NAME
radmod - A preprocessor to the program MODFLOW for simulation of
axisymmetric problems
ABSTRACT
Cylindrical (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.
METHOD
In 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.
HISTORY
Version 1.1 1996/01/19 - first wrdapp release - minor code clean-up
Version 1.0 1992/04/01 - original release
DATA REQUIREMENTS
The 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 OPTIONS
The 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 REQUIREMENTS
RADMOD 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.
APPLICATIONS
RADMOD is used to generate the data input to enable MODFLOW to
simulate well hydraulic problems or circular recharge basins.
DOCUMENTATION
Reilly, 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 DOCUMENTATION
Bennett, 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.
CONTACTS
Operation:
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/radmod
SEE ALSO
modflow(1) - Modular three-dimensional ground-water flow
model