Hypoxia and eutrophication resulting from excessive nutrient loading are one of the most significant environmental issues around the world. Although the 1972 Clean Water Act has effectively reduced point source loadings of nutrients to surface waters, controlling diffuse, nonpoint source pollution continues to be a challenge. Anthropogenic activities, including land use change, are considered some of the main reasons for the excessive riverine nitrogen (N) loading. Temperature, stream discharge, the structure of the drainage network as well as soil moisture are among the important factors influencing nitrogen transport and transformation in watersheds. Of particular interest is temperature, which was found to be a key factor, influencing nitrogen transformation processes; however, modeling temperature in watersheds is challenging due a large number of coupled processes involved. Stream thermal regimes are primarily driven by climatic conditions and influenced by a host of other factors, including topographic conditions, stream discharge, land cover near the stream and interactions with the subsurface. Riparian vegetation processes close to the stream banks control canopy shading, as do factors such as the spatial heterogeneity of vegetation density and temporal aspects of vegetation growth. Vegetation type affects stream temperature while also influencing the riparian microclimate including air temperature, wind speed and relative humidity. These complexities call for an integrated model that can describe coupled hydrologic-vegetation processes. This dissertation research involves the development and application of an integrated and fully process-oriented water-temperature-nitrogen model based on the modeling framework of PAWS (Process-based Adaptive Watershed Simulator). The integrated model was tested using data from two watersheds of different sizes and climatic conditions - the Wood Brook watershed in central England located at the Birmingham Institute of Forest Research (BIFOR) and the Kalamazoo River watershed in Michigan. The phenology and surface energy modules in the coupled model were used to quantify the impacts of vegetation processes on radiation fluxes (e.g., canopy shading and the effect of vegetation growth on optical parameters). The integrated temperature model enabled accurate simulations of the movement and partitioning of water and thermal fluxes in stream, soil, streambed, and groundwater domains and allowed the identification of gaining and losing portions of stream reaches. Nitrogen transport and transformations on the landscape were modeled by representing multiple sources and processes (fertilizer / manure application, WWTPs, atmospheric deposition, Nitrogen retention and removal in wetlands and other lowland storage, temperature-dependent transformation rates etc.) across multiple hydrologic domains (streams, groundwater, soil water). The coupled model provides a tool to examine Nitrogen budgets and to quantify the impacts of human activities and agricultural practices on the riverine export of nitrogen species.
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