Theses and Dissertations from UMD
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Item LONGITUDINAL DYE DISPERSION AND SALT FLUX IN ESTUARIES(2017) Liu, Wei; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Estuarine dispersion plays an important role in determining the fate of waterborne materials. It is a long-standing question in estuarine dynamics that is still not well understood. This dissertation revisits this problem by utilizing two tracers: dye and salt. Dye-release experiments and numerical modeling are conducted to investigate horizontal dispersion in a partially mixed estuary. Longitudinal dispersion of a dye patch shows strong flood-ebb asymmetry at early times after a dye release, with most of the dispersion occurring during ebb tides. Tidal straining enhances vertical current shear on ebb tides and promotes longitudinal dispersion. There are also large differences in the dispersion rate between spring and neap tides. Due to strong spring mixing, a dye patch quickly extends from the bottom to the surface, exposing to the full vertical shear in the water column and leading to strong longitudinal dispersion. In contrast most of the dye patch is limited to bottom few meters during neap tides. Although weak vertical mixing facilitates longitudinal dispersion, the vertical shear across the thin dye patch is much weaker, leading to weak longitudinal dispersion during neap tides. In first four tidal cycles, the second moment of the dye patch in the along-channel direction increases with time at a power of between 2 and 3. The longitudinal dispersion rate varies as the four-third power of the dye patch size, indicating scale-dependent diffusion. Salt dispersion and transport are examined in a comparative numerical modeling study between the partially-mixed Chesapeake Bay and the well-mixed Delaware Bay. To investigate how different physical mechanisms drive the salt transport into the estuaries, the longitudinal salt fluxes are decomposed using the Eulerian and quasi-Lagrangian methods. Under the Eulerian framework, the salt flux is decomposed into three parts: an advective term associated with the barotropic forcing, a steady shear dispersion term associated with the estuarine exchange flow, and a tidal oscillatory salt flux. In both estuaries, the advective term is dominant over steady shear dispersion and tidal oscillatory salt flux in the temporal variation of total salt flux. In Chesapeake Bay, the steady shear dispersion is the dominant mechanism and the tidal oscillatory salt fluxis small. In Delaware Bay, the steady shear dispersion and tidal dispersion are comparable. The along-channel variation of tidal oscillatory salt flux is mainly due to changes of the phase difference between the tidal current and salinity. Isohaline analysis using the quasi-Lagrangian methodology yields a new interpretation of the estuarine exchange flows and describes the evolution path of salinity classes.Item PERCEIVED ORGANIZATIONAL SUPPORT: LINKING HUMAN RESOURCE MANAGEMENT PRACTICES WITH IMPORTANT WORK OUTCOMES(2004-06-16) Liu, Wei; Bartol, Kathryn M.; Management and Organization; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is widely believed that the implementation of progressive HR practices that affect employee skills, motivation, and behaviors can create strategic advantage for the organization (e.g., Arthur, 1994; Delaney and Huselid, 1996; Snell & Dean, 1992). However, the intervening mechanisms of this effect are yet to be examined. Given the strategic value of HR in successfully cultivating organizational culture and social relationships that cannot be readily replicated (Becker & Gerhart, 1996), researchers have pointed to the need to investigate the role of social relationships in the HR literature (Uhl-Bien, Graen, & Scandura, 2000). Drawing on social exchange theory (Blau, 1964) and organizational support theory (Eisenberger, Huntington, Hutchinson, & Sowa, 1986), this dissertation aims to meet this need by examining the role of perceived organizational support (POS) in linking employee perceptions of HR practices and important work outcomes. It is hypothesized that employee perceptions of the HR practices implemented that demonstrate organizational recognition of employee contribution and caring about employee well-being will increase employee POS, which in turn, will lead to positive employee attitudes and behaviors. Further, it is also proposed that employees' professional commitment will interact with POS to influence the outcomes. Survey data were collected from 193 information technology workers and their direct supervisors within a large corporation. Structural equation modeling was used to examine the hypothesized model, and hierarchical regression analysis was conducted to test the interaction effects. As expected, it was found that employee perceptions of HR practices, including pay level, career development opportunities, work-family support, and relationships with the leader, are positively related to their POS. Higher levels of POS, in turn, resulted in lower turnover intention, and a sense of obligation within the individuals to reciprocate the organization with better job performance and increased OCBs, as well as stronger organizational commitment. In addition, the effects of POS on turnover intention, felt obligation, and organizational commitment were found to be moderated by professional commitment. The findings of this study shed some light on the process through which the implementation of appropriate HR practices may influence employee attitudes and behaviors.