Browsing by Subject "decomposition"
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Item Burial and decomposition of particulate organic matter in a temperate, siliciclastic, seasonal wetland(2009-05-15) Welsh, Lisa WilliamsonUnderstanding the role of freshwater wetlands in the global carbon cycle has become more important as evidence of climate change grows. In this paper, we examine the burial and decomposition of particulate organic matter (POM) in a temperate, siliciclastic, seasonal wetland. High POM abundances are found in silt layers, while sand units preserve very little POM. The POM distribution with depth is compared to the biogeochemistry of sediment porewater with depth. POM acts as a driver for reduction reactions within the wetland soil. Porewater biogeochemistry and POM decomposition are controlled by seasonal changes in the level of the water table which cause seasonal shifts in the oxic/anoxic boundary. At the oxic/anoxic boundary, reoxidation of FeS minerals in the soil cause rapid POM decomposition at the average minimum water table level in the late summer and early fall. Variation in the minimum depth of the water table from year to year may account for fluctuating POM numbers in the upper silt layers. The results from this study can be used to predict seasonal water level fluctuations in ancient wetland and to explain recurrence horizons in peat.Item Computational Study of Mean-Risk Stochastic Programs(2013-05-01) Cotton, Tanisha GreenMean-risk stochastic programs model uncertainty by including risk measures in the objective function. This allows for modeling risk averseness for many problems in science and engineering. This dissertation addresses gaps in the literature on stochastic programs with mean-risk objectives. This includes a need for a computational study of the few available algorithms for this class of problems. The study was aimed at implementing and performing an empirical investigation of decomposition algorithms for stochastic linear programs with absolute semideviation (ASD) and quantile deviation (QDEV) as mean-risk measures. Specifically, the goals of the study were to analyze for specific instances how algorithms perform across different levels of risk, investigate the effect of using ASD and QDEV as risk measures, and understand when it is appropriate to use the risk-averse approach over the risk-neutral one. We derive two new subgradient based algorithms for the ASD and QDEV models, respectively. These algorithms are based on decomposing the stochastic program stage-wise and using a single (aggregated) cut in the master program to approximate the mean and deviation terms of the mean-risk objective function. We also consider a variant of each of the algorithms from the literature in which the mean-risk objective function is approximated by separate optimality cuts, one for the mean and one for the deviation term. These algorithms are implemented and applied to standard stochastic programming test instances to study their comparative performance. Both the aggregated cut and separate cut algorithms have comparable computational performance for ASD, while the separate cut algorithm outperforms its aggregate counterpart for QDEV. The computational study also reveals several insights on mean-risk stochastic linear programs. For example, the results show that for most standard test instances the risk-neutral approach is still appropriate. We show that this is the case due to the test instances having random variables with uniform marginal distributions. In contrast, when these distributions are changed to be non-uniform, the risk-averse approach is preferred. The results also show that the QDEV mean-risk measure has broader flexibility than ASD in modeling risk.Item Development of Spatio-Temporal Wavelet Post Processing Techniques for Application to Thermal Hydraulic Experiments and Numerical Simulations(2012-07-16) Salpeter, NathanielThis work focuses on both high fidelity experimental and numerical thermal hydraulic studies and advanced frequency decomposition methods. The major contribution of this work is a proposed method for spatio-temporal decomposition of frequencies present in the flow. This method provides an instantaneous visualization of coherent frequency ?structures? in the flow. The significance of this technique from an engineering standpoint is the ease of implementation and the importance of such a tool for design engineers. To validate this method, synthetic verification data, experimental data sets, and numerical results are used. The first experimental work involves flow through the side entry orifice (SEO) of a boiling water reactor (BWR) using non-intrusive particle tracking velocimetry (PTV) techniques. The second experiment is of a simulated double ended guillotine break in the prismatic block gas cooled reactor. Numerical simulations of jet flow mixing in the lower plenum of a prismatic block high temperature gas cooled reactor is used as a final data set for verification purposes as well as demonstration of the applicability of the method for an actual computational fluid dynamics validation case.Item Essays on the Effectiveness of Environmental Conservation and Water Management Policies(2012-10-19) Mezzatesta, MarianoAn awareness of the effect of agricultural production on the environment has led to the development of policies to mitigate its adverse effects. This dissertation provides analyses of agri-environmental policies designed to protect environmental assets, as well as analytical decision-making tools useful for conducting policy evaluations. The first essay employs propensity score matching techniques to estimate the additionality of federal agricultural