| Abstract: |
The Raccoon River Watershed (RRW) in West-Central Iowa has been recognized as
exporting some of the highest nitrate-nitrogen loadings in the United States
and is a major source of sediment and other nutrient loadings. An integrated
modeling framework has been constructed for the RRW that consists of the Soil
and Water Assessment Tool (SWAT) model, the interactive SWAT (i_SWAT) software
package, Load Estimator (LOADEST) computer program, and other supporting
software and databases. The simulation framework includes detailed land use
and management data such as different crop rotations and an array of nutrient
and tillage management schemes, derived from the U.S. Department of
Agriculture's National Resources Inventory databases and other sources. This
paper presents the calibration and validation of SWAT for the streamflow,
sediment losses, and nutrient loadings in the watershed and an assessment of
land use and management practice shifts in controlling pollution. Streamflow,
sediment yield, and nitrate loadings were calibrated for the 1981-1992 period
and validated for the 1993-2003 period. Limited field data on organic
nitrogen, organic phosphorus, and mineral phosphorus allowed model validation
for the 2001-2003 period. Model predictions generally performed very well on
both an annual and monthly basis during the calibration and validation
periods, as indicated by coefficient of determination (R2) and Nash-Sutcliffe
simulation efficiency (E) values that exceeded 0.7 in most cases. A set of
land use change scenarios based on taking cropland out of production indicated
a significant benefit in reducing sediment yield at the watershed outlet. A
second scenario set found that relatively small reductions in nutrient
applications resulted in significant reductions in nitrate loadings at the
watershed outlet, without affecting crop yields significantly. |