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Twombly, Cameron Robert

Civil & Mechanical Engineering

2019

M.S.

Agricultural runoff is one of largest contributors of phosphorus (P), nitrogen (N), and sediment affecting freshwater systems in watersheds across the Northeastern U.S., including the Lake Champlain Basin in Vermont. Agricultural cropping systems, such as corn silage and haylands, used for dairy feed production have been shown to impact watershed hydrology and water quality. Agricultural best management practices (BMPs) have the potential to decrease runoff volumes and flow rates and the associated export of nutrients and sediment from agricultural fields. Many states in the Northeastern U.S., including Vermont, are beginning to require farmers to implement water quality BMPs and further improve risk evaluation of export of P in runoff using evolving P site assessment tools, such as the Phosphorus Index (P-Index). Quantifying the effects of BMPs on hydrologic and nutrient exports from fields is critical for informing site assessment tools that aid in the development of nutrient management plans and to help design agroecosystems that do not degrade water quality. However, there is a lack of data on the effects of BMPs on edge-of-field hydrologic and nutrient fluxes, especially in cold-climate regions with snow-melt induced runoff events. This thesis consists of four chapters, Chapter 1 is a comprehensive literature review on agricultural hydrology and water quality, BMP effectiveness, and P site assessment tools. Chapters 2 and 3 address research objectives related to the evaluation of BMP and P site assessment tool effectiveness. Chapter 4 is a summary of the conclusions drawn from the work done in Chapters 2 and 3, and suggestions for future work. Chapter 2 evaluates the effects of soil aeration prior to manure application on edge-of-field hydrology, water quality, and P fluxes in haylands with clay soils during both precipitation and snow-melt induced runoff events. Edge-of-field water quality monitoring techniques and passive-capillary lysimeter systems were used to continuously measure the losses of surface runoff, subsurface leachate, and the associated export of nutrients (total phosphorus, total dissolved phosphorus, total nitrogen, and total dissolved nitrogen) and total suspended solids resulting from runoff events year-round from 2012 to 2018. Annual P fluxes in the form of vegetative uptake and removal, manure additions, and soil test P were also recorded. Results from this study indicated that soil aeration had the potential to reduce nutrient and sediment exports from haylands with poorly-drained, high runoff producing soils in the Northeastern U.S. where winter freeze-thaw conditions exist. However, potential increases in surface and subsurface hydrologic flows can accompany these reductions; these implications should be considered before implementation. Chapter 3 identifies potential P-Index improvements through the representation of topographic controls on phosphorus (P) transport by comparing results from the Vermont P-Index (VT P-Index) and a more complex process-based model, TopoSWAT, across topographic regions in a small agricultural watershed (360 ha) in the Lake Champlain Basin. Scenarios of varying P management strategies were modeled for corn silage production fields with poorly-drained soils and rolling topography. Modeled outputs of P risk assessments and edge-of-field dissolved and particulate P losses were compared. Results from this study suggest that the VT P-Index could improve its ability to support farm nutrient management planning and other P-based management decisions by incorporating topographic controls of runoff production into its estimation of P transport.