Potential Watershed Nitrate Load Reduction with Drainage Water Management under Varied Implementation Options

The potential impact of drainage water management (DWM) on the nitrate load from subsurface drainage in the Hoagland watershed in west central Indiana was assessed using the DRAINMOD 6.0 model. The watershed was divided into 6460 grid cells of 180 x 180 m size and the drain spacing, soil parent material and cropping pattern in each of the grid cells were identified from the analysis of high resolution aerial photographs and from the GIS-based analysis of soil, crop land and land cover datasets. The DRAINMOD model was applied to each cell and the nitrate loss through subsurface drains was estimated for the conventional and DWM cases. The delivery ratio (DR), which is the fraction of the nitrate load delivered from the field edge to the watershed outlet, was estimated for each of the grid cells using measured nitrate attenuation rates and calculated travel times. A 44% reduction from the current average (1987-2006) annual nitrate load of 155 metric tons was predicted with the implementation of DWM in all subsurface drained fields in the watershed. The effects of multiple management scenarios of DWM implementation based on drain spacing, soil parent material, DR and distance from the ditches/streams were evaluated with respect to the percent reduction in annual nitrate load at the watershed outlet. The highest percent reduction in average annual nitrate load, on a per unit area basis, was predicted from implementation of DWM in the grid cells formed from the Eolian sand parent material (58%) followed by the cells with