Phosphorus Risk Assessment Index Evaluation Using Runoff Measurements

An index to evaluate the phosphorus (P) pollution potential of agricultural fields was proposed by Lemunyon and Gilbert in 1993. Data .from three rainfall simulation studies were used to evaluate the relative importance of the variables in the P index. These studies included plots containing sorghum (Sorghum bicolor L.), winter wheat (Triticum aestivum L), and corn (Zea mays L) residues on which chemical fertilizer and composted or noncomposted beef cattle feedlot manure were applied under no-till and disked conditions. The factors of erosion, runoff, soil P level P application source, and method, and rate of P addition were weighted and considered in the index calculation. Measured soil erosion accounted for 18% and 88% of the variability in total and particulate P losses,respectively, indicating the importance of erosion in the lossof sediment bound P. Bioavailable and dissolved P losseswere unrelated to measured soil erosion but were influenced by tillage, runoff amount, and P source. The P index of Lemunyon and Gilbert was modified based on the results of these experiments. The correlation coefficient value (r) between total P loss and the modified P index values, was 0.74 as compared to 0.52for the Lemunyon and Gilbert'sP index.Additive index valuesweremorecloselycorrelatedwith total P lossthan the index valuescalculatedbymultiplyingthe weightof eachfactor. If erosionand runoff are accuratelypredicted, the P index can serve as a usefUltoolfor identifying sites where transport of P to surface water can be a potential concern.