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Item Tracing interactions of hydrogeology and land use in two Montana watersheds(Montana State University - Bozeman, College of Agriculture, 2024) Keeshin, Skye Ilan; Chairperson, Graduate Committee: Stephanie A. EwingHydrogeologic systems dictate the introduction, transport, processing, and mixing of groundwaters, with implications for both groundwater and surface water quality. Land use can transform hydrogeologic processes and water quality through contributions of human amendments, alteration of soil and aquifer materials, and redistribution and consumption of water resources. Groundwater transit times are also orders of magnitude longer than those of surface water systems, resulting in lag times in water quality changes. In this thesis, I examine water quality consequences of land use in Montana at two headwater sites using geochemical tracers in groundwater and surface water. The first is within the Powder River Basin, the largest coal producing region in the US, where manmade aquifers composed of spoils from mine overburden replace existing unconsolidated and bedrock aquifers with salinity effects on downgradient waters. In a reclaimed coulee supplying groundwater to Rosebud Creek, a tributary of the Yellowstone River, geochemical and isotopic tracers reveal lateral contributions that contribute to dilution of high salinity mine-derived waters. These contributions include local inputs from shallow unconsolidated and bedrock aquifers and inputs of water on the order of 10,000 years from regional bedrock systems. The second site is the Gallatin Valley, a rapidly urbanizing intermountain basin in the Upper Missouri headwaters, where groundwater and surface water transects reveal mixing of water with a range of ages from a few years to 100,000 years. While not associated with spatial variation in nitrate concentration, these contributions likely attenuate rising nitrate concentrations in the valley aquifer as a whole over time, reflecting spatially variable loading from a legacy of agricultural fertilization and increasingly prevalent septic wastewater systems. A large component of the Gallatin Valley aquifer is decades old water sourced from higher elevation precipitation, consistent with long travelled mountain front stream losses. This component may diminish over time with an increasingly limited snowpack. Overall, hydrogeologic systems in these two land use regimes limit but do not eliminate effects of human-derived water quality concerns, and documenting them will improve water quality forecasting with impending changes in snowpack and precipitation.Item Understanding hydrogeomorphic influences on stream network denitrification and temperature dynamics(Montana State University - Bozeman, College of Agriculture, 2020) Carlson, Samuel Paul; Chairperson, Graduate Committee: Geoffrey PooleThe removal of nitrate from stream networks through the process of denitrification is an important component of local and regional nutrient cycles, but the controls on stream network denitrification rates remain poorly understood. Previous work has demonstrated general effects of stream channel size and nitrate loading rates on network-scale denitrification rates, but has been unable to elucidate connections between the complex environmental template of streams, and resulting denitrification rates. Understanding links between land use and management practices, physical characteristics of streams, and stream denitrification rates is critical to interpreting observed patterns of nitrate in freshwater systems and forming holistic management strategies for reducing the negative effects of elevated nitrate concentrations. To address these critical uncertainties, I developed a stream network simulation model that incorporates the effects of whole-stream aerobic respiration on biotic denitrification demand. This model is applied to a small, subalpine stream network under scenarios designed to explore: 1) the implications of temperature-controlled, network scale patterns of respiration rates on the distribution and overall magnitude of stream network denitrification, and 2) the effect of logging-induced channel simplification on whole network denitrification rates. The first analysis is complimented by an evaluation of controls on stream temperature across this network, revealing the spatially and temporally variable influence of in-network lakes on stream temperatures. Results from the first analysis suggest that reach- and network-scale denitrification rates are strongly influenced by respiration rate and temperature when nitrate supplies are high relative to removal rates, indicating an increased contribution of lower, warmer streams to whole-network denitrification. The second analysis reveals that historical logging has caused a ~30% loss of stream network denitrification capacity, which is manifested as a corresponding reduction in whole-network denitrification rates when nitrate supplies are not limiting. In sum, this work emphasizes the diverse set of factors that influence reach- and watershed-scale biogeochemical characteristics and processes, and suggests that land management actions which influence stream morphology may also alter stream denitrification rates.Item Investigating the impacts of agricultural land use change on regional climate processes in the northern North American Great Plains(Montana State University - Bozeman, College of Agriculture, 2021) Bromley, Gabriel Trees; Chairperson, Graduate Committee: Paul C. Stoy; Jack Brookshire (co-chair); Tobias Gerken, Andreas F. Prein and Paul C. Stoy were co-authors of the article, 'Recent trends in the near-surface climatology of the northern North American Great Plains' in the journal 'Journal of climate' which is contained within this dissertation.; Andreas F. Prein, Shannon Albeke and Paul C. Stoy were co-authors of the article, 'Simulating the impacts of agricultural land use change on the climate of the northern North American Great Plains: validating a convection-permitting climate model' submitted to the journal 'Climate dynamics' which is contained within this dissertation.; Andreas F. Prein, Shannon Albeke and Paul C. Stoy were co-authors of the article, 'The decline in summer fallow in the northern plains cooled near-surface climate but had minimal impacts on precipitation' submitted to the journal ' ' which is contained within this dissertation.; Andreas Prein