Changes to the PCG7 Package for MODFLOW-2005 version 1.9: Added the IHCOFADD option. When an active cell is surrounded by dry cells, the cell normally converts to dry. The new option causes the cell to convert to dry only if storage and head-dependent boundary flow are also 0. IHCOFADD is a 4th integer input item read on the first input line. A non-zero value activates the option. An unspecified or 0 value turns the option off. Changes to the PCG7 Package for MODFLOW-2005 version 1.5: The option was added to the PCG7 Package to have separate damping factors (input variable DAMP, Harbaugh, 2005, p 8-53) for steady-state and transient stress periods within a single simulation. In some cases, a lower damping factor is needed for steady state stress periods than for transient stress periods. The single variable DAMP is replaced by two variables, DAMPPCG and DAMPPCGT. DAMPPCG is required, and DAMPPCGT is optional. The option of having two damping factors is activated by specifying a negative value for DAMPPCG, indicating that a second damping factor used for transient stress periods will be read. The value of DAMPPCG is made into a positive value before being used, and this values is used for steady-state stress periods. The two damping factors are included in Item 2 of the PCG7 input file. The complete input instructions for PCG follow. REVISED PCG INPUT INSTRUCTIONS Input to the Preconditioned Conjugate-Gradient (PCG) Package is read from the file that is type "PCG" 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. FOR EACH SIMULATION 0. [#Text] Item 0 is optional -- "#" must be in column 1. Item 0 can be repeated multiple times. 1. MXITER ITER1 NPCOND [IHCOFADD] 2. HCLOSE RCLOSE RELAX NBPOL IPRPCG MUTPCG DAMPPCG [DAMPPCGT] Explanation of Variables Read by the PCG Package 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 written to the Listing File. MXITER-- is the maximum number of outer iterations. For a linear problem MXITER should be 1, unless more than 50 inner iterations are required, when MXITER could be as large as 10. A larger number (generally less than 100) is required for a nonlinear problem. ITER1-- is the number of inner iterations. For nonlinear problems, ITER1 usually ranges from 10 to 30; a value of 30 will be sufficient for most linear problems. NPCOND-- is the flag used to select the matrix conditioning method: 1-- is for Modified Incomplete Cholesky (for use on scalar computers) 2-- is for Polynomial (for use on vector computers or to conserve computer memory) IHCOFADD-- is a flag that determines what happens to an active cell that is surrounded by dry cells: 0-- cell converts to dry regardless of HCOF value. This is the default, which is the way PCG2 worked prior to the addition of this option. Not 0-- cell converts to dry only if HCOF is 0 (no head-dependent stresses or storage terms). HCLOSE-- is the head change criterion for convergence, in units of length. When the maximum absolute value of head change from all nodes during an iteration is less than or equal to HCLOSE, and the criterion for RCLOSE is also satisfied, iteration stops. RCLOSE-- is the residual criterion for convergence, in units of cubic length per time. The units for length and time are the same as established for all model data. (See LENUNI and ITMUNI input variables in the Discretizations File.) When the maximum absolute value of the residual at all nodes during an iteration is less than or equal to RCLOSE, and the criterion for HCLOSE is also satisfied, iteration stops. For nonlinear problems, convergence is achieved when the convergence criteria are satisfied for the first inner iteration. RELAX-- is the relaxation parameter used with NPCOND = 1. Usually, RELAX = 1.0, but for some problems a value of 0.99, 0.98, or 0.97 will reduce the number of iterations required for convergence. RELAX is not used if NPCOND is not 1. NBPOL-- is used when NPCOND=2 to indicate whether the estimate of the upper bound on the maximum eigenvalue is 2.0, or whether the estimate will be calculated. NBPOL=2 is used to specify the value is 2.0; for any other value of NBPOL, the estimate is calculated. Convergence is generally insensitive to this upper bound. NBPOL is not used if NBPOL is not 2. IPRPCG-- is the printout interval for PCG. IPRPCG, if equal to zero, is changed to 999. The maximum head change (positive or negative) and residual change are printed for each iteration of a time step whenever the time step is an even multiple of IPRPCG. This printout also occurs at the end of each stress period regardless of the value of IPRPCG. MUTPCG-- is a flag that controls printing of convergence information from the solver: 0-- is for printing tables of maximum head change and residual each iteration 1-- is for printing only the total number of iterations 2-- is for no printing 3-- is for printing only if convergence fails DAMPPCG-- is the damping factor. It is typically set equal to one, which indicates no damping. A value less than 1 and greater than 0 causes damping. >0 -- applies to both steady-state and transient stress periods. <0 -- applies only to steady-sate stress periods. The absolute value is used as the damping factor. DAMPPCGT-- is the damping factor for transient stress periods. DAMPPCGT is enclosed in brackets indicating that it is an optional variable that only is read when DAMPPCG is specified as a negative value. If DAMPPCGT is not read, then the single damping factor, DAMPPCG, is used for both transient and steady-state stress periods. References 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, variously paginated.