Introduction
MODFLOW is the name that has been given the USGS Modular ThreeDimensional GroundWater Flow Model. Because of its ability to simulate a wide variety of systems, its extensive publicly available documentation, and its rigorous USGS peer review, MODFLOW has become the worldwide standard groundwater flow model. MODFLOW is used to simulate systems for water supply, containment remediation and mine dewatering. When properly applied, MODFLOW is the recognized standard model used by courts, regulatory agencies, universities, consultants and industry.
MODFLOW's VERSATILITY
The main objectives in designing MODFLOW were to produce a program that can be readily modified, is simple to use and maintain, can be executed on a variety of computers with minimal changes, and has the ability to manage the large data sets required when running large problems. The MODFLOW report includes detailed explanations of physical and mathematical concepts on which the model is based and an explanation of how those concepts were incorporated in the modular structure of the computer program. The modular structure of MODFLOW consists of a Main Program and a series of highlyindependent subroutines called modules. The modules are grouped in packages. Each package deals with a specific feature of the hydrologic system which is to be simulated such as flow from rivers or flow into drains or with a specific method of solving linear equations which describe the flow system such as the Strongly Implicit Procedure or Preconditioned Conjugate Gradient. The division of MODFLOW into modules permits the user to examine specific hydrologic features of the model independently. This also facilitates development of additional capabilities because new modules or packages can be added to the program without modifying the existing ones. The input/output system of MODFLOW was designed for optimal flexibility.
MODFLOW SIMULATION
Groundwater flow within the aquifer is simulated in MODFLOW using a blockcentered finitedifference approach. Layers can be simulated as confined, unconfined, or a combination of both. Flows from external stresses such as flow to wells, areal recharge, evapotranspiration, flow to drains, and flow through riverbeds can also be simulated.
MODFLOW PACKAGES
The following packages are also included in most versions of MODFLOW.
TRANSIENT LEAKAGE  The TLK1 Package is a new method of simulating transient leakage in the MODFLOW model. It solves these equations that describe the flow components across the upper and lower boundaries of confining units. The exact equations are approximated to allow efficient solution for the flow components. The flow components are incorporated into the finitedifference equations for model cells that are adjacent to confining units. Confiningunit properties can differ from cell to cell and a confining unit need not be present at all locations; however, a confining unit must be bounded above and below by model layers in which head is calculated or specified.
IBS1 (Compaction Package)  This recent addition to MODFLOW permits calculation of both elastic and inelastic release of water from finegrained beds. This is especially useful in areas where land surface is subject to subsidence.
CHD1 (TimeVariant SpecifiedHead Package)  This package for MODFLOW permits specification of fixed head for boundary cells that vary from time step to time step during a stress period.
STR1 (Streamflow Routing Package)  The Stream Package permits representation of intermittent streams in MODFLOW. It is especially useful in systems in the headwaters of small streams. The program limits the amount of groundwater recharge to the available streamflow. It permits two or more streams to merge into one with flow in the merged stream equal to the sum of the tributary flows. The program also permits diversions from streams.
PCG2 (Preconditioned Conjugate Gradient Solver)  PCG2 uses the preconditioned conjugate gradient method to solve the equations produced by MODFLOW for hydraulic head. Linear or nonlinear flow conditions may be simulated. PCG2 includes two preconditioning options: modified incomplete Cholesky preconditioning which is efficient on scalar computers; and polynomial preconditioning which requires less computer storage and, with modifications that depend on the computer used, is most efficient on vector computers. Convergence of the solver is determined using both headchange and residual criteria. Nonlinear problems are solved using Picard iterations.
ZONEBUDGET  The MODFLOW Zonebudget Package calculates subregional water budgets using results from the USGS MODFLOW Model. It uses cellbycell flow data saved by the model in order to calculate the budgets. Subregions of the modeled region are designated by zone numbers. The user assigns a zone number for each cell in the model. Composite zones can also be defined as combinations of the numeric zones.
BCF3  As originally published, MODFLOW could simulate the desaturation of variablehead model cells which resulted in their conversion to noflow cells but could not simulate the resaturation of cells. That is, a noflow cell could not be converted to variable head. However, such conversion is desirable in many situations. For example, one might wish to simulate pumping that desaturates some cells followed by the recovery of water levels after pumping is stopped. This program allows cells to convert from noflow to variablehead. A cell is converted to variable head based on the head at neighboring cells.
GFD1 (Generalized FiniteDifference Package)  This package for the advanced user of MODFLOW permits specification of interblock conductance. It is essential for use with RADMOD.
RADMOD  A preprocessor for assembling files needed to use MODFLOW to simulate radial flow towards a well. Although MODFLOW permits simulation of flow toward a well, it does so with a rectilinear grid. RADMOD permits simulation using a twodimensional cross section.
HORIZONTAL FLOW BARRIER PACKAGE  This package for MODFLOW simulates thin, vertical lowpermeability geologic features that impede the horizontal flow of ground water. These geologic features are approximated as a series of horizontalflow barriers conceptually situated on the boundaries between pairs of adjacent cells in the finitedifference grid. The key assumption underlying this package is that the width of the barrier is negligibly small in comparison with the horizontal dimensions of the cells in the grid. Barrier width is not explicitly considered in the package but is included implicitly in a hydraulic characteristic defined as either (1) barrier transmissivity divided by barrier width if the barrier is in a constanttransmissivity layer or (2) barrier hydraulic conductivity divided by barrier width if the barrier is in a variabletransmissivity layer. Furthermore, the barrier is assumed to have zero storage capacity. Its sole function is to lower the horizontal branch conductance between the two cells that is separates.
MODFLOW ATTRIBUTES
MODFLOW is most appropriate in those situations where a relatively precise understanding of the flow system is needed to make a decision. MODFLOW was developed using the finitedifference method. The finitedifference method permits a physical explanation of the concepts used in construction of the model. Therefore, MODFLOW is easily learned and modified to represent more complex features of the flow system.
MODFLOW INPUT REQUIREMENTS
A large amount of information and a complete description of the flow system is required to make the most efficient use of MODFLOW. In situations where only rough estimates of the flow system are needed, the input requirements of MODFLOW may not justify its use. To use MODFLOW, the region to be simulated must be divided into cells with a rectilinear grid resulting in layers, rows and columns. Files must then be prepared that contain hydraulic parameters (hydraulic conductivity, transmissivity, specific yield, etc.), boundary conditions (location of impermeable boundaries and constant heads), and stresses (pumping wells, recharge from precipitation, rivers, drains, etc.).
