NAMEmodflow - Modular three-dimensional finite-difference ground-water flow modelABSTRACTMODFLOW is a three-dimensional finite-difference ground-water flow model. 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. This version includes all the major capabilities that were documented as of September 1996. MODFLOW 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 direction aligned with the grid axes and the anisotropy ratio between horizontal coordinate directions is fixed in any one layer), and the storage coefficient may be heterogeneous. The model requires input of the ratio of vertical hydraulic conductivity to distance between vertically adjacent block centers. 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. MODFLOW is currently the most used numerical model in the U.S. Geological Survey for ground-water flow problems. An efficient contouring program is available (Harbaugh, 1990) to visualize heads and drawdowns output by the model.METHODThe ground-water flow equation is solved using the finite-difference approximation. The flow region is considered to be subdivided into blocks in which the medium properties are assumed to be uniform. The plan view rectangular discretization results from a grid of mutually perpendicular lines that may be variably spaced. The vertical direction zones of varying thickness are transformed into a set of parallel "layers". Several solvers are provided for solving the associated matrix problem; the user can choose the best solver for the particular problem. Mass balances are computed for each time step and as a cumulative volume from each source and type of discharge.HISTORYVersion 2.6 1996/09/20 - Added Reservoir package (RES1) as documented in U.S. Geological Survey Open-File Report 96-364. Problem fixed for IBS package. Although subsidence is only meant to be active for layers in which IBQ>0, sometimes MODFLOW performed subsidence calculations when IBQ<0. Note that this was a problem only if negative IBQ values were specified. That is, the code has always worked correctly for IBQ=0 and IBQ>0. Version 2.5 1995/06/23 - Added direct solution package (DE45). Version 2.4 1995/06/15 - Added transient leakage package (TLK1). Version 93/08/30 - Release with PCG2, BCF3, STR1, HFB1, ISB1, CHD1, and GFD1 additions. Version 87/07/24 - Fortran 77 version published in U.S. Geological Survey Techniques of Water-Resources Investigations 6-A1. Version 83/12/28 - Fortran 66 version published in U.S. Geological Survey Open-File Report 83-875.DATA REQUIREMENTSIn order to use MODFLOW, initial conditions, hydraulic properties, and stresses must be specified for every model cell in the finite- difference grid.OUTPUT OPTIONSPrimary output is head, which can be written to the listing file or into a separate file. Other output includes the complete listing of all input data, drawdown, and budget data. Budget data are printed as a summary in the listing file, and detailed budget data for all model cells can be written into a separate file.SYSTEM REQUIREMENTSMODFLOW-88 is written in Fortran 77 with the following extensions: use of variable names longer than 6 characters and the CARRIAGECONTROL option in OPEN statements. By default, the software is dimensioned for use with models having up to 90,000 cells. MODFLOW-88 uses preallocated files, which means that the file names are not assigned by Fortran OPEN statements. Instead, the compiler or operating system must provide a way to open the files. Example script files are provided to illustrate this procedure. Generally, the program is easily installed on most computer systems. The code has been used on UNIX-based computers and DOS-based 386 or greater computers having a math coprocessor and 4 mb of memory.PACKAGESThis version of MODFLOW includes the following packages: BAS1 -- Basic Package BCF3 -- Version 3 of Block-Centered Flow Package RIV1 -- River Package DRN1 -- Drain Package WEL1 -- Well Package GHB1 -- General Head Boundary Package RCH1 -- Recharge Package EVT1 -- Evapotranspiration Package SIP1 -- Strongly Implicit Procedure Package SOR1 -- Slice Successive Over-Relaxation Package UTL1 -- Utility Package PCG2 -- Version 2 of Preconditioned Conjugate Gradient Package STR1 -- Stream Package IBS1 -- Interbed-Storage Package CHD1 -- Time-Variant Specified-Head Package GFD1 -- General Finite Difference Flow Package