MODFLOW-NWT: a Newton Formulation for MODFLOW-2005
The USGS MODFLOW-NWT is a Newton-Raphson formulation for MODFLOW-2005
to improve solution of unconfined groundwater-flow problems.
MODFLOW-NWT is a standalone program that is intended for solving
problems involving drying and rewetting nonlinearities of the
unconfined groundwater-flow equation.
MODFLOW-NWT must be used with the Upstream-Weighting (UPW) Package for
calculating intercell conductances in a different manner than is done
in the Block-Centered Flow (BCF), Layer Property Flow (LPF), or
Hydrogeologic-Unit Flow (HUF) Packages. Flow-property input for the UPW
Package is designed based on the LPF Package and material-property
input is identical to that for the LPF Package except that the
rewetting and vertical-conductance correction options of the LPF
Package are not available with the UPW Package. Input files constructed
for the LPF Package can be used with slight modification as input for
the UPW Package.
The NWT linearization approach generates an asymmetric matrix, which is
different from the standard MODFLOW formulation that generates a
symmetric matrix. Because all linear solvers presently available for
use with MODFLOW-2005 solve only symmetric matrices, MODFLOW-NWT
includes two previously developed asymmetric matrix-solver options. The
matrix-solver options include a generalized-minimum-residual (GMRES)
Solver and an Orthomin / stabilized conjugate-gradient (CGSTAB) Solver.
MODFLOW-NWT is described in the documentation report by Niswonger and
others (2011). The Surface-Water Routing (SWR1) and Seawater Intrusion
(SWI2) Packages, which are included with MODFLOW-NWT are documented in
Hughes and others (2012) and Bakker and others (2013), respectively.