Prediction of Phosphorus Losses in Runoff from Southern Plains Watersheds

The concentrations of soluble and particulate (sediment) P in runoff from several cropland and grassland watersheds across the Southern Plains were predicted from relationships obtained from simulated rainfall‐runoff studies. Soluble P was predicted from the water‐extractable P content of surface soil (0–1 cm) collected in March each year, using a general P‐extraction coefficient covering all the soils found at the watersheds studied. For individual soils the P‐extraction coefficient was related to the depth of interaction between surface soil and runoff. Particulate P concentration was predicted from the total P content of surface soil collected in March each year, using P‐enrichment ratios calculated from sediment discharge. Predictions of mean annual soluble and particulate P concentrations were good over a wide range in concentrations (50–500 and 200–5,100 µ g P/liter, respectively), fertilizer and watershed management practices, soil types, and vegetative covers. The results obtained indicate that relationships used in the CREAMS model can provide accurate predictions of mean annual P concentrations.