MODFLOW-USG: An Unstructured Grid Version of MODFLOW for Simulating
Groundwater Flow and Tightly Coupled Processes Using a Control Volume
Finite-Difference Formulation
A version of MODFLOW, called MODFLOW-USG (for UnStructured Grid), was
developed to support a wide variety of structured and unstructured grid
types, including nested grids and grids based on prismatic triangles,
rectangles, hexagons, and other cell shapes. Flexibility in grid design
can be used to focus resolution along rivers and around wells, for
example, or to subdiscretize individual layers to better represent
hydrostratigraphic units.
MODFLOW-USG is based on an underlying control volume finite difference
(CVFD) formulation in which a cell can be connected to an arbitrary
number of adjacent cells. To improve accuracy of the CVFD formulation
for irregular grid-cell geometries or nested grids, a generalized Ghost
Node Correction (GNC) Package was developed, which uses interpolated
heads in the flow calculation between adjacent connected cells.
MODFLOW-USG includes a Groundwater Flow (GWF) Process, based on the GWF
Process in MODFLOW-2005, as well as a new Connected Linear Network
(CLN) Process to simulate the effects of multi-node wells, karst
conduits, and tile drains, for example. The CLN Process is tightly
coupled with the GWF Process in that the equations from both processes
are formulated into one matrix equation and solved simultaneously. This
robustness results from using an unstructured grid with unstructured
matrix storage and solution schemes.
MODFLOW-USG also contains an optional Newton-Raphson formulation, based
on the formulation in MODFLOW-NWT, for improving solution convergence
and avoiding problems with the drying and rewetting of cells. Because
the existing MODFLOW solvers were developed for structured and
symmetric matrices, they were replaced with a new Sparse Matrix Solver
(SMS) Package developed specifically for MODFLOW-USG. The SMS Package
provides several methods for resolving nonlinearities and multiple
symmetric and asymmetric linear solution schemes to solve the matrix
arising from the flow equations and the Newton-Raphson formulation,
respectively.