NAMEradmod - A preprocessor to the program MODFLOW for simulation of axisymmetric problemsABSTRACTCylindrical (axisymmetric) flow to a well is an important specialized topic of ground-water hydraulics and has been applied by many investigators to determine aquifer properties and determine heads and flows in the vicinity of the well. A recent modification to the U.S. Geological Survey Modular Three-Dimensional Finite- Difference Ground-Water Flow Model--MODFLOW (McDonald and Harbaugh, 1988) provides the opportunity to simulate axisymmetric flow to a well. The theory involves the conceptualization of a system of concentric shells that are capable of reproducing the large variations in gradient in the vicinity of the well by decreasing their area in the direction of the well. The computer program RADMOD serves as a preprocessor to the U.S. Geological Survey model by creating the input data file needed to implement the axisymmetric conceptualization.METHODIn axisymmetric flow to a well, the flow occurs through concentric shells that decrease in area in the direction of the well. The head gradient increases approaching the well, because the cross-sectional area for flow to pass through is decreasing. Thus, finer discretization (or closer spacing of nodes) is required near the well to accurately represent this increasing gradient. The conductance that represents the porous media between nodes must account for the changing grid spacing and the radial geometry. This preprocessor calculates the conductances and storage capacity more precisely for the changing geometries and head gradients of axisymmetric flow to a well. The conductances and storage capacity blocks are concentric shells that decrease in area in the direction of the well. This program then outputs these coefficients as an input data file for the General Finite-Difference (GFD) Package (Harbaugh, 1992). The simulation of the axisymmetric model requires that all other data sets necessary for MODFLOW must be prepared by the user. These would include as a minimum the Basic Package, a solver package, and the Well Package. For the Basic Package, the number of layers (NLAY), rows (NROW), and columns(NCOL), would be specified as: one layer (NLAY), the number of layers simulated in the axisymmetrical model (NROW), and the number of radial shells (NCOL). Any constant-head boundary conditions (which must be radially symmetric) would be specified in MODFLOW's IBOUND array. The Well Package would be used to specify the discharge rate at nodes representing the well along the inner radial boundary. The well nodes will always be located in column one when using the axisymmetrical conceptualization. If a well is screened in more than one layer, the user must specify the discharge for each layer. The implementation of the axisymmetric discretization in RADMOD for use with MODFLOW assumes a constant saturated thickness. This assumption impacts the utility of the model for the simulation of water-table aquifers. The model does have the capability to simulate the water released from storage at the water-table boundary, but this boundary does not move in the simulation and shells do not go dry. Thus, when applying this implementation to water-table problems, the user must evaluate the drawdowns at the top layer to check if any changes in saturated thickness are significant enough to invalidate the assumption used in the preprocessor.HISTORYVersion 1.1 1996/01/19 - first wrdapp release - minor code clean-up Version 1.0 1992/04/01 - original releaseDATA REQUIREMENTSThe required data set contains the radial grid multiplier, the radius of the well, the number of radial shells, the number of layers, and the unit number that MODFLOW will use to read the data. The data set also contains the thickness, horizontal hydraulic conductivity, vertical hydraulic conductivity, and the specific storage for each layer (within MODFLOW, each radial layer will be considered a row).OUTPUT OPTIONSThe RADMOD program calculates conductances and storage capacity and then outputs these coefficients as an input data file for the General Finite-Difference (GFD) Package (Harbaugh, 1992) to be used with MODFLOW. Lateral node locations, although not needed for model input, are printed in a separate file for use in interpreting results.SYSTEM REQUIREMENTSRADMOD is written in Fortran 77. 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 2 mb of memory.APPLICATIONSRADMOD is used to generate the data input to enable MODFLOW to simulate well hydraulic problems or circular recharge basins.DOCUMENTATIONReilly, T.E., and Harbaugh, A.W., 1993, Simulation of cylindrical flow to a well using the U.S. Geological Survey Modular Finite- Difference Ground-Water Flow Model: Ground Water, v. 31, no. 3, p. 489-494. Reilly, T.E., and Harbaugh, A.W., 1993, Source code for the computer program and sample data set for the simulation of cylindrical flow to a well using the U.S. Geological Survey Modular Finite- Difference Ground-Water Flow Model: U.S. Geological Survey Open- File Report 92-659, 7 p.RELATED DOCUMENTATIONBennett, G.D., Reilly, T.E., and Hill, M.C., 1990, Technical training note in ground-water hydrology: Radial flow to a well: U.S. Geological Survey Water-Resources Investigations Report 89-4134, 83 p. 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. McDonald, M.G., and Harbaugh, A.W., 1988, A modular three- dimensional finite-difference ground-water flow model: Techniques of Water-Resources Investigations of the U.S. Geological Survey, book 6, chap. A1, 586 p.CONTACTSOperation: U.S. Geological Survey Office of Ground Water Thomas E. Reilly 411 National Center Reston, VA 20192 tereilly@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 RADMOD software can be retrieved are, respectively: http://water.usgs.gov/software/radmod.html --and-- /pub/software/ground_water/radmodSEE ALSOmodflow(1) - Modular three-dimensional ground-water flow model

The URL for this page is: http://water.usgs.gov/cgi-bin/man_wrdapp?radmod

Send questions or comments to h2osoft@usgs.gov