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The P index (PI) is a proposed tool for rating the potential for offsite losses of P from a field or other management unit. The eight site characteristics comprising the PI were evaluated on several best management practice (BMP) demonstration plots in the Dairy‐McKay Hydrologic Unit Area (HUA) in western Oregon and on an entire subbasin in the Granger Drain HUA in eastern Washington. An estimate of site vulnerability is obtained by adding the weighted values of the site characteristics. Site vulnerability ranged from low to medium in the Dairy‐McKay HUA. The medium values were due to P fertilizer application rates of > 66 lb P/acre and surface sidedress application. Site vulnerability ratings by soil type for fields in the Granger Drain HUA were concentrated in the medium and high categories. This was due to high values for site characteristics of irrigation erosion and surface runoff of irrigation water, high soil test P levels, and application of organic P from animal wastes. This study demonstrates that the PI can be used to rank soils as to their potential for P loss between sites. Validations studies are needed to determine if high vulnerability sites are significantly affecting surface waters. Research Question Increased concern over the movement of P from the landscape into surface waters has brought about a need to assess the potential of a given landscape to affect P movement to surface waters. A field‐level P index (PI) is needed to assess and to assist in the development of management practices to lessen the potential for P movement. The object of this study was to determine if the current rating system would provide a logical numerical separation of sites with differing characteristics. Literature Summary Attempts have been made to develop models and procedures that would estimate the actual P loading from a watershed to a body of surface water. The PI reported here has been developed to assess a site's potential to harm surface water resources through P movement off the site. This PI does not attempt to estimate quantities of P moving from the site, but only the potential for movement. Study Description The proposed PI was used to determine the site vulnerability of production areas that were part of ongoing water quality research and education efforts. The PI was applied to a study area located in the Dairy‐McKay Hydrologic Unit Area (HUA) in Tualatin River basin of western Oregon in an area characterized by moist, mild winters and dry summers with 39 to 47 in. annual precipitation. Two areas designated MR and SU were selected for the study. The PI was applied to plots established to investigate the use of red clover as a winter cover crop, and management practices needed to control runoff and erosion from strawberry fields. A second study area was in the Granger Drain HUA located in southern Yakima Valley of Washington. This irrigated agriculture area is characterized by hot dry summers and cold dry winters, with 7 to 9 in. annual precipitation. Producer fields were used in this study area. The PI...

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