HFB1 -- Horizontal Flow Barrier Package TLK1 -- Transient Leakage Package DE45 -- Direct solver RES1 -- Reservoir Package The user must specify values for the appropriate IUNIT array element to include packages not in the original model. The IUNIT array is interpreted according to the following table: BCF3 -- IUNIT(1) -- same IUNIT as used for BCF1 because BCF3 replaces BCF1 TLK1 -- IUNIT(6) DE45 -- IUNIT(10) PCG2 -- IUNIT(13) GFD1 -- IUNIT(14) HFB1 -- IUNIT(16) RES1 -- IUNIT(17) STR1 -- IUNIT(18) IBS1 -- IUNIT(19) CHD1 -- IUNIT(20) The input unit for the Basic Package is unit 5, which is defined by the assignment of variable INBAS in the MAIN program. DEPENDENCIES AMONG PACKAGES As documented in Open-File Report (OFR) 94-59, the Transient Leakage (TLK) Package does not simulate flow through a confining unit at any horizontal grid location at which a cell on either side of the confining unit is dry. When this situation occurs as a result of initial conditions, the user can determine if this is appropriate before making a simulation. However, a cell can go dry at any time during a simulation when using the water-table or convertible layer options in the Block-Centered Flow (BCF) Package. When a cell goes dry on either side of a confining unit, the transient leakage through the confining unit immediately becomes zero at that horizontal location. Users should check simulations to see if cells on either side of a confining unit are going dry at any time during a simulation and determine if it is acceptable for the transient leakage to switch to zero. Further complications can result when using the wetting capabilities of version 2 of the BCF Package. If dry cells convert to wet so that cells on both sides of a confining unit are wet, then transient leakage calculations will be started; however, the equations will not be properly formulated to simulate the previous conditions, so the transient flow will not be correct. Thus, the wetting capability should not be used for any model layers that connect to a confining unit that is being simulated with the TLK Package. The Time-Variant Specified-Head (CHD) Package can potentially cause the TLK Package to operate incorrectly if the CHD Package is being used to specify constant heads at cells on either side of a confining unit. The TLK Package relies on initial head as defined by the Basic Package to setup initial parameters. If the data for the CHD Package define initial heads (i.e., head for the first time step of the simulation) on either side of a confining unit to be different than defined by the Basic Package, the transient leakage calculations will be incorrect. To avoid this conflict, do not use the CHD Package to define constant head cells on either side of a confining unit, or be sure that the initial head in the Basic Package exactly matches the initial head defined by the CHD Package.DOCUMENTATIONMcDonald, M.G., and Harbaugh, A.W., 1988, A modular three- dimensional finite-difference ground-water flow model: U.S. Geological Survey Techniques of Water-Resources Investigations, book 6, chap. A1, 586 p. Version 2 of Preconditioned Conjugate Gradient Package is documented in: Hill, M.C., 1990, Preconditioned conjugate-gradient 2 (PCG2), a computer program for solving ground-water flow equations: U.S. Geological Survey Water-Resources Investigations Report 90-4048, 43 p. The Stream 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 Interbed-Storage and Time-Variant Specified-Head Packages are documented in: Leake, S.A., and Prudic, D.E., 1988, Documentation of a computer program to simulate aquifer-system compaction using the modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 88-482, 80 p. The General Finite Difference Flow Package is documented in: Harbaugh, A.W., 1992, A generalized finite-difference formulation for the U.S. Geological Survey modular three-dimensional finite- difference ground-water flow model: U.S. Geological Survey Open- File Report 91-494, 60 p. The Version 2 of the Block-Centered Flow Package is documented in: McDonald, M.G., Harbaugh, A.W., Orr, B.R., and Ackerman, D.J., 1992, A method of converting no-flow cells to variable-head cells for the U.S. Geological Survey modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 91-536, 99 p. The BCF3 Package is documented in three pieces. It builds on two previous