Estimating the environmental effects of winter cover crops in the Tuckahoe Creek watershed using cost-share enrollment data, satellite remote sensing, and SWAT modeling

Authors: HIVELY, W.D. - Us Geological Survey (USGS); LEE, S. - University Of Maryland; Sadeghi, Ali; McCarty, Gregory; LAMB, B.T. - New York University; SOROKA, A. - Us Geological Survey (USGS); KEPPLER, J. - Maryland Department Of Agriculture; YEO, I.Y - University Of Newcastle; Moglen, Glenn

Submitted to: Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2019
Publication Date: 5/12/2020
Citation: Hively, W., Lee, S., Sadeghi, A.M., McCarty, G.W., Lamb, B., Soroka, A., Keppler, J., Yeo, I., Moglen, G.E. 2020. Estimating the environmental effects of winter cover crops in the Tuckahoe Creek watershed using cost-share enrollment data, satellite remote sensing, and SWAT modeling. Soil and Water Conservation Society. 75(3):362-375. https://doi.org/10.2489/jswc.75.3.362.
DOI: https://doi.org/10.2489/jswc.75.3.362

Interpretive Summary: Winter cover crops (WCC) have been shown to be an effective best management practice (BMP) for taking up residual nitrogen that remains in the top soils after summer crops are harvested, reducing nitrogen losses to groundwater, and reducing soil lost to erosion. The Maryland Department of Agriculture strongly promotes winter cover crop BMPs through cost-share programs, with the goal of reducing excess nitrate loads from agricultural fields into the waterways that would otherwise enter the Chesapeake Bay estuary. Over the past ten years we have studied winter cover crop performance as part of the USDA Choptank River Conservation Effects Assessment Project (CEAP), using remote sensing and modeling approaches. The focus of this manuscript is to provide a synthesis of WCC research and modeling outcomes, linking estimated water quality impacts to observed trends in wintertime ground cover and cover crop implementation on working farmland. Results show that for an effective WCC BMP implementation: I) plant earlier, II) plant effective species, and III) increase implementation acreage. Furthermore, our findings reveal that the difference in nitrogen reduction efficiency among planting dates is substantial, modeled at approximately 35% reduction from early (Oct 1) to late (Nov 1) planting. Findings from our WCC studies confirm the Chesapeake Bay U.S. EPA Program partnership’s characterization of a matrix of variable efficiencies for winter cover crops within the Chesapeake Bay water quality model, accounting for planting date, as well as species and planting method.

Technical Abstract: This study combined winter cover crop cost-share enrollment data, satellite remote sensing of wintertime vegetation, and results of Soil and Water Assessment Tool (SWAT) water quality simulations to estimate the environmental performance of winter cover crops (WCC) at the watershed scale. The SWAT model was calibrated to streamflow and nutrient loading from the Tuckahoe Creek sub-watershed of the Choptank River. The Choptank is a major river basin within the Chesapeake Bay watershed and, as a focus watershed for the U.S. Department of Agriculture’s Conservation Effects Assessment Project (CEAP), has been the subject of considerable study linking land use to water quality. Simulation of the growth of WCC within SWAT was calibrated based upon results of satellite remote sensing by linking modeled WCC performance to species and planting date (elapsed growing degrees). The calibrated model was subsequently used to demonstrate the effectiveness of WCC on reducing nitrate loads at the watershed scale. Farm enrollment data from the Maryland Agricultural Cost Share (MACS) program documented a strong increase in the use of WCC within the Tuckahoe Creek watershed, from 30% of corn fields and 4% of soybean fields in 2008 to 65% of corn fields and 48% of soybean fields in 2017. Satellite remote sensing of wintertime ground cover detected increased wintertime vegetation following corn crops, in comparison to full season and double cropped soybean, consistent with patterns of cover crop implementation. Overall, 62% of corn fields, 50% of full season soybean fields, and 27% of double cropped soybean fields showed medium to high levels of wintertime vegetation, compared to 80% of hay fields and 85% of alfalfa fields. However, expected multi-year temporal trends toward increased wintertime vegetation were partly obscured by inter-annual variation in climate with warm winters resulting in increased vegetative cover. The predominant WCC species recorded by the MACS program as planted in the Tuckahoe Creek watershed were wheat (67.6%), barley (17.6%), and rye (7.1%), and the majority of wheat cover crops (50.8%) were planted late (after October 15). When WCC enrollment data were combined with output from the SWAT model, to estimate water quality impacts based on known distribution of cover crop species and planting date (2008 to 2017), results indicated a 22% overall 10-year reduction in nitrate leaching from cropland resulting from cover crop adoption, rising to an estimated 32% load reduction in 2017 when 56% of fields were planted to cover crops). Increased environmental benefits associated with cover crops would be achieved by shifting agronomic methods away from late-planted wheat.