Impact of flow pathway and source water connectivity on subsurface sediment and particulate phosphorus dynamics in tile-drained agroecosystems

Abstract

Subsurface tile drainage is recognized as a significant source of sediment and particulate phosphorus (PP) in the midwestern U.S. However, the role of subsurface flow pathway and source water connectivity dynamics on sediment transport is poorly understood. The overarching objective of this study was to investigate sediment and PP loading dynamics for a midwestern subsurface tile drained agroecosystem and assess the governing flow pathway and water sources impacting subsurface sediment loads. In this study, we used a recently-developed framework that couples event-based hydrograph recession and specific conductance-end-member mixing analysis (SC-EMMA) to assess governing drivers of sediment transport through tile. We collected high-frequency specific conductance, turbidity, and subsurface discharge data from an edge-of-field (EOF) tile main located in northwestern Ohio for 15 months. Multiple linear regression (MLR) analysis and hysteresis analysis were employed to evaluate the impact of pathway-connectivity dynamics on flow-weighted mean Total Suspended Solids (TSS) concentrations. The MLR analysis showed that quickflow of new water (Qquick-new) had the highest flow-weighted mean sediment concentrations, and that concentrations associated with quickflow of old water (i.e., matrix-macropore exchange) were variable. Analysis using the hysteresis index (HI) showed that hysteresis characteristics (magnitude and direction) for separated hydrographs using the pathway-connectivity framework deviated from HI values of subsurface discharge (Qtile) and highlighted the importance of Qquick-new through much of the monitoring period. For events immediately following tillage and cover crop application in Fall 2019, we found Qquick-old was the primary form of preferential flow, peak sediment concentrations coincided with Qquick-old, and event sediment loadings during these events decreased relative to the previous fall. The findings suggest that reducing preferential transport of new water may be an effective strategy for reducing sediment and particulate P loadings at the edge-of-field.