration experiments at six soil water pressures (?? = -0.2 to 0 m) in an agricultural field near College Station over a 21 month period. The aim was to determine steady infiltration rate, if, saturated hydraulic conductivity, Ksat, unsaturated hydraulic conductivity, K(??) and unsaturated flux density ??(??). Moreover, the effect of varying disc diameters on steady state infiltration rates (if) was also studied. Results demonstrated that infiltration occurred in a bi-modal fashion consisting of preferential flow and matrix flow. Macropores and roots present in the soil resulted in gravity dominated flow at ?? = -0.05 to 0 m for all experiments. Statistical analysis suggested that the soil did not exhibit spatial variability within the plot and the five different disc diameters had no effect on if. Statistically significant differences in if were observed between 0.2 and 0.24 m disc diameters at saturation over the 21 month period. The if values illustrated strong temporal variations based on natural conditions over the 21 month period.Item Linking Burn Severity to Soil Infiltartion and Runoff in a Montane Watershed: Boulder, Colorado(2012-11-28) Ahlstrom, Anna 1988-Forest fires have an enormous impact on biotic and abiotic variables that control runoff and soil properties in watersheds. Because wildfires do not have a uniform effect on the burned area, significant variability occurs between areas of different burn severity and likely elicits different hydrologic responses within watersheds. Much of the control on this hydrologic response stems from the variability of soil between burned and unburned watersheds. Establishing a linkage between soil infiltration and burn severity may therefore, offer insight into the likelihood of elevated levels of runoff and the likelihood of floods. Although previous studies have sought to establish a quantitative relationship between runoff and burn severity, this relation has not been evaluated with respect to soil moisture and infiltration and varying degrees of burn severity. The Loretta-Linda Basin presents a unique opportunity to compare areas with different burn severities (with the right fork of the drainage experiencing a much higher burn severity than the left), while eliminating most other variables that may occur with greater spatial variability such as elevation, temperature, precipitation, underlying geology, and soil type. Rainfall, soil moisture, runoff, and infiltration data collected over a two-month period were used to evaluate the relationship between burn severity, runoff, and infiltration for the Loretta-Linda basin as a whole as well as for the individual forks of the basin. The impact of varying burn severity on the two sub drainages was further investigated by creating a dynamic simulation model in TopoFlow?. Comparative analysis between the two forks did not show a dramatic difference in the runoff and infiltration relationship between the two burn severities. Variability of field conditions, the presence of parameters affecting runoff not accounted for, and the limitations of point measurements, are reflected by the data analysis and lack of a strong correlation between burn severity, infiltration, and runoff. The use of spatial hydrologic modeling allowed for the investigation of the relative importance of the infiltration parameters as well as the impact of Manning?s n on the response of the basin to rainfall. The modeling results indicate a strong correlation between high burn severity, low infiltration capacity, and elevated discharge volumes.Item On the Predictive Uncertainty of a Distributed Hydrologic Model(2009-05-15) Cho, HuidaeWe use models to simulate the real world mainly for prediction purposes. However, since any model is a simplification of reality, there remains a great deal of uncertainty even after the calibration of model parameters. The model?s identifiability of realistic model parameters becomes questionable when the watershed of interest is small, and its time of concentration is shorter than the computational time step of the model. To improve the discovery of more reliable and more realistic sets of model parameters instead of mathematical solutions, a new algorithm is needed. This algorithm should be able to identify mathematically inferior but more robust solutions as well as to take samples uniformly from high-dimensional search spaces for the purpose of uncertainty analysis. Various watershed configurations were considered to test the Soil and Water Assessment Tool (SWAT) model?s identifiability of the realistic spatial distribution of land use, soil type, and precipitation data. The spatial variability in small watersheds did not significantly affect the hydrographs at the watershed outlet, and the SWAT model was not able to identify more realistic sets of spatial data. A new populationbased heuristic called the Isolated Speciation-based Particle Swarm Optimization (ISPSO) was developed to enhance the explorability and the uniformity of samples in high-dimensional problems. The algorithm was tested on seven mathematical functions and outperformed other similar algorithms in terms of computational cost, consistency, and scalability. One of the test functions was the Griewank function, whose number