Simulating coupled surface and subsurface water flow in a tile-drained agricultural catchment

The impact of shallow tile drainage networks on groundwater flow patterns, and associated transport of chemical species or sediment particles, needs to be quantified to evaluate the effects of agricultural management on water resources. A current challenge is to represent tile drainage networks in numerical models at the scale of a catchment for which proper simulation and reproduction of subsurface drainage water dynamics are essential. This study investigates the applicability of various tile drainage modeling concepts by comparing their results to a reference model. The models were setup for a 3.5. ha regularly monitored tile-drained area located in the clayey till Lillebæk catchment in Denmark, where the main input of water to the streams originates from subsurface drainage. Several simulations helped identify the most efficient way of modeling tile-drained areas using the integrated surface water and groundwater HydroGeoSphere code, which also simulates one-dimensional water flow in tile drains. The aim was also to provide insight on the design of larger scale models of complex tile drainage systems, for which computational time can become very large. A reference model that simulated coupled surface flow, groundwater flow and flow in tile drains was setup and showed a rapid response in drain outflow when precipitation is applied. A series of additional simulations were performed to test the influence of flow boundary conditions, temporal resolution of precipitation data, conceptual representations of clay tills and drains, as well as spatial discretization of the mesh. For larger scale models, the simulations suggest that a simplification of the geometry of the drainage network is a suitable option for avoiding highly discretized meshes. Representing the drainage network by a high-conductivity porous medium layer reproduces outflow simulated by the reference model. This approach greatly reduces the mesh complexity and simulation time and thus represents an alternative to a discrete representation of drains at the field scale, but specifying the hydraulic properties of the layer requires calibration against observed drain discharge.