DRN - Drain Package

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Abbreviation in Name file

DRN

Purpose

The Drain package is used to simulate head-dependent flux boundaries.  In the Drain package if the head in the cell falls below a certain threshold, the flux from the drain to the model cell drops to zero.

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: Head-Dependent Flux Boundary Packages

•Drain Observation input file

•Drain Return package

Supported in

•MODFLOW-2000

•MODFLOW-2005

•MODFLOW-LGR

•MODFLOW-CFP

•MODFLOW-NWT

•MODFLOW-OWHM

Other Notes

Special care must be taken when constructing the DRN package input file if the observations are used.  See discussion of "Observations at Cells Having More Than One Head-Dependent Boundary Feature Represented by the Same Package" in the DROB input file.

Input Instructions

Input to the Drain (DRN) Package is read from the file that has type "DRN" 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 NPDRN MXL]

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

NPDRN—is the number of drain parameters.

In MODFLOW-2000, the maximum number of allowed parameters is 999.  In MODFLOW-2005,-LGR, and -CFP the maximum number of allowed parametersis 2000. In MODFLOW-OWHM, the maximum number of allowed parameters is specified in the Basic package.

MXL—is the maximum number of drain cells that will be defined using parameters.

Data Set 2

MXACTD IDRNCB [Option]

MXACTD—is the maximum number of drain cells in use during any stress period, including those that are defined using parameters.

IDRNCB—is a flag and a unit number.

•If IDRNCB > 0, it is the unit number to which cell-by-cell flow terms will be written when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control.

•If IDRNCB = 0, cell-by-cell flow terms will not be written.

•If IDRNCB < 0, drain leakage for each drain cell will be written to the listing file when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control.

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 drain 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 Groundwater Flow Process, but they will be available for use by other processes. The auxiliary variable values will be read after the Cond variable.

Auxiliary parameters are used with MODPATH and the Groundwater Transport process.  In MODPATH, the variable IFACE is specified with an auxiliary parameter.  In the Groundwater 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.

•CBCALLOCATE or CBC—indicates that memory should be allocated to store cell-by-cell flow for each drain in order to make these flows available for use in other packages. (This option is only available in MODFLOW-2000.)

•NOPRINT—suppresses the printing of cell lists.

Repeat Items 3 and 4 for each NPDRN 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. For the DRN Package, the only allowed parameter type is DRN, which defines values of the drain hydraulic conductance.

Parval—is the parameter value. This parameter value may be overridden by a value in the PVAL file, the Sensitivity Process (SEN) input file, or by a value generated by the Parameter-Estimation (PES) Process.

NLST—is the number of drain 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 drain 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 Elevation  Condfact [xyz]

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

Layer—is the layer number of the cell containing the drain.

Row—is the row number of the cell containing the drain.

Column—is the column number of the cell containing the drain.

Elevation—is the elevation of the drain.

Condfact—is the factor used to calculate drain hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value.

See also "What is conductance?"

[xyz]—represents any auxiliary variables for a drain 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.

FOR EACH STRESS PERIOD

Data Set 5

ITMP NP

ITMP—is a flag and a counter.

•If ITMP < 0, non-parameter drain data from the last stress period will be reused.

•If ITMP ≥ 0, ITMP will be the number of non-parameter drains 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 Elevation  Cond [xyz]

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

Layer—is the layer number of the cell containing the drain.

Row—is the row number of the cell containing the drain.

Column—is the column number of the cell containing the drain.

Elevation—is the elevation of the drain.

Cond—is the hydraulic conductance of the interface between the aquifer and the drain.

See also "What is conductance?"

[xyz]—represents any auxiliary variables for a drain 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.

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.