Sensitivity Analysis of the Ohio Phosphorus Risk Index

Abstract. The Phosphorus (P) Index is a widely used tool for assessing the vulnerability of agricultural fields to P loss. This study is focused on the Ohio P Index, which was developed in the mid-1990s and has yet to be evaluated or revised. The objective of the study was to complete a stochastic sensitivity analysis of the Ohio P Index in order to determine the input variables to which the P Index score is most sensitive and identify variables for which future research and development are needed. Input variable probability distributions were created using the best available data from five agricultural watersheds in Ohio. Monte Carlo simulation was then used to generate 10,000 iterations of the P Index score based on the input variable probability distributions for each watershed. Results showed that three variables (connectivity to water, runoff class, soil-test P) explained 78% to 81% of the variance in the P Index score. Phosphorus application rate, P application method, soil erosion, and filter strip variables each explained