Drainage water management effects on tile discharge and water quality

Authors: Williams, Mark; King, Kevin; Fausey, Norman

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2014
Publication Date: 9/28/2014
Publication URL: http://handle.nal.usda.gov/10113/61314
Citation: Williams, M.R., King, K.W., Fausey, N.R. 2014. Drainage water management effects on tile discharge and water quality. Agricultural Water Management. 148:43-51.

Interpretive Summary: Subsurface tile drains have been widely installed in Midwestern headwater watersheds during the last 150 years to allow economical crop production. Many studies have shown that tile drains are an important source of nitrogen (N) to streams and drainage ditches, but it is not known how tile drains influence N loss at the watershed scale. This study monitored the watershed outlet as well as all of the tile drains in a central Ohio headwater watershed to determine the effect of tile drainage on watershed N export over an 8 year period. Results indicate tile drainage is the primary factor controlling watershed N transport. Understanding tile drainage contributions to watershed N loads is an important step in assessing the impact of agricultural practices on water quality. Findings suggest that practices such as drainage water management, reduced N application rates, and cover crops may decrease N concentrations in tile discharge and ultimately improve downstream water quality.

Technical Abstract: Nitrogen (N) fluxes from tile drained watersheds have been implicated in water quality studies of the Mississippi River Basin, but the contribution of tile drains to N export in headwater watersheds is not well understood. The objective of this study was to ascertain seasonal and annual contributions of tile drainage to watershed discharge and N loading. Discharge and N concentration were measured at the outlet of an Ohio, USA agricultural headwater watershed and all tile drain outlets in the watershed from 2005 through 2012. Results showed that tile discharge accounted for 56% of annual watershed discharge over the 8-year study. Nitrate-N was the dominant form of N exported in both watershed and tile discharge and concentrations ranged from 0.1 to 70.5 mg L-1. Watershed NO3-N concentration increased with increasing discharge up to the 75th percentile of flow where NO3-N concentrations reached an asymptote. This pattern indicates that tile contributions to stream NO3-N concentration were increasingly important as discharge increased, but other sources of water (e.g., precipitation, surface runoff) likely decreased stream NO3-N concentrations at the highest discharge rates. Annual watershed NO3-N load ranged from 12.4 to 39.6 kg ha-1, with tile drainage contributing between 44 and 82% (mean: 62%) of annual watershed NO3-N export. Study results indicate that tile drainage is the primary factor controlling watershed discharge and N, especially NO3-N, export. Thus, in tile drained headwater watersheds, management practices that decrease NO3-N leaching to tile drains (e.g., reduced N application rates, cover crops) and reduce NO3-N loads (e.g., drainage water management) have the potential to improve downstream water quality.