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Abstract
Nitrogen pollution in groundwater and surface water can cause eutrophication and harmful algal blooms in receiving waterbodies. Riparian zones can effectively reduce nitrogen loading via denitrification, the conversion of nitrate to nitrogen gas under saturated, anoxic conditions with sufficient carbon. Stream incision can reduce denitrification by lowering groundwater below the carbon-rich root zone. Few studies have characterized the interactions between stream incision and riparian processes, which are also often underrepresented in models. The goal of this research is to further investigate the role of incision on riparian groundwater dynamics and nitrogen cycling by analyzing existing models and collecting field data in watersheds with contrasting soil types and land uses, then coupling denitrification and channel evolution models to estimate long-term changes in nitrogen loading. I evaluated well-known riparian nitrogen models for their robustness in simulating hydrologic processes, vegetation, soils, nutrients, and channel morphodynamics. I performed global, time-varying sensitivity analyses of the Riparian Ecosystem Management Model (REMM) and the Soil and Water Assessment Tool+ (SWAT+) and found that soil and topographic parameters were most influential for estimating groundwater and nitrogen dynamics. The influence of stream channel depth and incision was underrepresented in all models.
I collected a novel dataset by studying groundwater-surface water interactions and nitrogen dynamics at paired incised and unincised streams at two sites in the southeastern US. Incision lowered the riparian groundwater table and affected the gaining and losing dynamics at each stream. Denitrification was prevalent, removing an estimated 40-95% of available nitrogen. I developed statistical models to predict groundwater depth and denitrification for both the overall dataset and individual sites. I simulated channel morphodynamics over 30 years at each site using the River Erosion Model (REM) and linked the results with the statistical models to estimate the long-term effects of incision on network-scale riparian nitrogen loading. The results of this research indicate that incision can substantially increase long-term nitrate loading in small watersheds, underscore the importance of flow management and stream restoration for water quality improvement, and illustrate the need for fully-coupled channel evolution and riparian nitrogen models to improve estimation of nitrogen loading in evolving stream networks.