Water Quality Impacts of the U.S. Renewable Fuel Standard

The U.S. Renewable Fuels Standard (RFS) - a federal policy requiring the use of biofuels (primarily corn ethanol to date) - has had a large impact on U.S. agriculture since its creation in 2005 and amendment in 2007. Particular attention has been given to potential RFS impacts on environmental outcomes, specifically water quality. However, attribution of water quality outcomes to the RFS via a causal link to economic and land use change processes has yet to be established. We developed a linked economic-land-use-biophysical modeling framework that estimates the impact of the RFS from 2007-2016 on phosphorus loss via runoff and nitrate leaching to groundwater relative to a counterfactual business-as-usual (BAU) scenario. We accounted for two separate mechanisms through which the RFS impacts water quality: intensification of existing cropland (e.g., more corn planted on existing fields) and extensification of non-cropland to cropland (e.g., conversion of grassland to corn). The hydrologic outcomes were estimated using a process-based agroecosystem model (AgroIBIS) to estimate annual phosphorus and nitrate losses, which required creation of new historical datasets for land-use/land-cover and nutrient applications to account for soil nutrient legacies. Model results indicate that implementation of the RFS has led to an increase in ~5% for nitrate leaching and ~3% for phosphorus runoff over the continental U.S. The increase due to intensification was higher than extensification for nitrate leaching; but the opposite was true for phosphorus runoff. These results are important for the public and decision-makers to consider when designing future renewable fuel policy.