Modelling groundwater/surface water interactions using MODFLOW-WhaT

The three-dimensional (3-D), modular finite-difference groundwater model MODFLOW has been modified to simulate 3-D variably saturated flow (using Richard's equation) and 2-D overland flow (using the kinematic wave approximation). Surface and subsurface flow are coupled at the iteration level and are contained within the MODFLOW finite difference grid. This approach treats the surface/subsurface flow as an integrated system which is solved by an implicit finite difference formulation that uses Picard iterations to reach convergence based on a head criterion. The resulting modifications retain the modular structure of the MODFLOW code and preserve the model's existing capabilities as well as its compatibility with commercial pre/post processors. Two conjunctive surface/subsurface flow simulations are presented; a soil flume experiment and a watershed flowpath simulation. The simulation cases both verify the model's performance and demonstrate the model's ability to capture seasonal groundwater/surface water interactions. The overall success of the model in simulating conjunctive surface/subsurface flow, mixed boundary conditions and variable soil types demonstrates its utility for future hydrologic investigations.