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Title: Sensitivity analysis of the Ohio phosphorus risk index

item Williams, Mark
item King, Kevin
item DAYTON, ELIZABETH - The Ohio State University
item LABARGE, GREG - The Ohio State University

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2014
Publication Date: 2/1/2015
Publication URL:
Citation: Williams, M.R., King, K.W., Dayton, E., LaBarge, G. 2015. Sensitivity analysis of the Ohio phosphorus risk index. Transactions of the ASABE. 58(1):93-102.

Interpretive Summary: The Ohio Phosphorus (P) Index was developed in the mid-1990s to help producers manage P and minimize the environmental impacts associated with P loss. However, the Ohio P Index has yet to be evaluated or revised. To make the process of evaluating and revising more efficient, a sensitivity analysis of the Ohio P Index was completed to help identify input variables within the P Index that need to be refined and prioritize additional data collection and research. Results showed that input variables associated with field characteristics and previous management practices were the most important for determining a field's risk of P loss. Other variables, such as P application rate and method, that reflect current management practices did not influence a field's risk of P loss. The results of this study provide a strong rationale for the collection of field data related to hydrology, site characteristics, and management practices in order to determine the accuracy of the Ohio P Index. They also indicate that changes to the current Ohio P Index may help producers better evaluate management alternatives to limit P loss and enhance the overall predicting capacity of the P Index. Producers and environmental planners will benefit from this study by having an enhanced understanding of how management practices within the Ohio P Index influence a field's risk of P loss. The results will also benefit scientists by helping identify future research needs on P loss within Ohio and serving as an example to other states across the U.S. interested in evaluating their P Index.

Technical Abstract: The Phosphorus (P) Index is a widely used tool for assessing the vulnerability of agricultural fields to P loss; yet, few of the P Indices developed in the U.S. have been evaluated for their accuracy. Sensitivity analysis is one approach that can be used prior to calibration and field-scale testing to gain an understanding of how the P Index responds to its input variables as well as prioritize data collection and research efforts. In this study, stochastic sensitivity analysis of the Ohio P Index was completed for five agricultural watersheds based on watershed-specific input variable estimations and distributions. Results showed that in all five watersheds the Ohio P Index is most sensitive to the surface hydrologic connection between a field and water body, which explained 39-55% of the variability in the P Index score. The Ohio P Index is also relatively sensitive to either soil-test P or runoff class depending on the watershed (20-39% of the explained variability). Other input variables, such as fertilizer and organic P application rates, application methods, soil erosion, and the presence of a filter strip each explained <10% of the variability in the P Index score. The Ohio P Index is designed to help planners and producers identify management alternatives that minimize P loss; however, these findings suggest that in the current P Index changes in management practices (i.e., lowering P application rates, etc.) will result in minimal changes to the P Index score. To increase the predictive capability of the Ohio P Index and better reflect the original goals of the P Index concept, future research should focus on identifying and quantifying how individual input variables and practices influence P loss through field-scale monitoring and assessment. This study also highlights the advantages and disadvantages of utilizing the P Index at the watershed-scale compared to a statewide P Index.