ECONOMIC AND ENVIRONMENTAL IMPACTS OF LSNT AND COVER CROPS FOR NITRATE-NITROGEN REDUCTION IN WALNUT CREEK WATERSHED, IOWA, USING FEM AND ENHANCED SWAT MODELS.

Nitrate nitrogen (NO3-N) enriched water originates from subsurface drains or "tiles" that underlay many fields in the Corn Belt and is the primary source of NO3-N to surface waters in this region. To better assess the fate and transport of nutrients, such as NO3-N, the tile drain and pothole components of SWAT (Soil and Water Assessment Tool) were enhanced and modified in the previous component of this study. In this study, the environmental and economic impacts of various best management practice (BMP) scenarios often adopted by local farmers to reduce sediment and nutrient loadings (in particular NO3-N) were evaluated using the modified SWAT (SWAT-M) and FEM (Farm-level Economic Model) models. Measured values of water quality indicators from the Walnut Creek watershed (WCW) located in central Iowa were used to verify the capability of SWAT-M to predict the impact of late-spring nitrate test (LSNT) and rye cover crop management on NO3-N reduction at the subbasin level. The results obtained from SWAT-M simulation results, similar to field measurement data, indicated a 25% reduction in NO3-N under the LSNT scenario. FEM results indicated a corresponding increased annual cost of $6/ha across all farms in the watershed. Simulation of other scenarios, including winter cover cropping and a combination of LSNT and cover cropping at different adoption rates within WCW, resulted in a progressive reduction in sediment and nutrient losses as adoption rates increased. Use of the rye cover crop added about $25/ha to $35/ha to the annual cost of the average farm, indicating that some cost-share support may be necessary to encourage farmers to use winter cover crops. [ABSTRACT FROM AUTHOR]