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Navigation: Online Guide to MODFLOW > Groundwater Flow Process > Boundary Condition Packages > Specified Head Boundaries

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

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report 00-92, 121 p.

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. https://doi.org/10.3133/tm6A16

Time-varying-parameters.pdf

release.txt

modpath.txt

Related Packages

See: IBOUND in the Basic package

Flow and Head Boundary (FHB) package

Constant-Head Flow Observation package

Supported in

MODFLOW-2000

MODFLOW-2005

MODFLOW-LGR

MODFLOW-CFP

MODFLOW-NWT

MODFLOW-OWHM

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.

Data Set 1

[ PARAMETER NPCHD MXL]

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

Data Set 2

MXACTC [Option]

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.

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.

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.

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

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.

 

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.