Runoff transport of phosphorus (P) is often predicted from simple equations with parameters determined from data applicable primarily to row-cropped and fallow cover conditions. The applicability, accuracy, and precision of such P transport prediction equations under pasture situations are less well defined. The objectives of this study were to determine parameters of simplified runoff P transport equations for pasture fields and to assess the accuracy and precision of the equations. Runoff, sediment yield, soluble P transport, and particulate P transport data were collected from four pasture fields in northwestern Arkansas. Runoff event enrichment ratios and extraction coefficients were computed, and confidence limits on respective predicted particulate and soluble P transport were determined. An inverse linear relationship between the natural logarithms of enrichment ratio and sediment yield was found significant for all fields, but the slopes were lower than values reported earlier for general use. Runoff event extraction coefficients were considerably higher than those typically used and were highest for runoff occurring shortly following animal manure application. The 95% confidence limits on predicted soluble and particulate P transport varied in some cases by more than an order of magnitude, indicating that significant imprecision was associated with those predictions. The data suggested that the simplified model of soluble P transport might be a reasonable description of the processes for the fields, but that modification to the particulate P transport prediction method might be necessary to improve the prediction accuracy for low event sediment yields (< 10 kg/ha).

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Published in Transactions of the ASAE, v. 39, issue 2, p. 489-496.

© 1996 American Society of Agricultural Engineers

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