APPLYING THE SWAT WATERSHED MODEL TO THE WILLOW RIVER, WESTERN WISCONSIN: LAND-USE CHANGE AND HYDROLOGIC SENSITIVITY

Marylee S. Murphy, University of Minnesota, Dept. of Geology and Geophysics, St. Croix Watershed Research Station
James E. Almendinger, St. Croix Watershed Research Station

We have employed the Soil and Water Assessment Tool (SWAT), a spatially-based, distributed watershed model, to examine the effects of land use and management changes in the Willow River watershed in western Wisconsin. The Willow River drains 292 mi2, predominately in St. Croix County, into the St. Croix River at city of Hudson. We have undertaken an analysis of land use changes between 1992 and 2004, hydrologic calibration of the SWAT model and associated sensitivity analysis, and preliminary examination of the current nutrient and sediment export from the Willow River watershed.

St. Croix County is a historically agricultural county with 45% of land in active cultivation in 1992, the year of the most recent land use map of Wisconsin. By 1999 about 10% of this farm land was lost to other land uses, primarily rural residential development. Between 1990 and 2000, St. Croix County increased in population by 25.7%, with an estimated 17.7% additional growth from 2000 to 2004. In addition to this increase in rural residential acreage, acreages of higher density urban residential development, commercial land, recreational land, and forested land all increased. Concurrently, management of agricultural land has shifted toward a corn/soybean rotation grown for cash crops and away from a corn/alfalfa rotation grown for dairy operations.

We have analyzed the land use changes associated with increased population and shifting land use and applied incremental changes to spatial data sets incorporated into the SWAT model. Also, through tabulating the results of a 2004 agricultural management survey, we have identified crop rotations that are representative of current agricultural management practices in the Willow River watershed and can be applied to reflect yearly consistent acreages of land uses. Each year of a crop rotation includes dates for planting, tilling, fertilizing and harvesting the crop in addition to information about the quantity and mix of manure and chemical fertilizer applied to the land. The incorporation of specific land use and management practices into SWAT results in greater model efficiency over the default management rotations provided by SWAT.

After incorporating specific land use and management practices into the SWAT model, we have been refining the final model calibration through the manipulation of groundwater, snowmelt, infiltration and other watershed hydrologic parameters based on parameter sensitivity analyses. Sensitivity analysis gauges model response to incremental shifts in model parameters and helps identify parameters for which a small change in value produces a large change in model response. Critical ranges of sensitive parameters have been examined to identify appropriate values to model the Willow River watershed. Some of the most influential parameters are curve number, which affects the quantity of runoff generated, and surface runoff lag coefficient, which affects how much of the runoff reaches the stream channel each day.