conservation programs for six conservation practices for farmers in Ohio. Additionality is an important measure of the effectiveness of conservation programs in inducing an increase in the conservation effort of farmers. Results suggest that additionality is positive and statistically significant for all six conservation practices. However, while programs achieve positive additionality for all practice types, a comparison between conservation practices reveals that certain practice types achieve higher percent additionality than others. Such results, coupled with information on the environmental benefits obtained per practice, could prove useful to program managers for improving the effectiveness of conservation programs. The second essay develops a new methodology to decompose the additionality measure into the two effects induced by conservation programs: expansion versus the new adoption of conservation practices. To do so, the relative contributions of two types of farmers, prior-adopters and new-adopters, are estimated. Results of the decomposition reveal that the additionality for prior-adopters is not significant for all practice types. Instead, additional conservation effort comes from new-adopters adopting new practices. Second, decomposition estimates suggest that practice types with a greater fraction of enrolled farmers that are new-adopters achieve greater percent additionality than those with greater proportions of prior-adopters. This suggests that a farmers? history in conservation adoption has a significant influence on additionality levels. The final essay analyzes the effect of recent instream flow diversion-guidelines on agricultural water security and streamflows within a decentralized water management regime. Spatially-explicit economic and hydrologic models are integrated to evaluate the tradeoffs between salmon bypass-flows and agricultural water security for three different diversion-guidelines within a northern-California watershed. Results indicate that the most restrictive diversion-guideline provides the greatest protection of bypass-flow days within smaller watersheds; however, within larger watersheds protection is not as significant. Water security, however, decreases sharply under the strict and moderate diversion-guidelines, especially during dry years. Overall, results indicate that greater focus should be given to protecting streamflows in the smallest watersheds, and meeting human water needs during dry years, when agricultural water security is impacted the most.Item Invasive Allochthonous Input: the Chinese Tallow Tree and Stream Food Webs(2017-11-10) Sadeghian, Cyrus; Hargrave, Chad W.The invasive Chinese Tallow tree (Triadica sebifera or Sapium sebiferum) was heavily introduced to the southern United States from Asia at the turn of the 20th Century. A Tallow invasion can reduce richness within the plant community by direct competition and can decrease density of consumer communities by limiting basal food resources. Additionally, tallow leaches rapidly into aquatic systems, where a sharp increase in aerobic microbial decomposition can simultaneously drop dissolved oxygen and pH levels, thus causing mortality in macroorganisms. I predicted an input of solely Tallow leaves into mesocosms (artificial streams) would temporarily increase N & P concentration, algae concentration, invertebrate density, and fishes because of increased rates of rapid decomposition, but would fail to sustain long-term and overall growth for that same reason. In contrast, sycamore leaves would provide a more sustained, long-term allochthonous subsidy to the mesocosms, thus resulting in increased growth of fishes and other response variables relative to that of tallow leaf treatments. Cellulose paper was used as a no-leaf control to account carbon input. Bullhead minnows (Pimephales vigilax) were stocked in half of our experimental stream mesocosms, where benthic algae, invertebrate density, and nutrient content were sampled for 16 weeks. After 16 weeks, 50% of the original Sycamore leaves remained, whereas less than 10% of the cellulose paper (control) and Tallow remained. The concentration of benthic algae was highly dependent on time as values increased significantly after week 6, and tended to be higher in treatments with no-fish and cellulose/tallow. Invertebrate density generally iv remained higher in no-fish treatments; densities were highest in tallow treatments before the experimental halfway point, and were highest in sycamore treatments past the halfway point. Nitrogen and phosphorous concentrations fluctuated highly throughout the experimental runtime, and showed no significant interactions among treatment groups. Fish were largest in mesocosms with the Sycamore leaves where either the undecomposed leaves provided additional surface area for food resources, or growth was higher relative to tallow treatments because of possible physiological inhibition. Tallow treatment fish growth was significantly smaller than sycamore, and relatively equal to cellulose. Herein, we discuss the reasons for the decreased fish growth in the presence of tallow leaves such as: (1) rapid decomposition providing a short-term nutrient pulse that moved through the food web quickly, and (2) physiological inhibition from the chemical composition of tallow leaves.Item The Political-Economic and Demographic Causes of Metropolitan Income Inequality and Its Components(2010-07-14) Chen, XIThis research project examines variations in inequality in individual earned incomes across U.S. metropolitan areas. The main analysis includes thirteen explanatory variables from three major perspectives - the political economy perspective, the demand-side perspective