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This article in JEQ

  1. Vol. 41 No. 6, p. 1730-1740
    Received: Jan 2, 2012
    Published: October 16, 2012October 16, 2012October 16, 2012

    * Corresponding author(s): lwgood@wisc.edu
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Testing the Wisconsin Phosphorus Index with Year-Round, Field-Scale Runoff Monitoring

  1. Laura W. Good *a,
  2. Peter Vadasb,
  3. John C. Panuskac,
  4. Carlos A. Bonillad and
  5. William E. Jokelae
  1. a Dep. of Soil Science, Univ. of Wisconsin-Madison, 1525 Observatory Dr., Madison, WI 53706
    b USDA Dairy Forage Research Center, 1925 Linden Dr. West, Madison, WI 53706
    c Biological Systems Engineering Dep., Univ. of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706
    d Dep. of Hydraulic and Environmental Engineering, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Macul, Santiago, Chile
    e USDA–ARS, 2615 Yellowstone Dr., Marshfield, WI 54449. Assigned to Associate Editor Douglas Smith


The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash–Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.

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Copyright © 2012. Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.