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SIP - Strongly Implicit Procedure Package

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SIP - Strongly Implicit Procedure Package

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SIP - Strongly Implicit Procedure Package

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Abbreviation in Name file



The Strongly Implicit Procedure package is used to solve the finite difference equations in each step of a MODFLOW stress period.


Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model -- the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16.

Related Packages


Supported in


Other Notes

If the Lake package is used, more iterations may be required.

Input Instructions

Input to the Strongly Implicit Procedure (SIP) Package is read from the file that is type "SIP" in the name file. All numeric variables are free format if the option “FREE” is specified in the Basic Package input file; otherwise, all the variables have 10-character fields.


Data Set 0


Item 0 is optional—“#” must be in column 1. Item 0 can be repeated multiple times.

Text—is a character variable (199 characters) that starts in column 2. Any characters can be included in Text. The “#” character must be in column 1. Except for the name file, lines beginning with # are restricted to these first lines of the file. Text is printed when the file is read.

Data Set 1


MXITER—is the maximum number of times through the iteration loop in one time step in an attempt to solve the system of finite-difference equations.

NPARM—is the number of iteration variables to be used. Five variables are generally sufficient.

Data Set 2


ACCL—is the acceleration variable. It must be greater than zero and is generally equal to one. If a zero is entered, it is changed to one.

HCLOSE—is the head change criterion for convergence. When the maximum absolute value of head change from all nodes during an iteration is less than or equal to HCLOSE, iteration stops.

IPCALC—is a flag indicating where the seed for calculating iteration variables will come from.

0—the seed entered by the user will be used.
1—the seed will be calculated at the start of the simulation from problem variables.

WSEED—is the seed for calculating iteration variables. It is always read, but it is used only if IPCALC is equal to zero.

IPRSIP—is the printout interval for SIP. If IPRSIP is equal to zero, it is changed to 999. The maximum head change (positive or negative) is printed for each iteration of a time step whenever the time step is an even multiple of IPRSIP. This printout also occurs at the end of each time step regardless of the value of IPRSIP.