Modeling flow on a tile-drained watershed using a GIS-integrated DRAINMOD

Many of the most productive agricultural watersheds in Illinois have mild topography (<1%) and poorly drained soils. The primary mechanism for removing excess water in these watersheds is subsurface (tile) drainage systems. Subsurface drainage systems are a rapid conduit for transporting contaminants, especially nitrate-nitrogen. To better understand the hydrologic response of tile-drained watersheds, DRAINMOD was coupled with Arcview to simulate the hydrologic response of a tile-drained watershed. In this modeling approach, a tile-drained watershed is subdivided into uniform cells, and DRAINMOD is run on each cell with inputs based on the individual characteristics of each cell. The result is a distributed parameter model based on the water balance of DRAINMOD that accounts for surface runoff, subsurface tile flow, and stream baseflow. Daily flow was simulated for the Upper Little Vermilion River watershed in east central Illinois from 1992 to 1997. Individual tile systems within the watershed were mapped by analyzing color infrared aerial photographs to determine the location and extent of subsurface drainage systems. The model was able to adequately simulate the hydrologic response of the watershed. Comparing observed flow with the model's output over the simulation period returned an R² value of 0.672 and a standard error of 0.465 mm/day.