U.S. Geological Survey (USGS) modflow(1) NOTE: Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. NAME modflow - Modular three-dimensional finite-difference ground-water model ABSTRACT MODFLOW is a three-dimensional finite-difference ground-water model that was first published in 1984. It has a modular structure that allows it to be easily modified to adapt the code for a particular application. Many new capabilities have been added to the original model. Harbaugh (2005) documents a general update to MODFLOW, which is called MODFLOW-2005 in order to distinguish it from earlier versions. MODFLOW-2005 simulates steady and nonsteady flow in an irregularly shaped flow system in which aquifer layers can be confined, unconfined, or a combination of confined and unconfined. Flow from external stresses, such as flow to wells, areal recharge, evapotranspiration, flow to drains, and flow through river beds, can be simulated. Hydraulic conductivities or transmissivities for any layer may differ spatially and be anisotropic (restricted to having the principal directions aligned with the grid axes), and the storage coefficient may be heterogeneous. Specified head and specified flux boundaries can be simulated as can a head dependent flux across the model's outer boundary that allows water to be supplied to a boundary block in the modeled area at a rate proportional to the current head difference between a "source" of water outside the modeled area and the boundary block. In addition to simulating ground-water flow, the scope of MODFLOW-2005 has been expanded to incorporate related capabilities such as solute transport and ground-water management; however, this distribution incorporates only the ground-water flow parts of MODFLOW. METHOD The ground-water flow equation is solved using the finite-difference approximation. The flow region is subdivided into blocks in which the medium properties are assumed to be uniform. In plan view the blocks are made from a grid of mutually perpendicular lines that may be variably spaced. Model layers can have varying thickness. A flow equation is written for each block, called a cell. Several solvers are provided for solving the resulting matrix problem; the user can choose the best solver for the particular problem. Flow-rate and cumulative-volume balances from each type of inflow and outflow are computed for each time step. HISTORY MODFLOW-2005 Version 1.6.01 3/4/2009 - Fixed SFR problem in which the new NUMTIM and WEIGHT variables that are read when using the transient routing option (IRTFLG>0) were ignored. Also added the FLWTOL input variable to SFR. MODFLOW-2005 Version 1.6 1/23/2009 - Added HYDMOD and LMT Packages. Added NOVFC option to LPF Package. Added a full double-precision executable version of MODFLOW for downloading (see readme.txt file). Also fixed several miscellaneous problems. All of the changes are documented in the release.txt file. MODFLOW-2005 Version 1.5 4/28/2008 - Updated SFR, UZF, and LAK Packages to the versions used in the GSFLOW coupled ground-water and surface-water model. Also added the capability to use different damping factors in the PCG Package for transient and steady-state stress periods. Added option "DDREFERENCE" to the Output Control Files, which was an option implemented in MODFLOW-2000. Also fixed several miscellaneous problems. All of the changes are documented in the release.txt file. MODFLOW-2005 Version 1.4 11/2/2007 - Added SWT Package, added adaptive damping to GMG, and fixed minor problems (see release.txt file). MODFLOW-2005 Version 1.3.01 8/2/2007 - Checks added to prevent using HUF with UZF Package or the unsaturated flow part of SFR Package. MODFLOW-2005 Version 1.3 8/1/2007 - Fixed several minor problems (see release.txt file). MODFLOW-2005 Version 1.2.01 10/24/2006 - Fixed bug in SFR. MODFLOW-2005 Version 1.2 10/16/2006 - SFR, LAK, UZF, and GAG packages were added. MODFLOW-2005 Version 1.1 05/18/2006 - DRT, ETS, RES, and SUB packages were added. Observations (OBS Process) as implemented in MODFLOW-2000 were also added. MODFLOW-2005 Version 1.0 01/15/2006 - Initial