Surface-to-tile drain connectivity and phosphorus transport: Effect of antecedent conditions

Authors: Williams, Mark; Penn, Chad; King, Kevin; McAfee, Scott

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2023
Publication Date: 3/1/2023
Citation: Williams, M.R., Penn, C.J., King, K.W., Mcafee, S.J. 2023. Surface-to-tile drain connectivity and phosphorus transport: Effect of antecedent conditions. Hydrological Processes. 37(3). Article e14831. https://doi.org/10.1002/hyp.14831.
DOI: https://doi.org/10.1002/hyp.14831

Interpretive Summary: Climate change is anticipated to alter precipitation patterns across the U.S. Midwest. As a result, it is important to understand the processes controlling subsurface tile drain discharge and water quality in order to better design and develop conservation practices that decrease nutrient loss. In this study, we use laboratory rainfall simulations with artificial macropores (wormholes created using dowel rods) to study water movement through the soil to tile drains over a range of antecedent wetness conditions. We also measured water, tracer, and dissolved phosphorus (P) concentration from a field site located in northeastern Indiana over a 2-year period to compare to laboratory results. Both laboratory and field results showed that when soil conditions were dry, tile discharge was minimal and was primarily comprised of precipitation that bypassed the soil matrix. Increasing wetness resulted in greater movement of water that was stored in the soil (i.e., groundwater) prior to the precipitation event. Dissolved P concentration decreased and loading increased with increasing soil wetness. Many widely used mathematical (computer) models currently are not able to predict subsurface P transport; thus, datasets collected in the current study are critical for improving water and nutrient processes in these models. Results also have direct applicability to conservation practice implication such as tillage practices, which alters soil-to-tile drain connectivity, and drainage water management, which increases the potential for water storage within fields. Research results combined with predicted future climate (e.g., wetter springs, drier summers) also suggest that management practices such as fertilizer application timing and the the risk of P loss will vary with changes in antecedent conditions.

Technical Abstract: Macropores connecting surface soils to tile drains can alter water and nutrient transport through the subsurface. In this study, laboratory rainfall simulations with artificial macropores combined with edge-of-field monitoring were used to evaluate surface-to-tile drain connectivity and phosphorus (P) transport as a function of antecedent conditions. Laboratory rainfall simulations using repacked soil boxes with different macropore layouts (i.e., no macropore, surface-connected macropores, disconnected macropores) were used to examine changes in water sources and flow pathways to tile drains with varying degrees of connectivity and antecedent wetness. Water, tracer, and P fluxes from a tile-drained field were also monitored to quantify linkages among water flow pathways, antecedent conditions, and P delivery to tile drains. Both laboratory and field results showed that surface-to-tile drain connectivity was important for water transport through the subsurface under both dry and wet antecedent conditions. When soil conditions were dry, discharge was minimal and primarily comprised of event water that bypassed the soil matrix. Increasing wetness resulted in similar event water transport, but greater mobilization of stored pre-event water and greater discharge; thus, the dominant source of tile water and the magnitude of tile discharge were substantially altered with changing antecedent conditions. Field data revealed that changes in drainage water source and discharge with increasing wetness impacted dissolved P transport. Dissolved P concentration decreased and loading increased with increasing wetness. Findings indicate that greater mobilization of pre-event water under wet antecedent conditions acted as both a hydrologic and chemical buffer for subsurface dissolved P transport. Comparison of study results to water quality data from a larger edge-of-field network suggest that relationships between antecedent conditions, water flow pathways, and P transport from the current study are broadly applicable across tile-drained fields. Understanding processes controlling P delivery to tile drains has direct applicability for conservation practice implementation and improving process representation in models.