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A METHOD OF CONVERTING NO-FLOW CELLS TO VARIABLE-HEAD CELLS
FOR THE U.S. GEOLOGICAL SURVEY MODULAR FINITE-DIFFERENCE
GROUND-WATER FLOW MODEL
By Michael G. McDonald, Arlen W. Harbaugh,
Brennon R. Orr, and Daniel J. Ackerman
U.S. GEOLOGICAL SURVEY
Open-File Report 91-536
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provide an accurate and correct document. The document is supplied
"as-is" without guarantee or warranty, expressed or implied.
To reduce the file size of this document, listings of source code,
which are available on some original open-file reports, are not
included.
Wen-Hsing Chiang
Hamburg, April 1998.
U.S. DEPARTMENT OF THE INTERIOR
MANUEL LUJAN, JR., Secretary
U.S. GEOLOGICAL SURVEY
Dallas L. Peck, Director
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U.S. Geological Survey
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A METHOD OF CONVERTING NO-FLOW CELLS TO VARIABLE-HEAD CELLS
IN THE U.S. GEOLOGICAL SURVEY MODULAR FINITE-DIFFERENCE
GROUND-WATER FLOW MODEL
BY
Michael G. McDonald, Arlen W. Harbaugh, Brennon R. Orr,
and Daniel J. Ackerman
ABSTRACT
The U.S. Geological Survey Modular Ground-Water Flow Model, commonly
referred to as MODFLOW, sfmulates ground-water flow through porous media
using the finite-difference method.
The region being modeled is divided
into a grid of cells, and each cell is defined to be either no-flow,
variable-head, or constant-head.
The model calculates a value for head at
all variable-head cells whereas head at constant-head cells is specified by
the user.
Cells are designated as no-flow cells if they contain impermeable
material or are unsaturated, and accordingly the flow of water is not
simulated in such cells.
As originally published, MODFLOW could simulate the desaturation of
variable-head model cells, which resulted in their conversion to no-flow
cells, but could not simulate the resaturation of cells.
cell could not be converted to variable head.
desirable in many situations.
That is, a no-flow
However, such conversion is
For example, one might wish to simulate
pumping that desaturates some cells followed by the recovery of water levels
after pumping is stopped.
An option that allows cells to convert from no-
flow to variable-head has been added to the model.
In this option, a cell
is converted to variable head based on the head at neighboring cells.
The
option is written in FORTRAN 77 and is fully compatible with the existing
model.
This report documents the new option,
including a description of the
concepts, detailed input instructions, and a listing of the code.
Example
problems illustrate the practical applications of the option. Although
solution of the modified flow equations can be difficult for the model
solvers, the example problems show that it is possible to solve a variety of
complex problems.