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We applied a SPARROW-based empirical watershed model to assess regional total phosphorus (P) export coefficients that can assist with nutrient mitigation projects and support adaptive watershed management. Limitations in the number of tributary monitoring stations were overcome by assembling multi-agency water quality data and extending the study area to include Lake Simcoe Region, Nottawasaga Valley and Grey Sauble Conservation Authorities. To analyze tributary nutrient loading during different flow regimes, such as dry and wet years, we applied a Bayesian hierarchical framework. Agriculture was identified as a major non-point P source representing between 30 and 48% of the delivered P loading. Our source apportionment predicted P loss rates from croplands that exceeded those from forested areas by 320% during dry years and by 360% during wet years, while low intensity agricultural areas (hay and pasture) exceeded P export from forests by a mere 20% and 30%, respectively. Our study identified urban runoff as another significant non-point nutrient source displaying the highest variability between dry and wet years. P export rates from urban areas were 3-4 times higher than those in intensive agriculture during high flow regimes. Importantly, owing to the massive urbanization in the Lake Simcoe watershed, urban runoff contributed nearly half of the delivered P loading from the tributaries into the lake. As a result of our analysis, we emphasize the importance of mitigating urban non-point sources as well as controlling agricultural runoff. [ABSTRACT FROM AUTHOR]