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Journal of Environmental Quality Abstract - Landscape and Watershed Processes

Assessing Site Vulnerability to Phosphorus Loss in an Agricultural Watershed


This article in JEQ

  1. Vol. 30 No. 6, p. 2026-2036
    Received: Jan 31, 2001

    * Corresponding author(s): ans3@psu.edu
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  1. Andrew N. Sharpley *,
  2. Richard W. McDowell,
  3. Jennifer L. Weld and
  4. Peter J. A. Kleinman
  1. USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Building 3702, Curtin Road, University Park, PA 16802-3702


A P index was developed as a tool to rank agricultural fields on the basis of P loss vulnerability, helping to target remedial P management options within watersheds. We evaluated two approaches, a soil P threshold and components of a P index, by comparing site vulnerability estimates derived from these two approaches with measured runoff P losses in an agricultural watershed in Pennsylvania. Rainfall–surface runoff simulations (70 mm h−1 for 30 min) were conducted on 57 sites representing the full range of soil P concentrations and management conditions found in the watershed. Each site was comprised of two, abutting 2-m2 runoff plots, serving as duplicate observations. For sites that had not received P additions for at least six months prior to the study, Mehlich-3 P concentration was strongly associated with dissolved P concentrations (r 2 = 0.86) and losses (r 2 = 0.83) in surface runoff, as well as with total P concentration (r 2 = 0.80) and loss (r 2 = 0.74). However, Mehlich-3 P alone was poorly correlated with runoff P from sites receiving manure within three weeks prior to rainfall. The P index effectively described 88 and 83% of the variability in dissolved P concentrations and losses from all sites in the watershed, and P index ratings exhibited strong associations with total P concentrations (r 2 = 0.81) and losses (r 2 = 0.79). When site-specific observations were extrapolated to all fields in the watershed, management recommendations derived from a P index approach were less restrictive than those derived from the soil P threshold approach, better reflecting the low P loads exported from the watershed.

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Copyright © 2001. American Society of Agronomy, Crop Science Society of America, Soil Science SocietyPublished in J. Environ. Qual.30:2026–2036.