**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PHOSPHORUS (P) TRANSPORTED VIA SURFACE AND SUBSURFACE FLOWS FROM AGRICULTURAL SYSTEMS PLAYS A MAJOR ROLE IN WATER QUALITY IMPAIRMENTS DOWNSTREAM, INCLUDING HARMFUL ALGAL BLOOMS (HABS). IN HEAVILY TILE-DRAINED AGRICULTURAL LANDSCAPES SUCH AS THE WESTERN LAKE ERIE BASIN (WLEB), UNDERSTANDING AND QUANTIFYING DYNAMICS OF DISSOLVED P (DP) TRANSPORT THROUGH TILE DRAINAGE IS CRITICAL. WHILE THE TRANSPORT OF TOTAL P (TP) AND DP HAVE BEEN EXTENSIVELY STUDIED, KNOWLEDGE AND MODELING GAPS EXIST IN RELATING SOIL PROPERTIES WITH SORPTION DYNAMICS AND P TRANSPORT THROUGH MATRIX AND PREFERENTIAL FLOW PATHWAYS. WHILE P TRANSPORT THROUGH SURFACE RUNOFF IS FAIRLY WELL-REPRESENTED IN COMMONLY-APPLIED MODELS, P TRANSPORT TO TILE DRAINAGE IS EITHER IGNORED OR POORLY REPRESENTED. POOR REPRESENTATION OF P SORPTION AND TRANSPORT PROCESSES IMPACT THE USEFULNESS OF WATERSHED MODELS IN GUIDING ENVIRONMENTAL POLICY. TO FILL THIS KNOWLEDGE GAP, WE AIM TO CHARACTERIZE THE PROCESS OF P DESORPTION FROM TOPSOILS AND SORPTION BY SUBSOILS USING NOVEL, PROVEN METHODS. WE WILL THEN FILL THE MODELING GAP BY USING KNOWLEDGE FROM LABORATORY STUDIES TO IMPROVE THE THEORETICAL FRAMEWORK OF A WATERSHED MODEL (SOIL WATER ASSESSMENT TOOL - SWAT), BY ADOPTING MATHEMATICAL MODELS OF P DYNAMICS AND IMPROVING THE REPRESENTATION OF PREFERENTIAL AND MATRIX FLOW TO TILE DRAINS. FINALLY, WE WILL EVALUATE MODEL IMPROVEMENTS BY CONDUCTING SENSITIVITY ANALYSIS AND COMPARING MODEL OUTPUTS OF A FIELD-SCALE SWAT MODEL AGAINST FIELD-MEASURED WATER QUANTITY AND QUALITY DATA. EXPECTED OUTCOMES INCLUDE (1) ROBUST EVIDENCE OF THE MECHANISMS CONTROLLING P TRANSPORT TO TILE DRAINS, (2) A TRANSFERABLE METHOD OF SIMULATING P TRANSPORT AND DYNAMICS WITHIN HYDROLOGIC MODELS.OUR RESEARCH WILL BE IMPACTFUL IN PROVIDING FARMERS AND OTHER STAKEHOLDERS WITH MORE ACCURATE ESTIMATIONS OF PHOSPHORUS LOADS IN THE WLEB, WHICH WILL PROMOTE BETTER MANAGEMENT OF NUTRIENT POLLUTION. FURTHER, A TRANSFERABLE METHOD OF SIMULATING PHOSPHORUS TRANSPORTWITHIN A HYDROLOGIC MODEL WILL FACILITATE BETTER MANAGEMENT OF THIS POLLUTION AND EUTROPHICATION IT CAUSES IN WATERSHEDS BEYOND THE WLEB. FURTHER, OUR PROJECT WILL IMPROVE THE CAPACITY TO REPRESENT AND EVALUATE P TRANSPORT PROCESSES WITHIN WATERSHED-SCALE HYDROLOGIC MODELS, WHICH ARE COMMONLY APPLIED TO INFORM STAKEHOLDER DECISION MAKING AND POLICY DEVELOPMENT.