Impact of Climate Variability and Change on Nitrogen Delivery from Watersheds to Waterbodies

Excess nitrogen in waterbodies is responsible for water pollution, eutrophication, and ecological deterioration around the world. Recent anthropogenic activities have greatly enhanced the release of fixed nitrogen to the environment. The nitrogen cycle has been altered even more than the carbon cycle due to the recent anthropogenic activities. Nitrogen export through streams and waterbodies depends on watershed nitrogen sources and transport mechanisms. Watershed attributes and hydrologic conditions affect nitrogen transport over the landscape to waterbodies. The goal of this work is to better understand these hydrologic transport mechanisms of nitrogen and, more specifically, how variability in hydrologic/climatic conditions affect past, present, and future nitrogen export to waterbodies. This dissertation includes three independent but related studies that target this overarching research goal.;The objective of the first study was to quantify the relative effects in nitrogen export due to changes in hydrology/climate conditions compared to changes in nitrogen source contributions over a decade for the contiguous United States using the USGS SPARROW model. The conclusion was that large-scale watershed nitrogen export can be heavily driven by hydrologic conditions so that, even without changes in source input, drier hydrologic conditions can result in an overall reduction of nitrogen export.;The second study aimed to answer the question of how future climate conditions might affect nitrogen export in the contiguous United States, given the importance of climatic conditions to nitrogen export from watersheds. An ensemble of downscaled Global Circulation Model (GCM) outputs of future temperature and precipitation were used within a SPARROW model to estimate changes in nitrogen export rates. Results of the study show an overall decrease in nitrogen export due mainly to increases in air temperature, which is responsible for increased denitrification in landscapes and shallow waters.;The third study applied a physically-based watershed model (SWAT), rather than a statistical hybrid model (SPARROW) that was applied in the second study for the contiguous United States, to address the potential impact of future climate conditions on streamflow and nitrogen export. Results of this study are in line with the second study also indicating an overall decrease in nitrogen export in future climatic conditions. This study also suggests seasonal patterns in change in nitrogen export with increases in fall and winter months and decreases in spring and summer months over the next century.;Ultimately this work contributes to the understanding of weather and climate variability and its impact on regional-scale nitrogen exported from watersheds. Through analysis of the past conditions, this research shows that climate and weather conditions are a major driver for nitrogen export through hydrologic transport, and that weather and climate variability attributes for more of the observed changes in nitrogen export rates than variability in nitrogen sources over the period of analysis. Given this finding, future climatic conditions will have significant impact on nitrogen export. This research found that, for most of the contiguous United States, future climatic conditions will lessen nitrogen yield, although there are some regions where yield is projected to increase. This finding was verified for a smaller forested watershed using a physically-based model. For water resources management, special attention should be paid to regions where future climatic conditions will be more favorable for increased nitrogen export and the associated water quality problems that result from excess nitrogen.