Nitrate-N loadings through subsurface environment to agricultural drainage ditches in two flat Midwestern (USA) watersheds

A study was conducted to understand the contributions of tile flow and baseflow to total nitrate-N (NO3-N) loadings in two subsurface (tile)-drained watersheds, namely the Big Ditch (BD) and the Upper Embarras River (UER) watersheds in Illinois. Two stream sections were selected in the watersheds and rectangular cutthroat flumes were installed at the upstream and downstream ends of the stream sections to calculate the flow mass balance for separating baseflow. The stream section at BD site had two tile outlets draining into it. The stream section at UER watershed did not have any tile drain. Tile flow was also measured along with stream flow. Water samples were collected not only from the stream sections using auto-samplers but also manually from the tile drains. Average baseflow rates per unit lengths of the stream sections at BD and UER sites were 3.5 × 10−04 and 9.4 × 10−05 m2 s−1, respectively. At BD site, for six study periods, the percentages of baseflow and tile flow contributions of NO3-N loads within the stream section were 90 and 10%, respectively. Annual NO3-N contributions by the upstream subwatersheds for BD and UER stream sections were 61,819 and 16,155 kg, respectively. Annual NO3-N loss from these two subwatersheds within BD and UER watersheds was 42.9 and 7.0 kg ha−1, respectively. For the stream section at BD site, baseflow seemed to play a more important role than tile flow in raising the NO3-N concentration level in the stream water. Land use seemed to play a major role in the significant difference in NO3-N concentrations at the two subwatersheds upstream from the project sites. Nitrate-N loadings primarily depended on precipitation, antecedent moisture condition (AMC), fertilizer application time, and evapotranspiration (ET).