and the labor force supply-side perspective. In addition, I applied path models to explain causalities between some independent variables and used the decomposition of the Theil index to show the between-group effects. The results indicate that most demand-side and supply-side factors significantly contribute to variances in metropolitan income inequalities, while the impact of political economic factors are very limited. The paper is organized in the following manner: Chapter I is the introduction; Chapter II reviews literature focusing on the level of earning inequality and its predictors; Chapter III describes data and measures of variables; Chapter IV introduces statistical methods (including OLS regression model, path analysis, and decomposition of the Theil index); Chapter V presents the results of OLS regression model and its explanations; Chapter VI explains path analysis and decomposition analysis and their results; and finally, Chapter VII discusses the current research project and its implications for future studies.Item The use of ?15N to examine past mangrove stand structures(2009-06-02) Gudeman, Stephanie M.Twin Cays, Belize, is dominated by Rhizophora mangle L. (red mangrove). Tall (>5m in height) R. mangle are located along the fringe of the island and dwarf R. mangle grow in the interior of the island. These stand structures can be differentiated using ?13C and ?15N analysis (mean tall ?13C = -28 ?, mean tall ?15N = 0?; mean dwarf ?13C = -25?, mean dwarf ?15N = -10?), which may also prove useful in examining past mangrove stand structures from sediment cores. 15N label was traced in R. mangle leaves in a laboratory and field experiment over three months. The 15N label was examined to determine distribution of nitrogen in various biochemical fractions of the leaf and to verify if nitrogen is fractionated in a predictable manner over time. This information could be beneficial in examining past mangrove stand structures. Experimental data indicate that nitrogen is mobile within each biochemical fraction of the R. mangle leaf over time and a measurable amount of nitrogen exists in each fraction after 3 months of incubation. Nitrogen immobilization was evident in each experiment, as the ?15N values decreased ~200? in each of the labeled fractions of the laboratory experiment, which was mirrored by an increase in ?15N in the control samples. The amount of nitrogen in the biochemical fractions of the field experiment varied over time either increasing or decreasing, which may be due to the various environmental conditions such as tidal fluctuation, temperature, oxygen concentrations and microbial activity. The ?15N signature of the residual nitrogen fraction (?15N = 87?) reflected that of the bulk fraction (?15N = 133?) in the laboratory experiment as well as in the field experiment (residual nitrogen ?15N = 759?, bulk ?15N = 770?). To use isotope analysis to examine past mangrove stand structures it is essential that the original signature be maintained over time. The results of this study indicated that the simple interpretation of nitrogen isotopes may not be useful in examining past mangrove stand structures due to the variation over time, although this type of analysis may be considered if coupled with additional proxies and diagenetic factors are taken into account.Item Thermocatalytic decomposition of vulcanized rubber(Texas A&M University, 2007-04-25) Qin, FengUsed vulcanized rubber tires have caused serious trouble worldwide. Current disposal and recycling methods all have undesirable side effects, and they generally do not produce maximum benefits. A thermocatalytic process using aluminum chloride as the main catalyst was demonstrated previously from 1992 to 1995 in our laboratory to convert used rubber tire to branched and ringed hydrocarbons. Products fell in the range of C4 to C8. Little to no gaseous products or fuel oil hydrocarbons of lower value were present. This project extended the previous experiments to accumulate laboratory data, and provide fundamental understanding of the thermocatalytic decomposition reaction of the model compounds including styrene-butadiene copolymers (SBR), butyl, and natural rubber. The liquid product yields of SBR and natural rubber consistently represented 20 to 30% of the original feedstock by weight. Generally, approximately 1 to 3% of the feedstock was converted to naphtha, while the remainder was liquefied petroleum gas. The liquid yields for butyl rubber were significantly higher than for SBR and natural rubber, generally ranging from 30 to 40% of the feedstock. Experiments were conducted to separate the catalyst from the residue by evaporation. Temperatures between 400 ????C and 500 ????C range are required to drive off significant amounts of catalyst. Decomposition of the catalyst also occurred in the recovery process. Reports in the literature and our observations strongly suggest that the AlCl3 forms an organometallic complex with the decomposing hydrocarbons so that it becomes integrated into the residue. Catalyst mixtures also were tested. Both AlCl3/NaCl and AlCl3/KCl mixtures had very small AlCl3 partial pressures at temperatures as high as 250 ????C, unlike pure AlCl3 and AlCl3/MgCl2 mixtures. With the AlCl3/NaCl mixtures, decomposition of the rubber was observed at temperatures as low as 150 ????C, although the reaction rates were considerably slower at lower temperatures. The amount of naphtha produced by the reaction also increased markedly, as did the yields of aromatics and cyclic paraffin. Recommendations are made for future research to definitively determine the economic and technical feasibility of the proposed thermocatalytic depolymerization process.