R. Brad Thoms


December 2003

Document Type


Degree Name



Dept. of Environmental and Biomolecular Systems


Oregon Health & Science University


The three-dimensional (3-D), modular finite-difference ground water model MODFLOW has been modified to simulate three-dimensional (3-D) variably saturated flow (using Richard’s equation) and two-dimensional (2-D) overland flow (using the kinematic wave approximation). Surface and subsurface flow are coupled at the iteration level and both are contained within the MODFLOW finite difference grid. The resulting modifications retain the modular structure of the MODFLOW code and preserve the model’s existing capabilities as well as its compatibility with existing transport models and commercial pre/post processors. Model performance is evaluated with an analytical solution for 1-dimensional constant head infiltration (Dirichlet boundary condition), results from a field experiment of 1-dimensional constant rainfall infiltration (Neumann boundary condition) and results from a 2-dimensional conjunctive surface-subsurface flow soil flume experiment. An investigation into the relative influence of model grid size and soil characteristics on model performance is also undertaken and it is determined that non-linear soils require fine spatial discretization for acceptable solution convergence. 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.




OGI School of Science and Engineering



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