Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item YEAR-ROUND DETERMINATION OF METHANE (CH4) SOURCES AND SINKS IN ARCTIC LAKES USING CONTINUOUS AND AUTONOMOUS SAMPLING(2020) McIntosh Marcek, Hadley; Lapham, Laura L; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Methane (CH4) is a potent greenhouse gas and its concentration has been increasing in the atmosphere. While natural emissions from inland water bodies are known to be important, there is large uncertainty in the amount of methane released from lakes to the atmosphere, especially from Northern latitudes. Part of this is due to limited sampling in these systems during dynamic periods, such as ice-over and ice-melt. To better understand these temporal dynamics, I used autonomous, continuous samplers (OsmoSamplers) to collect lake water year-round over two years (2015-2017). Lake water was collected at a fine temporal resolution to provide time-integrated (~1 week) samples from multiple Arctic lakes within the Mackenzie Delta. The Mackenzie Delta is a lake-rich, productive environment that is expected to be a significant source of methane to the atmosphere. Lakes spanning the central delta and outer delta were sampled for methane concentration and stable carbon isotope ratio (δ13C-CH4) changes, ion concentrations, and water column characteristics were measured with continuous sensor data (temperature, water pressure, conductivity, light, and dissolved oxygen). These unique time-series datasets show lakes exhibit a close coupling of dissolved oxygen, and other electron acceptors, with the timing of methane increasing during ice-cover. The increase in methane concentrations is primarily from diffusion out of sediments and possibly water-column methanogenesis. One lake in the outer delta exhibited thermogenic gas bubble dissolution that contributed to under-ice methane concentration increases. Following ice-melt, lake depth appears to impact methane release to the atmosphere. Shallower lakes exhibit rapid fluxes followed by significant microbial methanotrophy. Deeper lakes in the central delta are connected to groundwater, though it does not appear groundwater transports methane. This is the first study of dissolved methane and gas bubble 14C-age in the Mackenzie Delta and shows that dissolved methane is produced primarily from modern carbon sources, such as macrophyte biomass and terrestrial material, but some methane transported in gas bubbles is significantly older, with seeps in the outer delta rapidly releasing radiocarbon-dead, thermogenic methane. This study demonstrates the importance of multi-lake studies particularly with fine scale temporal sampling to understand methane processes in seasonally ice-covered lakes.Item INTERCOMPARISON AND VALIDATION OF CONTINENTAL WATER LEVEL PRODUCTS DERIVED FROM SATELLITE RADAR ALTIMETRY, MODELING AND FORECASTING TROPICAL LAKE LEVELS(2012) Ricko, Martina; Carton, James A; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on validating the use of satellite radar altimetry products to observe and forecast water level in lakes and reservoirs. Satellite measurements of lake and reservoir water levels complement in situ observations by providing stage information for ungauged basins and by filling data gaps in gauge records. Yet different satellite radar altimeter-derived continental water level products may differ significantly due to choice of satellites, geophysical corrections, etc. To explore the impacts of these differences, in the first part of this dissertation a direct comparison between three different altimeter-based lake level estimates is presented and validated with lake level gauge time series for lakes of a variety of sizes and conditions (e.g. whether they freeze seasonally). This comparison provides quantitative estimates of the error in lake levels as well as advice on product choices to end users. The largest discrepancies among the altimeter products occur for the lakes that freeze. In the second part of this dissertation a simple water balance model is developed relating net freshwater flux on a catchment basin to lake level. The model is constructed with two empirical parameters: effective catchment to lake area ratio and time delay between freshwater flux and lake level response. This model allows comparison of observed net freshwater flux with the lake level estimates from altimetry for a series of 12 tropical lakes distributed across three continents. The results show encouraging agreement between these independent datasets. The third part of this dissertation uses the simple lake model, developed in the second part of this dissertation, and applies it to NOAA's Climate Forecast System (CFS) coupled model thus allowing us to produce seasonal lake level forecasts based on seasonal predictions of net freshwater flux. In the CFS net freshwater flux data bias with respect to the independent reanalysis is determined. One example of such a lake level model forecast is presented, showing promising significant results over most examined tropical lakes, but failing for reservoirs and smaller lakes. Model forecast bias with respect to altimeter observations is proposed to be further investigated for multiple lead times.