Modeling Cover Crop Effectiveness on Maryland's Eastern Shore

Cover cropping has become a widely used conservation practice on Maryland's Eastern shore. It is one of the main practices funded by the Maryland Department of Agriculture's (MDA) Maryland Agricultural Water Quality Cost Share (MACS) program. The major benefits of this practice include reduction of nutrient runoff and leaching to surface and ground waters, and control of soil erosion. Although cover crops are increasingly being implemented, the long term effectiveness of this practice is not well known, especially on a watershed-scale basis. Since many watershed/water quality models are designed to measure long-term, large-scale effects of management practices, the Soil Water Assessment Tool (SWAT) model was employed to evaluate the environmental impact of cover crop implementations. This project is part of the U.S. Department of Agriculture's (USDA) Conservation Effects Assessment Project (CEAP) program which was established to specifically quantify the environmental benefits from conservation practices. The study is being carried out on the Choptank, one of the nine major Maryland river basins within the Chesapeake Bay watershed. Several tributaries in the Choptank river basin have been identified as "impaired waters" under Section 303(d) of the Federal Clean Water Act due to high levels of nutrients and sediments. SWAT was first utilized to build a model for the German Branch (GB) subbasin (similar to 50 km2), a non-tidal tributary basin of the larger Choptank River watershed. The study period was 18 years (1990-2007). The streamflow component of SWAT was calibrated on a daily basis using years 2005 and 2006 with one year of spin-up (2004). Validation was conducted using the 1/1/07-4/15/07 time period. Cover cropping was first implemented in the GB watershed in 2002. Changes in nitrate loading were examined to measure improvements in the reduction of nitrate loads by increasing cover crop implementation. Model simulations were run to estimate nitrate loads for two scenarios: (1) no cover crop implementation during the entire study period, and (2) increasing cover crop implementation starting from 2002 through 2007.