versions of the Block-Centered Flow (BCF) Package. Documentation for the BCF1 Package describes the fundamental function of all BCF Packages. This documentation is contained in the basic model documentation (McDonald and Harbaugh, 1988). BCF2 documentation describes the addition of the capability to convert dry cells to wet: McDonald, M.G., Harbaugh, A.W., Orr, B.R., and Ackerman, D.J., 1992, A method of converting no-flow cells to variable-head cells for the U.S. Geological Survey modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 91-536, 99 p. BCF3 documentation describes the addition of alternate interblock transmissivities. The BCF3 code includes the capabilities of BCF1 and BCF2: Goode, D.J., and Appel, C.E., 1992, Finite-difference interblock transmissivity for unconfined aquifers and for aquifers having smoothly varying transmissivity: U.S. Geological Survey Water- Resources Investigations Report 92-4124, 79 p. The HFB1 Package is documented in: Hsieh, P.A., and Freckleton, J.R., 1993, Documentation of a computer program to simulate horizontal-flow barriers using the U.S. Geological Survey modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 92-477, 32 p. The Transient-Leakage Package (TLK1) is documented in: Leake, S.A., Leahy, P.P., and Navoy, A.S., 1994, Documentation of a computer program to simulate transient leakage from confining units using the modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 94-59, 70 p. The DE45 Package is documented in: Harbaugh, A.W., 1995, Direct solution package based on alternating diagonal ordering for the U.S. Geological Survey modular finite- difference ground-water flow model: U.S. Geological Survey Open- File Report 95-288, 46 p. The RES1 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.RELATED DOCUMENTATIONHarbaugh, A.W., 1990, A simple contouring program for gridded data: U.S. Geological Survey Open-File Report 90-144, 37 p.REFERENCESMODFLOW is widely used in the USGS and throughout the world. Belitz, K., and Phillips, S.P., 1993, Numerical simulation of ground-water flow in the central part of the western San Joaquin Valley, California: U.S. Geological Survey Water-Supply Paper 2396, 69 p. Prince, K.R., Franke, O.L., and Reilly, T.E., 1988, Quantitative assessment of the shallow ground-water flow system associated with Connetquot Brook, Long Island, New York: U.S. Geological Survey Water-Supply Paper 2309, 28 p.TRAININGModeling of Ground-Water Flow Using Finite-Difference Methods (GW2096TC), offered annually at the USGS National Training Center. Advanced Finite-Difference Modeling of Ground-Water Flow (GW3099TC), offered annually at the USGS National Training Center.CONTACTSOperation: U.S. Geological Survey Office of Ground Water Arlen Harbaugh 411 National Center Reston, VA 20192 harbaugh@usgs.gov Distribution: U.S. Geological Survey Hydrologic Analysis Software Support Program 437 National Center Reston, VA 20192 h2osoft@usgs.gov Official versions of U.S. Geological Survey water-resources analysis software are available for electronic retrieval via the World Wide Web (WWW) at: http://water.usgs.gov/software/ and via anonymous File Transfer Protocol (FTP) from: water.usgs.gov (path: /pub/software). The WWW page and anonymous FTP directory from which the MODFLOW software can be retrieved are, respectively: http://water.usgs.gov/software/modflow.html --and-- /pub/software/ground_water/modflow See http://water.usgs.gov/software/ordering_documentation.html for information on ordering printed copies of USGS publications.SEE ALSOcontour(1) - A contouring program for gridded data mmsp - Modular Model Statistical Processor moc(1) - Two-dimensional method-of-characteristics ground-water flow and transport model modfe(1) - Modular finite-element model for areal and axisymmetric ground-water flow problems Modflowp(1) - Parameter-estimation version of the modular model modpath(1) - Particle-tracking postprocessor program for the modular three-dimensional finite-difference ground-water flow model sutra(1) - Saturated and (or) unsaturated, constant or variable-density fluid flow, and solute or energy transport (2-dimensional finite-element code) vs2di(1) - A graphical software package for simulating fluid flow and solute or energy transport in variably saturated porous media

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