release. DATA REQUIREMENTS In order to use MODFLOW, initial conditions, hydraulic properties, and stresses must be specified for every model cell in the finite- difference grid. Input data are read from files. SYSTEM REQUIREMENTS MODFLOW-2005 is written primarily in Fortran 90. Only the GMG solver package is written in C. The code has been used on UNIX-based computers and personal computers running various forms of the Microsoft Windows operating system. FUNTIONALITY This version of MODFLOW includes the following functionality that is documented in Harbaugh (2005). BAS -- Basic Package BCF -- Block-Centered Flow Package LPF -- Layer-Property Flow Package HFB -- Horizontal Flow Barrier Package CHD -- Time-Variant Specified-Head Option RIV -- River Package DRN -- Drain Package WEL -- Well Package GHB -- General Head Boundary Package RCH -- Recharge Package EVT -- Evapotranspiration Package SIP -- Strongly Implicit Procedure Package PCG -- Preconditioned Conjugate Gradient Package DE4 -- Direct solver The following functionality is also included. This functionality is documented in separate reports for use in earlier versions of MODFLOW. Conversion of this functionality to work with MODFLOW-2005 is documented in separate files that are provided with the MODFLOW-2005 distribution. STR -- Streamflow-Routing Package FHB -- Flow and Head Boundary Package IBS -- Interbed Storage Package GMG -- Geometric MultiGrid Solver Package HUF -- Hydrogeologic-Unit Flow Package MNW -- Multi-Node Well Package ETS -- Evapotranspiration with a Segmented Function Package DRT -- Drains with Return Flow Package RES -- Reservoir Package SUB -- Subsidence Package OBS -- Observation Process SFR -- Streamflow-Routing Package LAK -- Lake Package UZF -- Unsaturated Zone Package GAG -- Gage Package SWT -- Subsidence and Aquifer-System Compaction Package LMT -- Link to the MT3DMS contaminant-transport model HYDMOD -- Hydrograph capability Unlike MODFLOW-2000, MODFLOW-2005 does not support a parameter-estimation process. Users of MODFLOW-2005 are encouraged to take advantage of 'clip-on' parameter-estimation programs such as UCODE_2005. UCODE_2005 and supporting documentation are available at: http://typhoon.mines.edu/freeware/ucode/ DOCUMENTATION The basic documentation for MODFLOW-2005 is contained in: 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 p. The STR Package is documented in: Prudic, D.E., 1989, Documentation of a computer program to simulate stream-aquifer relations using a modular, finite-difference, ground-water flow model: U.S. Geological Survey Open-File Report 88-729, 113 p. The FHB Package is documented in: Leake, S.A., and Lilly, M.R., 1997, Documentation of a computer program (FHB1) for assignment of transient specified-flow and specified-head boundaries in applications of the modular finite- difference ground-water flow model (MODFLOW): U.S. Geological Survey Open-File Report 97-571, 50 p. The IBS Package is documented in: Leake, S.A. and Prudic, D.E., 1991, Documentation of a computer program to simulate aquifer-system compaction using the modular finite-difference ground-water flow model: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 6, Chapter A2, 68 p. The GMG Package is documented in: Wilson, J.D. and Naff, R.L., 2004, The U.S. Geological Survey modular ground-water model -- GMG linear equation solver package documentation: U.S. Geological Survey Open-File Report 2004-1261, 47 p. The HUF Package is documented in: Anderman, E.R., and Hill, M.C., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- Documentation of the Hydrogeologic- Unit Flow (HUF) Package: U.S. Geological Survey Open-File Report 00-342, 89 p. Anderman, E.R., Kipp, K.L., Hill, M.C., Valstar, Johan, and Neupauer, R.M., 2002, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- Documentation of the Model-Layer Variable-Direction Horizontal Anisotropy (LVDA) capability of the Hydrogeologic-Unit Flow (HUF) Package: U.S. Geological Survey Open-File Report 02-409, 60 p. Anderman, E.R., and Hill, M.C., 2003, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- Three additions