Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2007
Publication Date: 8/8/2007
Citation: Saleh, A., Osei, E., Jaynes, D.B., Du, B., Arnold, J.G. 2007. Economic and environmental impacts of selected BMP's for nitrate-nitrogen reduction in Walnut Creek Watershed, Iowa, using FEM (farm economic model) and enhanced SWAT (soil and water assessment tool) models. Transactions of the ASABE. 50(4):1251-1259. Interpretive Summary: Changes in land use or farm management practices can affect water outflow, sediment, nutrient, and pesticide losses from agricultural watersheds. There is an increasing demand by decision makers and planners for information regarding the magnitude of these changes due to adoption of various agricultural practices. Because of the complexity of watershed systems and the large number of possible management practices to be considered, the development and use of computer models is required. We verified that a modified version of the ARS Soil and Water Assessment Tool (SWAT) model could successfully simulate the water flow and nitrate concentrations within the Walnut Creek watershed of central Iowa - a watershed that has been extensively tile drained and is representative of much of the U.S. cornbelt. We then used the validated model to evaluate the water quality impact of various levels of adoption of three management scenarios over a 30-year period. We found that switching from fall application of nitrogen fertilizer to a spring sidedress application guided by soil testing reduced nitrate losses in the stream by more than 20% - the same result observed in an earlier watershed experiment. We also found that following corn or corn and soybean with a fall-planted rye cover crop and combining the cover crop with sidedressing of nitrogen fertilizer resulted in a progressive reduction in sediment and nutrient losses. These findings and the improved watershed model will be of interest and use to a wide range of scientists, regulators, and decision makers.
Technical Abstract: Nitrate nitrogen (NO3-N) enriched water originates from subsurface drains or “tiles” that underlay many fields in the cornbelt 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 Soil and Water Assessment Tool (SWAT) were enhanced and modified in the previous component of this study (Du et al., 2005). 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 Farm Economic Model (FEM) models. FEM is a whole-farm annual model that simulates the economic impacts of a wide range of management practice scenarios on agricultural operations. Measured values of water quality indicators from 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) management on NO3-N reduction at the subbasin level. The results obtained from SWAT-M simulation results, similar to field measurement data, indicated a 26% reduction in NO3-N under the LSNT scenario, which is very similar to what was obtained during the Jaynes et al. (2004) study. FEM results indicate 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.