IMPACT OF PREFERENTIAL FLOW, SOURCE WATER CONNECTIVITY, AND AGRICULTURAL MANAGEMENT PRACTICES ON SEDIMENT AND PARTICULATE PHOSPHORUS DYNAMICS IN MIDWESTERN TILE-DRAINED LANDSCAPES

Tile drainage is recognized as a significant transporter of sediment and particulate phosphorus (PP) in the Midwestern U.S., leading to proliferation of Harmful Algal Blooms (HABs). Numerous studies have focused on Dissolved Reactive Phosphorus (DRP) and Nitrogen (N) flux dynamics in tile-drained landscapes; however, the impact of preferential flow and agricultural management practices on fate and transport of sediment and PP has remained poorly understood. The overarching objective of this study was to improve understanding of sediment P delivery in tile-drained landscapes. This dissertation focuses on four studies. In the first study, forms and flow pathway dynamics of total phosphorus (TP) loading in midwestern tile-drained landscapes was investigated. A dataset including 5 years of surface and tile discharge P and N concentrations from two Edge-of-Field (EOF) study sites with contrasting soil and management practices were investigated. Hydrograph recession techniques were coupled with multiple linear regression (MLR) for understanding hydrologic flow pathways, and empirical mode decomposition (EMD) time-series analysis was used to determine the significance of PP seasonality processes and the effect of management practices. The results showed that macropore flow plays a significant role in PP delivery to subsurface P loading which was significantly affected by environmental conditions and management practices. In the second study, a new framework that couples hydrograph recession and specific conductance end-member mixing analysis (SC-EMMA) was developed to quantify both flow pathway dynamics and source connectivity of drainage water in tile-drainage. Statistical analysis was employed to evaluate the impact of pathway-connectivity dynamics on DRP concentrations. The results highlighted that pathway-connectivity hydrograph components improved prediction of DRP concentrations over hydrograph recession and SC-EMMA results in isolation. The findings also highlighted the importance of matrix-macropore exchange and preferential flow of new water to groundwater recharge to impact drainage hydrographs and DRP concentrations. In the third study, our new pathway-connectivity framework was combined with high-frequency turbidity data to investigate sources and pathways of sediment delivery in tiles. MLR analysis was performed to evaluate impacts of pathway connectivity on sediment concentration and seasonal dynamics were assessed using hysteresis analysis. The results showed that new water that routes through quickflow reservoir is the main hydrograph fraction for sediment and PP delivery in these landscapes. Results showed that hydrograph partitioning can improve prediction of sediment concentration and quickflow of new water was the major sediment and PP delivery pathway to tiles. Sediment concentrations were different in dry season with promoted macropores as compared to cold season with higher soil moisture and freezing and thawing effects. In the fourth study, the impacts of drainage water management (DWM) on flow pathway-connectivity and PP dynamics were investigated. Before-After-Control-Impact (BACI) assessment, long-term EMD, and hysteresis analysis of data from a paired controlled (CD) and free-drainage (FD) field site was performed. The results showed that tile discharge, preferential flow and sediment P are significantly impacted by DWM at the event timescale. Results also suggested that DWM can change time-to-peak of hydrograph, preferential flow, thus impacting sediment pathway and transport processes in subsurface flow. Cumulatively, DWM was found to decrease sediment and PP concentration and loadings at the study site through enhancement of subsurface filtration and decreases in preferential transport of new water. The processes elucidated in this study should be considered and used in agroecosystem models for improving representation of subsurface sediment delivery processes, and for model evaluation. Future studies should consider use of more robust tracers to elucidate spatial and temporal distribution of sediment sources and erosion mechanisms from subsurface pathways