of minima is not well defined although the function serves as the basis for evaluating multi-modal optimization algorithms. Numerical and analytical methods were proposed to count the exact number of minima of the Griewank function within a hyperrectangle. The ISPSO algorithm was applied to the SWAT model to evaluate the performance consistency of optimal solutions and perform uncertainty analysis in the Generalized Likelihood Uncertainty Estimation (GLUE) framework without assuming a statistical structure of modeling errors. The algorithm successfully found hundreds of acceptable sets of model parameters, which were used to estimate their prediction limits. The uncertainty bounds of this approach were comparable to those of the typical GLUE approach.Item Restoration of resaca wetlands and associated wet prairie habitats at Palo Alto Battlefield National Historic Site(Texas A&M University, 2006-08-16) Margo, Michael RayCultivation and drainage projects associated with livestock production have substantially disturbed resaca wetlands and wet prairie habitats in southern Texas. As a consequence of the anthropogenic disturbances, the area of these wetlands has been reduced and the ecological integrity of the remaining wetlands has been compromised. The goal of this study was to explore effective strategies for ecological restoration of coastal prairie and resaca ecosystems in south Texas and provide restoration recommendations to the National Park Service at Palo Alto Battlefield National Historic Site (NHS). Field experiments were conducted to evaluate the effectiveness of different approaches for restoring Spartina spartinae on disturbed saline flats. A resaca hydrologic study was initiated to evaluate the groundwater hydrology in disturbed versus undisturbed resaca wetlands and explore potential restoration strategies. Transplanting S. spartinae in the fall season was more successful (80% survivability) than seeding (0% initial establishment), spring transplanting (0% survival), spring and fall mechanical transplanting (0% and 6% survivability, respectively). Soil disturbance significantly affected (p < 0.05) survival of transplanted tillers and basal diameter of both the bare root and container-grown transplants in the fall manual treatments. The initial hydrologic study of the resaca wetlands found that vegetation rooting zone hydrology was likely dependent on surface water rather than groundwater. These findings suggest that strategies that restore surface hydrologic regimes will likely restore the ecosystem structure and function of disturbed resacas. Manually transplanting bare-root stock of S. spartinae in the late fall season without soil disturbance will increase the likelihood of successful saline flat restoration.Item The distribution of dams in Costa Rica and their hydrologic impacts(Texas A&M University, 2006-04-12) Laurencio, Laura RichardsDam construction has increased exponentially over the past century, primarily in temperate environments. While the impacts of dams in temperate regions have been well-documented, a parallel level of research on dam impacts has not been achieved in tropical environments. The overall objective of this research was to understand the hydrologic impacts of dams in Costa Rica, a representative case study in a tropical environment. To achieve this objective, the following specific objectives were developed: 1) examine the spatial and temporal trend of large dam development within the country; 2) assess large-scale hydrologic impacts (at the national scale); 3) analyze downstream flow of individual dams to determine regional impacts. Analysis of the spatial trend of dam development utilized a geographic information system. The spatial distribution showed no apparent relation to hydroclimate, and additional land-use analysis indicated that basins containing large dams are primarily covered by either forest or crop. Assessment of large-scale impacts used potential reservoir storage to represent the hydrologic impact. Results indicate that large dams in Costa Rica are having a relatively low impact on the surface water component of the hydrologic cycle compared to temperate regions. However, this analysis revealed that two dams, Arenal and Sandillal, are having a disproportionately significant impact on their individual basins. Analysis of flow regime for individual dams followed standard hydrologic analyses of comparing pre- and post-dam discharge data. Variables analyzed included mean, minimum, and peak flows. Results of these analyses revealed that the Arenal- Corobic-Sandillal dam project have resulted in severe disruption to downstream hydrology for all three dams. In contrast, downstream of Ventanas Dam changes in downstream discharge were smaller than those documented for dams in temperate regions. The results of this research indicate that dam impacts in the tropics may be very different from those documented in temperate environments. Consequently, theories developed for temperate areas regarding expected dam impacts may not apply to tropical regions. This has important implications for hydrology, geomorphology and ecology. This study should serve as a step toward development of a more generalized theory of dam impacts in the tropics.