Submitted to: SERA-IEG 17 Bulletin
Publication Type: Other
Publication Acceptance Date: 1/3/2013
Publication Date: 1/15/2013
Citation: Sharpley, A., Bolster, C.H., Conover, C., Dayton, E., Davis, J., Easton, Z., Good, L., Gross, C., Kleinman, P., Mallinaro, A., Moffitt, D., Nelson, N., Norfleet, L., Osmond, D., Perry, R., Thompson, A., Vadas, P.A., White, M.J. 2013. Technical guidance for assessing phosphorus indices. SERA-IEG 17 Bulletin. 417:78. Interpretive Summary:
Technical Abstract: Disparate nutrient and land management recommendations generated by P Indices among states, a perceived lack of change in P-based management, and persistent P loading problems in many of the nation’s waters, led to a revision of the 590 Standard. The revision requires an assessment of P Indices across the U.S. to ensure that each state Index is correctly ranking the potential for P delivery to surface water. This report describes the rationale behind P Index assessment, how assessment should be conducted, what data and models should be used, and how assessments may be interpreted and incorporated into P Index revisions. At the end of 2012, NRCS funded six CIG to assess P Indices across the U.S. This report does not review their objectives, but provides as Appendices 1, 2, and 3, regional CIG methodologies and ongoing NRCS P Index assessments using APEX. The overarching intent of assessing Indices is to ensure they appropriately rank risk of actual P loss for any given site relative to other sites; are directionally and magnitudinally correct, in that as factors influencing P loss change to increase or reduce that loss, P Indices correctly estimate the extent of change in P loss; interpretations based on assigned risk are equivalent across state borders, given similar site and water resource conditions; and where inadequacies exist, the causes can be identified and rectified. The main recommendations are; • Runoff monitoring data are required to build confidence in P Index representation of site P loss potential, as well as validate nonpoint source models. Databases should include at a minimum runoff, site conditions, climate, management, and P loss over the planning / rotation period under natural rainfall. • Several models are available, such as APEX, APLE, and DrainMod. • Baseline management scenarios must be developed, against which to compare Index performance and source and transport factors influencing P loss ranked as locally-relevant low, medium, and high loss. • Conditions must be defined that result in both unacceptable P loss within a model and high P Index ratings that limit or preclude P applications run under the same set of conditions. • Determination of uncertainty or variability of Indexed risk and P loss is recommended.