Phosphorus runoff risk assessment in karstic regions of the United States

Abstract

The Phosphorus (P) Index risk assessment tool has been widely adopted across the United States to identify and rank site vulnerability to P runoff as part of the Natural Resources Conservation Service nutrient management planning (NMP) process. However, limited success has been achieved in addressing the risk of P loss by subsurface flow pathways, despite its relative importance in certain areas of the United States, particularly in those U.S. states dominated by karst terrain. Here we review how states with varying land areas classified as having karst features address the risk of P runoff during the NMP process. Indices adopted in Illinois and Indiana require setbacks (widths 15–72 m) around surface karst features. The remaining states with karst address the risk of P loss in NMP development rather than the application of a P Index. Given the spatially variable hydrogeologic properties of karst, technically rigorous field‐scale factors are unlikely to be developed in the near future.