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CHD - Time-Variant Specified-Head Package

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CHD - Time-Variant Specified-Head Package

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CHD - Time-Variant Specified-Head Package

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Abbreviation in Name file

CHD

Purpose

The Time-Variant Specified-Head package is used to simulate specified head boundaries that can change within or between stress periods.

Documentation

Related Packages

See: IBOUND in the Basic package

Supported in

MODFLOW-2000
MODFLOW-2005
MODFLOW-LGR
MODFLOW-CFP
MODFLOW-NWT
MF2005-FMP2

Other Notes

If two or more CHD Boundary cells are at the same location, the specified head at that location will be the sum of the specified heads as specified by the Time-Variant Specified-Head package.
Once a cell has been designated as a constant-head cell, it remains one for the rest of the simulation even if it is not respecified as a constant-head cell in a later stress period.  It's head will be the last head that was specified.
Specified heads specified by the Time-Variant Specified-Head package are overridden by those specified in the Flow and Head Boundary package.
Specified heads specified in the Basic package are overridden by those specified in the Time-Variant Specified-Head package.
Programmers should know that the Time-Variant Specified-Head package changes the value assigned to IBOUND in the Basic package.
The Time-Variant Specified-Head package is sometimes referred to as the Constant Head package. The name "Constant Head" is not entirely appropriate because the heads it specifies can vary with time.

Input Instructions

Input to the Time-Variant Specified-Head (CHD) package is read from the file that has file type "CHD" in the name file. Optional variables are shown in brackets. All variables are free format if the option “FREE” is specified in the Basic Package input file; otherwise, the non-optional variables have 10-character fields and the optional variables are free format.

FOR EACH SIMULATION

Data Set 0

[#Text]

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

[ PARAMETER NPCHD MXL]

This optional item must start with the word “PARAMETER”.

NPCHD—is the number of constant-head boundary parameters.

The maximum number of parameters in all packages taken together is 2000.

MXL—is the maximum number of constant-head-boundary cells that will be defined using parameters. MXL must equal or exceed the sum of NLST×N for all parameters, where N is the greater of 1 and NUMINST.

Data Set 2

MXACTC [Option]

MXACTC—is the maximum number of constant-head boundary cells in use during any stress period, including those that are defined using parameters.

Option—is an optional list of character values.

AUXILIARY abc” or “AUX abc”—defines an auxiliary variable, named "abc", which will be read for each constant-head boundary as part of Items 4 and 6. Up to 20 variables can be specified, each of which must be preceded by "AUXILIARY" or "AUX." These variables will not be used by the Ground-Water Flow Process, but they will be available for use by other processes. The auxiliary variable values will be read after the Ehead variable.

Auxiliary parameters are used with MODPATH and the Ground-Water Transport process.  In MODPATH, the variable IFACE is specified with an auxiliary parameter.  In the Ground-Water Transport process, concentration is specified with an auxiliary parameter.  The option COMPACT BUDGET AUXILIARY must be used in the Output Control when IFACE is used for MODPATH.

NOPRINT—suppresses the printing of cell lists.

Repeat Items 3 and 4 for each NPCHD parameter.

Data Set 3

[PARNAM PARTYP Parval NLST [INSTANCES NUMINST]]

If PARNAM is to be a time-varying parameter, the keyword “INSTANCES” and a value for NUMINST must be entered.

PARNAM—is the name of a parameter. This name can consist of 1 to 10 characters and is not case sensitive; that is, any combination of the same characters with different case will be equivalent.

All parameter names must be unique.

PARTYP—is the type of parameter to be defined. For the CHD Package, the only allowed parameter type is CHD, which defines values of the start and end head at the boundary.

Parval—is the parameter value. This parameter value may be overridden by a value in the PVAL file.

NLST—is the number of constant-head cells that are included in a non-time-varying parameter or in each instance of a time-varying parameter.

INSTANCES – is an optional keyword that designates a parameter as time varying. The keyword is case-insensitive; that is, it may be entered in any combination of upper- and lower-case letters. If INSTANCES is present, it must be followed by a value for NUMINST. If INSTANCES is absent, PARNAM is non-time-varying and NUMINST should not be present.

NUMINST – is the number of instances that are included in the definition of a time-varying parameter, where each instance is a list of constant-head cells and associated properties. If the keyword INSTANCES is present, NUMINST must be present and must be at least 1. If the keyword INSTANCES is absent, NUMINST should not be present.

Data Set 4a

INSTNAM

Item 4a is read only if PARNAM is time-varying. NUMINST repetitions of Item 4 (parts a and b) are read. After each repetition of Item 4a, NLST repetitions of Item 4b are read.

INSTNAM – is the name of an instance associated with the parameter PARNAM specified in the corresponding Item 3. The name can consist of 1 to 10 characters and is not case sensitive. That is, any combination of the same characters with different case will be equivalent. Names entered for INSTNAM must be unique for any given parameter, but names may be reused for instances associated with different parameters.

Data Set 4b

Layer Row Column Shdfact  Ehdfact [xyz]

NLST repetitions of Item 4b are required; they are read by module ULSTRD. (SFAC of the ULSTRD utility module applies to Shdfact and Ehdfact). The NLST repetitions of Item 4b follow each repetition of Item 4a when PARNAM is time-varying.

Layer—is the layer number of the constant-head boundary.

Row—is the row number of the constant-head boundary.

Column—is the column number of the constant-head boundary.

Shdfact—is the factor used to calculate the head at the boundary at the start of the stress period from the parameter value. The head is the product of Shdfact and the parameter value.

Ehdfact—is the factor used to calculate the head at the boundary at the end of the stress period from the parameter value. The head is the product of Ehdfact and the parameter value.

[xyz]—represents any auxiliary variables for a constant-head boundary that have been defined in Item 2. The auxiliary variables must be present in each repetition of Items 4 and 6 if they are defined in Item 2.

If the UZF package is used, IUZFBND should be set to zero for all specified head cells.

FOR EACH STRESS PERIOD

Data Set 5

ITMP NP

ITMP—is a flag and a counter.

If ITMP < 0, non-parameter CHD data from the preceding stress period will be reused.
If ITMP 0, ITMP is the number of non-parameter constant-head boundaries read for the current stress period.

NP—is the number of parameters in use in the current stress period.

Data Set 6

Layer Row Column Shead Ehead [xyz]

ITMP repetitions of Item 6 are read by module ULSTRD if ITMP > 0. (SFAC of the ULSTRD utility module applies to Shead and Ehead.) Item 6 is not read if ITMP is negative or 0.

Layer—is the layer number of the constant-head boundary.

Row—is the row number of the constant-head boundary.

Column—is the column number of the constant-head boundary.

Shead—is the head at the boundary at the start of the stress period.

Ehead—is the head at the boundary at the end of the stress period.

[xyz]—represents any auxiliary variables for a constant-head boundary that have been defined in Item 2. The auxiliary variables must be present in each repetition of Items 4 and 6 if they are defined in Item 2.

 

If the UZF package is used, IUZFBND should be set to zero for all specified head cells.

Data Set 7

[Pname [Iname]]

(Item 7 is repeated NP times. It is not read if NP is negative or 0.) Iname is read if Pname is a time-varying parameter.

Pname—is the name of a parameter that is being used in the current stress period. NP parameter names will be read.

Iname – is an instance name that is read only if Pname is a time-varying parameter. Multiple instances of the same time-varying parameter are not allowed in a stress period.