Tile drains are widely used in the Midwestern United States to improve the productivity of poorly drained agricultural fields. Since a tile drain reduces vadose zone soil moisture by lowering the water table, and its outlets feed directly into streams and ditches, tile flow can affect various hydrologic, biotic and biogeochemical processes in the watershed the streams. However, the effects of spatially resolved micro-topographic variability, such depressions and roadside ditches, on tile flow and their accumulated impact on ecohydrologic and nutrient dynamics remain poorly understood. Here we present an explicit model of tile flow and incorporated into the integrated ecohydrologic-flow model, MLCan-GCSFlow, to investigate the impacts of tile drain on ecohydrologic and nutrient dynamics in intensively managed agricultural fields at lidar-resolution scales. Explicit coupling between subsurface and tile flow is obtained by modifications of variably saturated Richards equation to capture the impacts of tile drain on soil moisture. The coupling between subsurface and overland flow is obtained by prescribing a boundary condition switching approach at the top surface of the computational domain. Model results for study sites in Critical Zone Observatory for Intensively Managed Landscapes (IMLCZO) show the significance of tile drain flow on the vertical and spatial soil moisture distribution and coupled surface - sub-surface flow dynamics
