Glossary

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Last modified on Wednesday, January 31, 1996 - 5:54:37 PM

Cell-by-cell flow

The flow between each pair of adjacent cells can be saved to disk. These flows are cell-by-cell flows. The cell-by-cell flows can be used by other programs but they are not printed on the main output file.

Column

In MODFLOW models, the model area is broken up into rows, columns, and layers. Each layer is broken up into rows and columns. Numbering of rows and columns starts in the upper left corner when seen from above.

Diversion

In the Streamflow-Routing Package, on segment can provide to more than one downstream segments. If a stream splits in this way, one of the downstream segments must be designated as a diversion.

Flag

A flag tells the MODFLOW program whether or not it should perform a specific action.

Integer

A number without a fractional part. In FORTRAN, Integers can not have decimal points

Layer

MODFLOW models contain one or more layers which each represent an aquifer, confining unit or part on an aquifer or confining unit. The different layers lay one on top of the other. The top of one layer is not required to be the same as the bottom of the layer above it. Instead, the properties of an intervening confining bed can be incorporated into a leakage term between the layers. Layer boundaries can follow natural geologic boundaries.

Reach

In the River and Streamflow Routing Packages, flow from a cell in the aquifer goes into a reach. Each reach is associated with one and only one cell but one cell can have more than one reach.

Real Number

A number that can have a fractional part. In FORTRAN, real numbers have a decimal point even if their fractional part is 0.

Right justification

Variable which are right justified have their rightmost digit in the rightmost space available. Blank spaces are added at the beginning of the variable to fill up any extra space.

Row

In MODFLOW models, the model area is broken up into rows, columns, and layers. Each layer is broken up into rows and columns. Numbering of rows and columns starts in the upper left corner when seen from above.

Segment

In the Streamflow-Routing Package, reaches are grouped together into segments. A segment can not have any branches in it so normally the ends of segments are where a stream branches. The reaches in a segment are labeled from upstream to downstream starting at 1 at the upstream end. Segments are also numbered and each segment that receives flow from another segment must have a higher number than the segments from which it receives flow.

Stress Period.

A period during which all the input parameters in the model remain constant. A stress period is divided into one or more time steps. Steady state models should have one stress period with one time step.

Time steps

The increment of time by which the model advances through time. Just as the physical space in the model is divided up into blocks, time is also divided up into increments. The time steps in a stress period may be of equal or unequal lengths. Generally, it is better to have shorter time steps at the beginning of a Stress period.

Unit number

All input and output files must be assigned a unit number. Unit numbers are arbitrary integers between 1 and 99. It would be good to avoid using 1, 5 or 6 because those numbers may have already been specified within the program. During execution of MODFLOW, the program will prompt you to supply a file name for each unit number you use. The program may tell you only the unit number when asking for a file name or it may give you more information. The unit numbers for most of the input files are specified in the input file for the Basic package. The unit numbers for the output files are specified in the Output Control option and near the beginnings of most of the other packages. The unit numbers for the Basic package input file is specified in the source code of MODFLOW probably as either 1 or 5. The unit number for a printer (or the main output file) is specified in the source code as unit 6.