and Paul C. Stoy were co-authors of the article, 'Recent enhancement of thermodynamic environments in the northern North American Great Plains' submitted to the journal 'Geophysical research letters' which is contained within this dissertation.The northern North American Great Plains (NNAGP) is the area defined by the Upper Missouri River Basin and the Canadian Prairies. It is a semi-arid region categorized by large stretches of grassland, pasture, and crops. During the last century and extending to the present day, a standard agricultural practice was to utilize a wheat-summer fallow rotation schedule, where the fields were left unplatted and an herbicide was often applied to keep weeds at bay. Concerns over soil health and profitability have led to the systematic decline of summer fallow, and nearly 116,000 km 2 that used to be fallow during the summer in the 1970s are now planted. An observational analysis discovered that from 1970-2015, during the early warm season, the NNAGP have cooled at -0.18 °C decade -1, nearly the same magnitude as the annual global warming rate. The near-surface atmosphere also moistened, evidenced by a decreasing vapor pressure deficit (VPD) trend, and monthly mean precipitation increased in excess of 8 mm per decade. Monthly mean convective available potential energy (CAPE) increased by 80% at Glasgow, MT and by 35% at Bismarck, ND based on atmospheric sounding observations. To test whether a reduction in summer fallow is responsible for these observed changes, a set of convection-permitting model experiments were performed over the NNAGP. Two sets (4 total) of three-year simulations were driven by ERA5 data with the vegetative fraction adjusted using satellite estimated fallow amounts for 2011 and 1984. The control simulations were extensively validated against an ensemble of observations with large temperature biases in Winter by ~ -3 °C and Summer by ~3°C. The areas where fallow area declined from 1984-2011 were cooler by about 1.5 °C and had a lower VPD by 0.15 kPa compared to where it did not. CAPE increased where fallow declined from 1984-2011 but so did convective inhibition (CIN). These findings insinuate that the observed change to monthly mean precipitation cannot be explained by summer fallow reduction alone. Trends in observed low level moisture transport show that the Great Plains Low Level Jet has been intensifying, bringing increased moisture to the NNAGP and partially responsible for the precipitation increase.Item Development of a Montana land cover map from Landsat imagery(Montana State University - Bozeman, College of Agriculture, 1981) Smith, Philip WilliamItem Spatial and seasonal variability of watershed response to anthropogenic nitrogen loading in a mountainous watershed(Montana State University - Bozeman, College of Agriculture, 2010) Gardner, Kristin Kiara; Chairperson, Graduate Committee: Brian L. McGlynn.; Brian L. McGlynn was a co-author of the article, 'Seasonality in spatial variability and influence of land use/land cover and watershed characteristics on streamwater nitrogen export in a developing watershed in the Rocky Mountain West ' in the journal 'Water resources research' which is contained within this thesis.; Brian L. McGlynn was a co-author of the article, 'A multi-analysis approach to assess the spatio-temporal patterns of watershed response to localized inputs of nitrogen' in the journal 'Water resources research' which is contained within this thesis.; Brian L. McGlynn, and Lucy A. Marshall were co-authors of the article, 'Quantifying watershed sensitivity to spatially variable nitrogen loading and the relative importance of nitrogen retention mechanisms' in the journal 'Water resources research' which is contained within this thesis.Anthropogenic activity has greatly increased watershed export of bioavailable nitrogen. Escalating levels of bioavailable nitrogen can deteriorate aquatic ecosystems by promoting nuisance algae growth, depleting dissolved oxygen levels, altering biotic communities, and expediting eutrophication. Despite these potential detrimental impacts, there is notable lack of understanding of the linkages between anthropogenic nitrogen inputs and the spatial and seasonal heterogeneity of stream network concentrations and watershed nitrogen export. This dissertation research seeks to more accurately define these linkages by investigating the roles of landscape position and spatial distribution of anthropogenic nitrogen inputs on the magnitude and speciation of watershed nitrogen export and retention and how these roles vary seasonally across contrasting landscapes in a 212 km ² mountainous watershed in southwest Montana. Results indicate localized inputs of anthropogenic nitrogen occurring in watershed areas with quick transport times to streams had disproportionate effects on watershed nitrogen export compared to spatially distributed or localized inputs of nitrogen to areas with longer transport times. In lower elevation alluvial streams, these effects varied seasonally and were most evident during the dormant winter season by amplified nitrate peaks, elevated dissolved organic nitrogen:dissolved organic nitrogen (DIN:DON) ratios and lower dissolved organic carbon (DOC):total dissolved nitrogen (DOC:TDN). During the summer growing season, biologic uptake of nitrogen masked anthropogenic influences on watershed nitrogen export; however, endmember mixing analysis of nitrate isotopes revealed significant anthropogenic influence during the growing season, despite low nitrate concentrations and DIN:DON ratios. In contrast, streams draining alpine environments consisting of poorly developed, shallow soils and small riparian areas exhibited yearlong elevated nitrate concentrations compared to other sites, suggesting these areas were highly nitrogen enriched. Watershed modeling revealed the majority of watershed nitrogen retention occurred in the upland environment, most likely from biological uptake or lack of hydrologic connectivity. This work has critical implications for watershed management, which include: 1) developing flexible strategies that address varying landscape characteristics and nitrogen loading patterns across a watershed, 2) avoiding clustering nitrogen loading in areas with quick travel times to surface waters, 3) seasonal monitoring to accurately gauge watershed nitrogen saturation status, and 4) incorporating spatial relationships into streamwater nitrogen models.