Nutrient enrichment is a critical issue affecting Iowa surface water bodies. The Upper Midwestern United States is heavily drained and subsurface drainage provides a direct pathway for nutrients to enter surface waters. Nitrogen is typically the focus of research on nutrients in drainage; however, current research has shown that drainage can be a significant source of phosphorous loading as well. The goal of the proposed project is to evaluate the impact of subsurface drainage on P concentrations in surface waters. Objectives of the study are (1) to determine intra-event contributions of different P pathways and (2) apply the ICECREAMDB phosphorous model to Iowa agricultural watersheds. ICECREAMDB is a graphical front-end for the ICECREAM model that also includes options to structure outputs. ICECREAMDB is a management oriented phosphorus loss model that quantifies runoff, erosion, and P losses at the field scale and has the capability to simulate phosphorus losses through drainage systems. Water quality monitoring and model simulations will be conducted on the Black Hawk Lake watershed located in Carroll and Sac counties in Iowa. The Black Hawk Lake watershed was selected for this study because its three unique monitoring locations allow comparison of overland surface flow to tile flow water quality. Results of the project include an intra-event analysis of total phosphorus and dissolved reactive phosphorus concentrations in grass waterway, surface runoff, and tile flow and testing of a new model for predicting phosphorous in tiles, ICECREAMDB, in the heavily drained Iowa landscape.