Winter cover crops are an effective management practice for taking up residual nitrogen in the soil after summer crops are harvested, thereby reducing nitrogen losses to groundwater and soil loss to erosion. To assess the environmental performance of winter cover crops from 2008 to 2017 at the Tuckahoe Creek watershed, a sub-basin within the Choptank River watershed in Maryland,, USGS scientists employed a novel combination of data, including cover crop cost-share enrollment records, satellite (Landsat) remote sensing of wintertime vegetation, and results of Soil and Water Assessment Tool (SWAT) water quality simulations.
The Tuckahoe Creek watershed is a tributary to the Choptank River, which is a major source of agricultural nutrient and sediment loading to the Chesapeake Bay. The Maryland Department of Agriculture subsidizes the use of cover crops to reduce the leaching of nitrate from farm fields over the winter months. Farm enrollment data from the Maryland Agricultural Cost Share (MACS) program documented a large increase in the use of cover crops within the Tuckahoe Creek watershed during the study period, rising from 27% of corn fields and 9% of soybean fields in 2008 to 89% of corn fields and 46% of soybean fields in 2016. 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. However, interannual variation in climate strongly affected observed levels of vegetation, with warm winters resulting in increased vegetative cover, obscuring the effects of cover crop implementation.
To detect greenness trends associated with increased cover crop adoption, it was necessary to construct a 30-year analysis of wintertime greenness (made possible using the Landsat imagery archives) that normalized for climate variation, ultimately revealing substantial increases in wintertime vegetation associated with increased adoption of cover crops. Finally, cover crop enrollment data (2008 to 2017) were combined with output from the SWAT model, calibrated to stream flow and nutrient loading from the Tuckahoe Creek watershed, to estimate water quality impacts based on known distribution of cover crop species and planting dates. Results indicated a 25% overall 10-year reduction in nitrate leaching from cropland attributable to cover crop adoption, rising to an estimated 38% load reduction in 2016 when 64% of fields were planted to cover crops. Results suggest that increased environmental benefits would be achieved by shifting agronomic methods away from late-planted wheat, which comprised 34.7% of all cover crops planted between 2008 and 2017.
Annual wintertime vegetation classification for cropland within the Tuckahoe Creek watershed, state?, from 2008 through 2017, based upon composite satellite Normalized Difference Vegetation Index (NDVI) threshold values for minimal, low, medium, and high levels of green vegetation. Crop type was determined using the Cropland Data Layer.