Phosphorus index (PI) is a risk-assessment tool that combines phosphorus (P) source factors and transport factors to rank the vulnerability of fields to P losses. Here we present the structure and concepts of conditional PI, developed as an educational and P-management tool adjusted for Swedish conditions. Because the significance of certain factors for P losses depends on their interplay with other factors, conditional rules are needed for a more accurate process description and quantification. Accounting for P losses through the soil profile, separate calculations for reactive and unreactive P and a changed approach to P loss assessment from erosion losses are some of the new features included in the tool presented here. The performance of the tool was tested by comparing the calculated PI values with measured annual P transport from seven observation fields included in a Swedish water quality monitoring program. This first test indicated that the tool could be used successfully for P loss risk assessment.
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Vol. 34 • No. 4