There is No Such Thing as a Quick Fix: Travel Times to Subsurface Drains

Abstract

Highlights Bromide tracer was applied at different distances from subsurface drains to assess travel times and preferential flow. Some bromide arrived at the drain outlet with the first water flow after application, indicating preferential flow. Total time for “cleanout” of bromide from the plots varied with drain spacing and relative distance from the drain. When changing management practices, the full impact of the new system will not be reflected until several years later. Abstract. Subsurface “tile” drains are an important water management practice for many productive agricultural soils, but drains also deliver nitrate-nitrogen and other soluble chemicals from the bottom of the rootzone to surface ditches and streams. Many studies have focused on determining different ways to reduce the delivery of nitrate and other chemicals to subsurface drains while still providing adequate drainage for crop production. One important consideration in such studies that is often overlooked is the response time of drainage outflow to changes in management at the soil surface. Drains integrate flow from distances immediately adjacent to the drain all the way to the midplane between parallel drains, which means changes in management will not be fully reflected in drainage waters for months to several years, depending on the drainage intensity, soil characteristics, and precipitation. This bromide tracer study was conducted at the Southeast Purdue Agricultural Center (SEPAC) in Indiana, USA. Bromide tracer was applied at the beginning of the drainage season (November) at different distances from the drains on multiple plots of different drain spacings. The bromide concentration in the drainflow was monitored over the next five years to determine breakthrough curves and the amount of time required to flush the bromide from the system. The initial breakthrough of bromide occurred during the first appearance of drainage after application, regardless of the location of the bromide strip, suggesting some preferential flow was occurring. This was further investigated with comparison to curves of theoretical travel time following Kirkham’s analytic solution for flow to a tile drain. The full transport of the bulk of the chemical took 2-3 years, but varied greatly among the different treatments, reflecting the longer travel times for locations further from the drain. Keywords: Keywords., Drainage, Preferential flow, Streamlines, Subsurface drainage, Travel times.