Nutrients originating from agricultural watersheds are known drivers of water quality impairment by causing eutrophication. The recently modified Ontario Phosphorus index (PLATO) is one P loss risk assessment tool to identify the areas vulnerable to P loss. Applicability and accuracy of PLATO is being tested in this study at two sites in Southwestern Ontario at field/intra-field scales. The main objective of this study is to identify the critical source areas to P loss within the Gully Creek watershed using this modified P index. Other P loss risk assessment tools (NCPLAT, APLE) are compared to enhance the results certainty at watershed level. The study also evaluates the spatial distribution of P loss risk within a field in the Bayfield river basin. The results indicate that at both field and sub-field scales the dominant P loss process in the rolling topography studied is particulate P through soil erosion loss.