Philip W. Gassman

An Application of SWAT for the Boone River Watershed in North Central Iowa

The Boone River Watershed covers over 237,000 ha in north central Iowa. The watershed is dominated by corn and soybean production, which together account for almost 84% of the land use. Fertilizer and livestock manure applications to cropland are key sources of nutrient loads to the watershed stream system. Nitrate losses are of particular concern, much of which escapes the cropland via subsurface tiles that drain the predominantly flat landscapes that persist throughout the watershed. A modeling framework has been constructed around the Soil and Water Assessment Tool (SWAT) model (version 2005) to support analyses of alternative management practice and/or cropping system scenarios for the Boone River Watershed. The required input files for the baseline SWAT simulation are initially constructed using the standard ArcView SWAT (AVSWATX) interface and supporting databases, including hydrologic response units (HRUs) that reflect homogeneous land use within different subwatersheds based on a 2002 Iowa land use data layer. The monoculture cropland HRUs are subsequently converted into more realistic crop rotations based on guidance provided by the 1997 USDA Natural Resources Inventory (NRI) cropping history data. Appropriate management data are also incorporated into the crop rotation HRUs. These updated HRUs and other input files are then imported into the interactive SWAT (i_SWAT) execution program, which facilitates execution of the SWAT simulations and storage of both the input and output data within a single Access database. A description of the modeling framework is provided, as well as initial baseline calibration/validation results and proposed forthcoming alternative scenarios.