Fully-coupled physically-based approach for modeling conventional and managed subsurface drainage

Subsurface drainage is an essential water management practice that is partly responsible for the legendary agricultural productivity of the "corn-belt" region in the United States. Recent researches indicate that the conventional drainage practice has contributed a large percentage of the surface water pollution by agricultural chemicals while the managed drainage practice can address the environmental concerns as well as the needs for crop yields. The fully-coupled physically-based hydrology models are important tools for evaluating these environmental issues and agroeconomic benefits by considering the subsurface drainage practices into the integrated surface and subsurface flow environment at the agricultural fields. Our research on the applications of the integrated hydrology model HydroGeoSphere to the real tile-drained farmlands in Indiana and Illinois yields a new tile drain module that can simulate both the conventional and managed drainage practices. The new tile drain module features the embedded node method for integrating tile drains into the subsurface, the resistance adjustment method for computing lateral flow into tile drains, the Preissmann slot method for combining free surface and pressurized flow routings inside tile drains, and a ghost node flow boundary condition for the free fall and submergence conditions at the drainage outlet. The new tile drain module is verified with both hypothetical examples and laboratory experiments. Applications of HydroGeoSphere with the new tile drain module to 2D and 3D agricultural drainage fields are demonstrated and discussed