A Year In the Life of An Agricultural Tile Drain.

Tile drains in agricultural fields tend to be highly transient hydrologic features that are influenced by a combination of climatic factors, soil properties and hydrogeologic fluctuations. These factors vary over the course of an annual cycle, particularly where a significant winter season is present.  As tiles pose an environmental risk because of the potential for contaminants to enter the shallow drains and be rapidly exported to surface water, understanding the nature of the transient flow characteristics associated with tiles is of interest. In this paper, the seasonal dynamics of tile drain flow and capture are examined on the basis of a year-long tracer experiment conducted in macroporous, agricultural soils. A mass balance approach is used to evaluate the effects of macropores and tile drains on the mobility of surface applied solute tracers in response to normal and extreme recharge events during an annual cycle. In the late fall, Br and Cl tracers were applied 0 to 2.3 m, and 2.3 to 4.6 m from a tile drain, respectively. This was immediately followed by a short artificial irrigation event, typical of a fall rain storm. Tile flow and tracer concentrations were subsequently monitored over the course of the year. Results suggest that in response to the irrigation event, the tile drain captured 8% of the applied Br mass within 48 hrs. A total of 27% of the Br mass reached the drain within 21 days. During a major winter melt event, the tile captured an additional 25% of the total Br mass. Seven months after tracer application, the majority of the Br was captured by the tile; however, the majority of the Cl remained in the soil. The study indicates that although the tile rapid capture zone is relatively small, solutes applied in the fall near the tiles can readily move to surface water via the tile and not be available for the following growing season. In addition, data indicate that seasonal melts are the most influential climatic events.