SEASONAL CLIMATIC INFLUENCES ON TILE DRAIN FLOW DYNAMICS AND IMPLICATIONS FOR AGRICULTURAL BMPS (Invited Presentation)

Tile drains are ubiquitous features within many North American agricultural landscapes, and the nature of flow through these important hydrologic conduits varies significantly over annual climate cycles. This flow variability imparts a seasonal signal on nutrient mass fluxes and on the partitioning of excess nutrients between groundwater and surface water receptors, yet the mechanisms that drive tile behavior under complex weather patterns and in macroporous soils is not well understood. In this investigation, tile flow dynamics including surface capture area and solute mass flux were quantified over an annual climate cycle by tracking distinct (Br and Cl) tracers that were applied as solutes at 0–2.3 m and 2.3–4.6 m from a tile drain respectively. The tracers were applied in early November on an agricultural field underlain by macroporous, silt loam soil and controlled irrigation was used to simulate a major precipitation event immediately following tracer application. The fate and distribution of the surface applied tracers were then monitored in tile effluent and within the subsurface during the course of the late-fall, winter, and spring climate cycles. Results indicated that the tile drain captured 8% of the Br mass (applied within 2.3 m of the tile) within 48 h of the irrigation event and 27% within 21 days of tracer application, while effectively none of the CL tracer (applied 2.3 – 4.6 m from the tile) was captured. During a major winter melt event, the tile captured an additional 25% of the total Br mass in less than 10 days, and by seven months after tracer application, approximately 95% of the Br tracer had been captured. However, even by the end of the monitoring period, the bulk of the Cl tracer mass appeared to remain in the soil. The implications for nutrient loading to surface and groundwater in the vicinity of tile drains based on the observed dynamic response is discussed along with potential impacts on the design of agricultural BMPs.