APPLICATION OF SIMPLIFIED PHOSPHORUS TRANSPORT MODELS TO PASTURE FIELDS IN NORTHWEST ARKANSAS

Runoff transport of phosphorus (P) is often predicted from simple equa tions 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 situ ations are less well defined The objectives of this study were to dete rmine parameters of simplified runoff P transport equations for pastur e fields and to assess the accuracy and precision of the equations. Ru noff, 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 comp uted, and confidence limits on respective predicted particulate and so luble P transport were determined. An inverse linear relationship betw een the natural logarithms of enrichment ratio and sediment yield was found significant for all fields, but the slopes were lower than value s reported earlier for general use. Runoff event extraction coefficien ts 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 transpor t varied in some cases by more than an order of magnitude, indicating that significant imprecision was associated with those predictions. Th e data suggested that the simplified model of soluble P transport migh t be a reasonable description of the processes for the fields, but tha t modification to the particulate P transport prediction method might be necessary to improve the prediction accuracy for low event sediment yields (< 10 kg/ha).