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.