| Abstract: | This study employed the Soil and Water Assessment Tool (SWAT) to evaluate the impacts of projected future
climate change scenarios on water balance, runoff, sediment, total nitrogen (N), and total phosphorus (P) at the field
scale for four locations in the Heartland region: Sioux City (Iowa) and Columbus, Mullen, and Harrison (Nebraska).
A conventional two-year corn-soybean rotation was assumed to be grown on each field. All fields were simulated
identically in terms of topographic and cover/land management conditions. Model inputs for the fields differed in only
three ways: the forcing conditions for existing and future climatic scenarios (SRES A2, A1B, and B1), soil and aquifer
properties, and calibrated parameters at each location. Model simulations indicate that for the Columbus and Sioux City
sites, where current average annual precipitation is about 740 and 650 mm, respectively, substantial increases in runoff
and pollutant loadings from a corn-soybean crop rotation are projected to occur during the spring under future climate
scenarios in comparison to existing conditions. At the Sioux City site, for example, increases in runoff of 213%, 124%, and
128% during the month of May are projected for the A2, A1B, and B1 scenarios, respectively, in comparison to the
baseline condition. Very large increases in attendant sediment and nutrient losses are also projected for that month at the
Sioux City site. Considerably greater attention in coming years will therefore likely be necessary to devise best
management practices and adaptation strategies that can be effectively employed to conserve soil and water resources and
to protect streams and receiving waters from the harmful effects of higher pollutant loadings. At the Harrison site, where
average annual precipitation is less than 450 mm, increases in average annual evapotranspiration of 29, 31, and 46 mm
under the A2, A1B, and B1 future climate scenarios are projected to occur for a corn-soybean crop. Relative to the
baseline at this site, water requirements are projected to be 37, 39, and 32 mm greater, respectively, for the peak irrigation
month of July under the A2, A1B, and B1 scenarios. For regions in western Nebraska with similar or lesser precipitation
levels, these anticipated changes could exacerbate already existing challenges for agricultural producers who primarily
rely upon groundwater for irrigation. SWAT was employed to simulate the impact of four best management practices
(BMPs) on changes in sediment, total N, and total P under the baseline and future climate change scenarios for each site.
These four treatments included conversion of the corn-soybean rotation to pasture, switchgrass, and no-till, and
implementation of a 10 m wide edge-of-field buffer strip. At all four sites, the pasture and switchgrass BMPs reduced
sediment and total P yields by 97% to 99% in comparison to the corn-soybean cover crop. Each of the BMP treatments
employed in this study appears to hold promise in providing potential reductions in sediment and nutrients for the two
eastern field sites. However, further analyses are needed to not only assess the impact of other types of BMPs, but their
cost effectiveness and sustainability as well. Model simulations suggest that, for the Harrison site, moderate decreases in
sediment, total N, and total P are projected to occur for the no-till BMP and modest decreases for the 10 m buffer strip
BMP. Model simulations also suggest that, of the four BMP types, only the pasture and switchgrass treatments appear to
provide appreciable reductions in sediment and nutrients at the Mullen site. |