Although evaluations of switchgrass as feedstock for the bioenergy industry have indicated significant benefits from an energy perspective, it also may adversely impact runoff, sediment and nutrient losses, and associated water quality effects. Field scale monitoring requires long-term measurements that are expensive and cannot be used practically for large-scale monitoring of many watersheds or regions (Harmel et al. 2006). Instead, crop models are gaining favor as they provide a practical alternative for assessing bioenergy cropping effects, due to their versatility and cost-effectiveness when compared to field experimentation.

Soil and Water Assessment Tool (SWAT) has been used at the plot and watershed scale to determine N loss, P loss, and soil erosion from switchgrass fields. Sarker (2009) used SWAT to compare N loss from agricultural systems growing switchgrass and cotton in the southeastern U. S. The plot scale modeling suggested that in the early years of growth there is a significant N loss from switchgrass to streamflow and groundwater, but N loss is significantly reduced as switchgrass matures. Nepple et al. (2002) conducted a similar SWAT watershed scale study in which 50,000 ac of cropland were converted to switchgrass production in the Rathbun Lake watershed in southern IA. Overall, the model indicated that conversion of 15.3% of the watershed area to switchgrass production would significantly reduce soil erosion, N, P, and atrazine loadings into the Rathbun Lake watershed, relative to a baseline of traditional row-crop agriculture (Table 5).

Chamberlin et al. (2011) applied the DAYCENT biogeochemistry model to calculate the nitrate in runoff water when converting land cover from cotton and unmanaged grasslands to a switchgrass system in the southern U. S. Long-term simulations predicted a reduction of nitrate runoff (up to 95%) for conversions from cotton to switchgrass with N application rates of 0-135 kg N ha1. A reduction of nitrate runoff ranging from 50-70% occurs at all levels of fertilization, when converting from unmanaged grasses. The simulated nitrate runoff values from DAYCENT fall within the observed range.

The impact of large-scale biofuel production on water quality is a growing concern. Wu et al. (2012) recently applied the SWAT model to simulate the impact of future biofuel production on water quality and water

cycle dynamics in the Upper Mississippi River basin. Converting pasture lands to switchgrass reduced soil erosion considerably and positively impacts N and P loadings at the projected yield and fertilizer input. In addition, switchgrass increases the water loss associated with evapotranspiration (1% of total precipitation), decreases the base flow (2%), and decreases the surface runoff flow to the basin. Nelson et al. (2006) used the SWAT model to predict reductions in four water quality indicators (sediment yield, surface runoff, nitrate nitrogen (NO3-N) in surface runoff, and edge-of-field erosion) associated with producing switchgrass on cropland in the DE basin in northeast KS. They determined that the production of switchgrass on conventional agricultural cropland had distinct environmental advantages versus traditional (e. g., corn-soybean) cropping rotations.

Field studies that support the findings that switchgrass production will improve surface water quality are slowly becoming available. Lee et al. (1998) compared switchgrass filter strips to cool-season grass filter strips and reported that switchgrass was more effective in removing P and N from runoff. Similarly, Sanderson et al. (2001) noted reductions in P and N runoff from a switchgrass filter strip treated with dairy manure, while Mersie et al. (1999) utilized switchgrass filter strips to reduce the amount of dissolved atrazine and metachlor herbicides by 52% and 59%, respectively. Entry and Watrud (1998) tested the ability of Alamo switchgrass to remediate soil contaminated by the radionuclides cesium-137 and strontium-90. Switchgrass captured 36% of the cesium and 44% of strontium over a five — month period. Overall, all models used predict that converting lands used for pastures or traditional row crop to perennial switchgrass for feedstock production will have a positive long-term environmental impact by reducing sediment loss and nutrient runoff, and improving water quality.