SUMMARY OF MODFLOW-NWT NOTE: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. ABSTRACT MODFLOW-NWT is a Newton formulation for MODFLOW-2005. 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. The UPW Package treats nonlinearities of cell drying and rewetting by use of a continuous function of groundwater head, rather than the discrete approach of drying and rewetting that is used by the BCF, LPF, and HUF Packages. This further enables application of the Newton formulation for unconfined groundwater-flow problems because conductance derivatives required by the Newton method are smooth over the full range of head for a model cell. 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. 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. MODFLOW-NWT is described in the documentation report by Niswonger and others (2011). HISTORY MODFLOW-NWT Version 1.0 05/16/2011 - Initial release. Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.1 05/27/2011 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.2 10/03/2011 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.3 01/03/2012 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.4 01/25/2012 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.5 05/14/2012 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.6 12/07/2012 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.7 01/15/2013 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.8 09/24/2013 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.0.9 07/01/2014 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.1.0 07/21/2016 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.1.1 07/28/2016 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.1.2 09/15/2016 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.1.3 08/01/2017 - Also see the file ‘release.txt’ for additional release notes. MODFLOW-NWT Version 1.1.4 04/01/2018 - Also see the file ‘release.txt’ for additional release notes. SYSTEM REQUIREMENTS MODFLOW-NWT is written in Fortran 90 programming language. The code has been used on personal computers running various forms of the Microsoft Windows operating system. FUNTIONALITY MODFLOW-NWT can be run using the Newton linearization (NWT Package) and the Upstream-Weighting (UPW) Package or it can be run using the standard MODFLOW-2005 Picard linearization method (Harbaugh, 2005). Thus, some MODFLOW-2005 Packages are supported by MODFLOW-NWT only when using the Picard linearization method of MODFLOW-2005. This version of MODFLOW-NWT includes the following MODFLOW-2005 packages: MODFLOW-2005 packages: BAS -- Basic Package BCF -- Block-Centered Flow Package (Picard only) CHD -- Time-Variant Specified-Head Option DE4 -- Direct solver (Picard only) DRN -- Drain Package EVT –- Evapotranspiration Package GAG -- Gage Package GHB -- General Head Boundary Package HFB -- Horizontal Flow Barrier Package HUF -- Hydrogeologic-Unit Flow Package (Picard only) LAK -- Lake Package LPF -- Layer-Property Flow Package (Picard only) MNW1 -- Multi-Node Well Package, version 1 MNW2 -- Multi-Node Well Package, version 2 OBS –- Observation Process PCG -- Preconditioned Conjugate Gradient Package (Picard only) RCH –- Recharge Package RIV –- River Package SFR -- Streamflow-Routing Package SIP -- Strongly Implicit Procedure Package (Picard only) SWR -- Surface-Water Routing Package UZF -- Unsaturated-Zone Package WEL -- Well Package SWT -- Subsidence-Water Table Package SWI -- Sea Water Intrusion Package LMT -- Link-MT3DMS Package Other packages that are included with the MODFLOW-NWT release but have not been tested include: MODFLOW-2005 packages: DRT -- Drain Return Package ETS -- Evapotranspiration Segments Package FHB -- Specified Flow Package Hydmod -- Hydrograph data for BAS Package IBS -- Interbed Storage Package RES -- Reservoir Package STR -- Stream Package SUB -- Subsidence Package DOCUMENTATION Niswonger, R.G., Panday, Sorab, and Ibaraki, Motomu, 2011, MODFLOW-NWT, A Newton formulation for MODFLOW-2005: U.S. Geological Survey Techniques and Methods 6–A37, 44 p. Available online at http://pubs.usgs.gov/tm/tm6a37/ REFERENCE Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model--the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16, variously paginated. Available online at http://pubs.water.usgs.gov/tm6A16/ CONTACT Richard Niswonger U.S. Geological Survey 345 Middlefield Rd. MS470 Menlo Park, CA 94025 (650) 329-4534