UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Essays on Climate Change Impacts and Adaptation for Agriculture
    (2013) Ortiz Bobea, Ariel; Just, Richard E; Agricultural and Resource Economics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Over the past twenty years economists have developed econometric approaches for estimating the impacts of climate change on agriculture by accounting for farmer adaptation implicitly. These reduced-form approaches are simple to implement but provide little insights into impact mechanisms, limiting their usefulness for adaptation policy. Recently, conflicting estimates for US agriculture have led to research with greater emphasis on mechanisms including renewed interest in statistical crop yield models. Findings suggest US agriculture will be mainly and severely affected by an increased frequency of high temperatures with crop yield suggested as a major driver. This dissertation is comprised of three essays highlighting methodological aspects in this literature. It contributes to the ongoing debate and shows the preeminent role of extreme temperature is overestimated while the role of soil moisture is seriously underestimated. This stems from issues related to weather data quality, the presence of time-varying omitted weather variables, as well as from modeling assumptions that inadvertently underestimate farmers' ability to adapt to seasonal aspects of climate change. My work illustrates how econometric models of climate change impacts on crop production can be improved by structuring them to admit some basic principles of agronomic science. The first essay shows that nonlinear temperature effects on corn yields are not robust to alternative weather datasets. The leading econometric studies in the current literature are based on a weather dataset that involves considerable interpolation. I introduce the use of a new dataset to agricultural climate change research that has been carefully developed with scientific methods to represent weather variation with one-hour and 14 kilometer accuracy. Detrimental effects of extreme temperature crucially hinge upon the recorded frequency at the highest temperatures. My research suggests that measurement error in short amounts of time spent at extreme temperature levels has disproportionate effects on estimated parameters associated with the right tail of the temperature distribution. My alternative dataset suggests detrimental temperature effects of climate change over the next 50-100 years will be half as much as in leading econometric studies in the current literature. The second essay relaxes the prevalent assumption in the literature that weather is additive. This has been the practice in most empirical models. Weather regressors are typically aggregated over the months that include the growing season. Using a simple model I show that this assumption imposes implausible characteristics on the technology. I test this assumption empirically using a crop yield model for US corn that accounts for differences in intra-day temperature variation in different stages of the growing season. Results strongly reject additivity and suggest that weather shocks such as extreme temperatures are particularly detrimental toward the middle of the season around flowering time, which corrects a disagreement of empirical yield models with the natural sciences. I discuss how this assumption tends to underestimate the range of adaptation possibilities available to farmers, thus overstating projected climate change impacts on the sector. The third essay introduces an improved measure of water availability for crops that accounts for time variation of soil moisture rather than season-long rainfall totals, as has been common practice in the literature. Leading studies in the literature are based on season-long rainfall. My alternative dataset based on scientific models that track soil moisture variation during the growing season includes variables that are more relevant for tracking crop development. Results show that models in the literature attribute too much variation in yields to temperature variation because rainfall variables are a crude and inaccurate measure of the moisture that determined crop growth. Consequently, I find that third of damages to corn yields previously attributed to extreme temperature are explained by drought, which is far more consistent with agronomic science. This highlights the potential adaptive role for water management in addressing climate change, unlike the literature now suggests. The fourth essay proposes a general structural framework for analyzing the mechanisms of climate change impacts on the sector. An empirical example incorporates some of the flexibilities highlighted in the previous essay to assess how farmer adaptation can reduce projected impacts on corn yields substantially. Global warming increases the length of the growing season in northern states. This gives farmers the flexibility to change planting dates that can reduce exposure of crops during the most sensitive flowering stage of the crop growth cycle. These research results identify another important type of farmer adaptation that can reduce vulnerability to climate change, which has been overlooked in the literature but which becomes evident only by incorporating the principles of agronomic science into econometric modeling of climate change impact analysis.
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    Morphology in Urbanized Streams of the Puget Sound Lowland Region
    (2004) Boyle, Pamela; Prestegaard, Karen; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The purpose of this research is to evaluate the effects of urbanization on channel morphology. Three hypotheses are tested: 1) Channel morphology measured from one cross section is not similar to reach-averaged values, 2) Channel shear stress ratios and erosivity increase with urbanization, and 3) Channel morphological complexity decreases with urbanization increases. Results indicate that single cross-section data do not adequately describe channel morphology. Shear stress and bed mobility did not increase with urbanization, perhaps due to the presence of large bed grain sizes that adjust to increases in flow. Similarly, channel complexity did not decrease with increased urbanization. These data indicate that channel changes resulting from urbanization are influenced by sediment supply as well as discharge, and that this should be taken into consideration in restoration design.
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    MORPHOLOGY IN URBANIZED STREAMS OF THE PUGET SOUND LOWLAND REGION
    (2004) Boyle, Pamela Roxana; Prestegaard, Karen; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Increased runoff from urbanization may result in erosion to the stream channel and banks, leading to channel incision, bed changes, loss of instream debris and habitat, and an overall reduction of heterogeneity and channel complexity. These impacts are especially evident in low gradient, gravel-bed, meandering streams - the major type of stream in the Puget Sound Lowland region. The failure of many stream restoration projects is due to a lack of understanding of how morphological features of a stream respond to hydrological changes. Single cross-section methods (instead of reach-level) are generally used and may not adequately portray the complexity, or variation, of the stream channel and bed. Three main hypotheses in this thesis are: 1) a single cross-section taken within a reach does not adequately describe a stream compared to a mean value calculated from several measurements; 2) urban streams with more urbanized drainage areas have higher shear stresses, and thus move larger bed particles and have higher reach mobility; and 3) urban channels have less channel complexity than non-urban channels. Results showed that a single cross-section may not adequately describe the morphological variables of a stream reach; however, this method may be appropriate for calculating reach shear stress. In addition, shear stress and mobility were not found to increase with increasing urbanization. Furthermore, complexity was not found to decrease with increasing urbanization. These two latter results indicate that urbanization (or percent imperviousness) alone cannot be used as a variable to investigate changes in stream morphology and hydraulics. In fact, a measure of sediment supply could be considered an additional independent variable by which to study urbanization impacts to streams. Substrate distributions from this thesis also support this finding.