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This article in JEQ

  1. Vol. 22 No. 1, p. 32-37
    Received: Nov 18, 1991

    * Corresponding author(s):
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Prediction of Bioavailable Phosphorus Loss in Agricultural Runoff

  1. Andrew N. Sharpley * and
  2. S.J. Smith
  1. USDA-ARS, National Agricultural Water Quality Laboratory, P.O. Box 1430, Durant, OK 74702-1430.



The loss of bioavailable P (BAP) in both soluble (SP) and participate (BPP, bioavailable particulate P) forms in agricultural runoff, can accelerate the eutrophication of surface waters. Bioavailable P represents P that is potentially available for algal uptake. Although prediction of BAP loss would provide a more reliable assessment of the impact of agricultural management on surface water eutrophication than just SP or total P (TP), currently no predictive equations are available. Bioavailable P transport in soluble and particulate forms in runoff from 20 agricultural watersheds in the Southern Plains over a 5-yr period, was predicted using kinetic and enrichment ratio approaches, respectively. Measured and predicted SP and BPP concentrations of individual runoff events were similar for all watersheds and management practices over a range of 7 to 3522 and 3 to 4059 µg L−1, respectively. Calculated as the sum of SP and BPP, BAP loss in runoff was predicted accurately, with only a 12% error in measured mean annual loss (662 g ha−1 yr−1). Although BAP loss below 100 g ha−1 yr−1 was underestimated, above this value measured and predicted values closely followed a 1:1 relationship. Inclusion of these equations in comprehensive water quality models will improve the prediction of agricultural management on the biological response of receiving water bodies.

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