Field scale discharge and water quality response, to drainage water management

Subsurface (tile) drainage, while necessary for viable agricultural crop production in the humid, poorly drained regions of the world, has been linked to offsite nutrient transport, culminating in harmful algal blooms and hypoxia in downstream waterbodies. Drainage water management (DWM), also known as controlled drainage, has been promoted as a method to reduce nitrogen (N) and phosphorus (P) loss from field scale tile drainage; however, subsequent changes in surface runoff and associated water quality have not been explored. Four years of surface and subsurface discharge, N, and P loss from two sets of paired field sites in northwest Ohio, USA were evaluated using a before-after control-impact (BACI) design to quantify the impact of DWM. Mean event surface runoff was greater (significant at one site) under DWM while tile drainage discharge was less at both sites but not significant. There was no significant difference in surface nitrate-N (NO3--N) loss; however, significant reductions in tile NO3--N loss under DWM were measured at both sites. Similar to discharge, dissolved reactive P (DRP) loss measured in surface runoff was greater under DWM (significant at one site), while no significant differences were noted in mean event tile drainage DRP loss. Total P (TP) surface runoff losses were significantly greater under DWM at the site with significantly greater surface runoff, while DWM reduced tile drainage TP loss at only one site. These findings suggest that DWM reduced tile NO3--N loss, while also highlighting that any benefit from DWM with respect to tile DRP or TP loss could be negated in part by increased surface losses. Furthermore, these findings emphasize the need to understand the natural resource concern to which the practice is targeted and also call for additional research on DWM implementation that includes surface runoff across a range of soil textures, cropping management, and climates.