to the Hydrogeologic-Unit Flow (HUF) Package: Alternative storage for the uppermost active cells, Flows in hydrogeologic units, and the Hydraulic-coductivity depth-dependence (KDEP) capability:U.S. Geological Survey Open-File Report 03-347, 36 p. The MNW Package is documented in: Halford, K.J. and Hanson, R.T., 2002, User guide for the drawdown-limited, multi-node well (MNW) package for the U.S. Geological Survey's modular three-dimensional finite-difference ground-water flow model, versions MODFLOW-96 and MODFLOW-2000: U.S. Geological Survey Open-File Report 02-293, 33 p. The DRT and ETS Packages are documented in: Banta, E.R., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- documentation of packages for simulating evapotranspiration with a segmented function (ETS1) and drains with return flow (DRT1): U.S. Geological Survey Open-File Report 00-466, 127 p. The RES Package is documented in: Fenske, J.P., Leake, S.A., and Prudic, D.E., 1996, Documentation of a computer program (RES1) to simulate leakage from reservoirs using the modular finite-difference ground-water flow model (MODFLOW): U.S. Geological Survey Open-File Report 96-364, 51 p. The SUB Package is documented in: Hoffmann, Jorn, Leake, S.A., Galloway, D.L., and Wilson, A.M., 2003, MODFLOW-2000 ground-water model -- User guide to the Subsidence and Aquifer-System Compaction (SUB) Package: U.S. Geological Survey Open-File Report 03-233, 46 p. The OBS Process is documented in: Hill, M.C., Banta, E.R., Harbaugh, A.W., and Anderman, E.R., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to the Observation, Sensitivity, and Parameter-Estimation Processes and three post-processing programs: U.S. Geological Survey Open-File Report 00-184, 209 p. See file obs.pdf for documentation of the OBS Process in MODFLOW-2005, including input instructions. The SFR Package is documented in: Prudic, D.E., Konikow, L.F., and Banta, E.R., A new Streamflow-Routing (SFR1) Package to simulate stream-aquifer interaction with MODFLOW-2000: U.S. Geological Survey Open File Report 2004-1042, 95 p. Niswonger, R.G., and Prudic, D.E., 2005, Documentation of the Streamflow-Routing (SFR2) Package to include unsaturated flow beneath streams -- a modification to SFR1: U.S. Geological Techniques and Methods Book 6, Chapter A13, 47 p. The UZF Package is documented in: Niswonger, R.G., Prudic, D.E., and Regan, R.S., 2006, Documentation of the Unsaturated-Zone Flow (UZF1) Package for modeling unsaturated flow between the land surface and the water table with MODFLOW-2005: U.S. Geological Techniques and Methods Book 6, Chapter A19, 62 p. The LAK Package is documented in: Merritt, M.L., and Konikow, L.F., 2000, Documentation of a computer program to simulate lake-aquifer interaction using the MODFLOW ground-water flow model and the MOC3D solute-transport model: Water-Resources Investigations Report 00-4167, 146 p. The GAG Package is documented as part of the above SFR and LAK documentation. The SWT Package is documented in: Leake, S.A. and Galloway, D.L., 2007, MODFLOW ground-water model -- User guide to the Subsidence and Aquifer-System Compaction Package (SUB-WT) for water-table aquifers: U.S. Geological Survey Techniques and Methods 6-A23, 42 p. The LMT Package is documented in: Zheng, Chunmiao, Hill, M.C., and Hsieh, P.A., 2001, MODFLOW-2000, the U.S. Geological Survey modular ground-water model - User guide to the LMT6 package, the linkage with MT3DMS for multi-species mass transport modeling: U.S. Geological Survey Open-File Report 01-82, 43 p. HYDMOD is documented in: Hanson, R.T. and Leake, S.A., 1999, Documentation of HYDMOD, a program for extracting and processing time-series data from the U.S. Geological Survey's modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 98-564, 57 p. A separate file, hydmod7.pdf, documents the conversion for MODFLOW-2005. CONTACTS U.S. Geological Survey Office of Ground Water 411 National Center Reston, VA 20192 (703) 648-5001 ogw_webmaster@usgs.gov See http://water.usgs.gov/software/ordering_documentation.html for information on ordering printed copies of USGS publications.