Browsing by Author "Lu, Jianbiao"

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    Modeling Hydrologic Responses to Forest Management and Climate Change in Contrasting Watersheds in the Southeastern United States
    (2006-04-07) Lu, Jianbiao; Dr. James D. Gregory, Committee Chair; Dr. Ge Sun, Committee Co-Chair; Dr. James M. Vose, Committee Member; Dr. Steven G. McNulty, Committee Member
    Hydrologic pathways and processes vary greatly from the coastal plain to the mountainous upland across the southeastern United States due to large physiographic and climatic gradients. The coastal plain is generally a groundwater dominated system with a shallow water table, while the mountainous upland is hillslope controlled system. It was hypothesized that these two different regions have different hydrologic responses to forest management and climate change due to different conditions: topography, climate, soil, and vegetation. The hydrologic impacts of climate change and forest management practices are complex and nonlinear, and a model is an advanced tool for addressing such tasks. The objectives of this study were: 1) to evaluate the applicability of a physically-based, distributed hydrologic modeling system - MIKE SHE/MIKE 11 - in the southeastern United States; and 2) to use the MIKE SHE/MIKE 11 modeling system to examine the hydrologic processes and responses to forest management practices and climate change on the coastal plain and the mountainous upland in the southeastern United States. Four experimental watersheds, three wetlands on the coastal plain and one Appalachian mountainous upland, were selected. The model was first evaluated to determine if it could sufficiently describe the hydrological processes in these diverse watersheds in two contrasting regions. Next, the model was applied to simulate the hydrologic impacts of forest management and climate change at the four study sites, four simulation scenarios per site. These included the base line, clearcut, 2 °C temperature increase, and 10% precipitation decrease scenarios. Water table level and streamflow amount were two responses used to evaluate the forest management and climate change impacts. This study indicated that forest management and climate change would have potential impacts on the wetland water table, especially during dry periods. The absolute magnitudes of streamflow reduction were larger in a wet year than in a dry year for the two watersheds under both climate change scenarios (2 °C temperature increase and 10% precipitation decrease). In terms of streamflow reduction percentages, the results seemed to suggest that climate change would have larger impacts on the coastal plain than the mountainous upland. However, more field data and research are needed to further test this hypothesis. This study showed that MIKE SHE over-predicted soil evaporation from harvested lands. Thus, the model may have underestimated the streamflow impacts under the clearcut scenario. A process-based evapotranspiration model is needed to fully describe soil evaporation processes under forest harvest conditions.
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    Modeling Regional Evapotranspiration for Forested Watersheds across the Southern United States
    (2002-06-20) Lu, Jianbiao; Ge Sun, Committee Chair
    Evapotranspiration (ET) is the process that returns water to the atmosphere and therefore completes the hydrologic cycle. ET is a major component in the hydrological balance, and therefore is important to understanding forest water yield, sediment and nutrient movement. However, direct measurement of forest ET for a large region is not possible. The objectives of this study were to develop a model to estimate long-term annual actual evapotranspiration (AET) for forested watersheds across the southern United States (U.S.) and to compare the differences among six potential evapotranspiration (PET) methods. The developed AET model will be used to study hydrologic effects of climate and landuse changes. The six compared PET methods include three temperature-based methods (the Thornthwaite, Hamon and Hargreaves-Samani method) and three radiation-based methods (the Turc, Makkink and Priestley-Taylor method). A GIS database including land cover, hydrology and climate was developed for thirty-nine forested watersheds across the southern U.S.. Based on these data, a long-term annual AET model was developed. The independent variables included in the model are rainfall, latitude, elevation and percentage of land cover of conifer forests and water body in the watersheds. The model has a R2 of 0.85 and is sufficient to predict long-term annual AET for forested watersheds across the southern U.S.. Six PET methods were highly correlated but significantly different from each other. Greater differences were found among the temperature-based PET methods than radiation-based PET methods. In comparisons of the six PET methods, the Priestley-Taylor, Hamon and Turc methods performed better than the Thornthwaite, Makkink and Hargreaves-Samani methods.