STR: Stream Package Pane

The STR: Stream package pane is on the MODFLOW Features tab of the Object Properties dialog box.

Segments must be defined with point or polyline objects. The first vertex in the object defining the stream is at the upstream end of the stream. You can reverse the order of the vertices in an object using "Object|Edit|Reverse Selected Objects."

The data that can be specified for the Stream package are the Segment number, Starting time, Ending time, Outflow segment, Diversion segment, Flow, Head, Conductance, Streambed bottom, Streambed top, Stream width, Stream slope and Manning's roughness coefficient. (The latter two are only specified if ICALC is selected on STR: Stream Package pane on the MODFLOW Packages and Programs dialog box.)

The Segment number for each stream must be unique. The Stream package allows the flow in one segment to flow into another segment. However, each segment that receives flow from another segment must have a higher segment number than the segment number from which it receives flow. If required, ModelMuse will renumber segments to ensure that this criterion is met.

If the Outflow segment, is greater zero, it represents a downstream segment that will receive flow from the current segment. The stream package allows a segment to receive flow from up to 10 segments.

If the Diversion segment is greater than zero, it represents an upstream segment from which flow is diverted.

Flow is the flow into the upstream end of a segment. Note that a segment that is the downstream segment of another segment can not have its flow specified.

Head is the head in a stream reach. If the Stream package calculates head, the specified head is ignored.

The Conductance is the factor that relates the difference in head to the rate of flow.

Streambed bottom is the elevation of the bottom of the stream bed.

Streambed top is the elevation of the top of the stream bed.

Stream width is the width of the stream reach. It is used to help calculate the head in the stream using the Manning's equation.

Stream slope is the slope of the stream reach. It is used to help calculate the head in the stream using the Manning's equation.

Manning's roughness coefficient is used to help calculate the head in the stream using the Manning's equation.

Water can leave the groundwater through the stream reach boundary when the head in the cell is higher than the Head. The rate of flow will be proportional to the difference between the Head and the head in the cell.

Water can enter the groundwater at a variable rate through the stream boundary when the head in the cell is below than the Head but higher than the Streambed bottom. The rate of flow will be proportional to the difference between the Head and the head in the cell.

Water can enter the groundwater at a constant rate through the stream boundary when the head in the cell is below than the Streambed bottom. The rate of flow will be proportional to the difference between the Head and the Streambed bottom.

Starting time and Ending time are explained in the help for the MODFLOW Features tab.

If stream parameters are used with an object, the conductance used in MODFLOW will be the parameter value (specified in the MODFLOW Packages and Programs dialog box) multiplied by the conductance specified for the object.

In MODFLOW, the Conductance is described as having units of L2/T and is equal to KLW/M where

K = the hydraulic conductivity of the sediment in the boundary condition such as a river or drain,

L = the length of the boundary condition in the cell,

W = the width of the boundary condition, and

M = the thickness of the sediment in the boundary condition perpendicular to flow between the boundary and the cell. Usually this will be vertical thickness of the sediment.

However, the conductance is also affected by the grid cell size independent of the above formula (Mehl and Hill, 2010).

For Polyline Objects, the length of the object in the cell can be determined with the function ObjectSectionIntersectLength.

One way to to set the conductance would be with a formula that includes ObjectSectionIntersectLength and user defined Global Variables for the width of the drain and the thickness of the sediment in the drain and perhaps the hydraulic conductivity of the sediment in the drain. For example, the user could define Global Variables named "DrainWidth" and "DrainSedimentThickness" and then set the Conductance of the drain using the following formula.  

((Kz * ObjectSectionIntersectLength) * DrainWidth) / DrainSedimentThickness

In the formula, it is assumed that the hydraulic conductivity of the sediment in the drain is the same of the vertical hydraulic conductivity of the cell that contains the drain. This may or may not be a valid assumption in any particular case.

A similar formula appropriate for Polygon Objects would be

(Kz * ObjectIntersectArea) / DrainSedimentThickness

However, if Conductance Interpretation is set to Calculated, as it is by default, ObjectSectionIntersectLength and ObjectIntersectArea should not be included in the formulas because they will be incorporated automatically for polyline and polygon objects. Thus, if Conductance Interpretation is set to Calculated for a Polyline Object, Conductance should be treated as if it were KW/M instead of KLW/M, its units would be L/t, and an appropriate formula would be

(Kz * DrainWidth) / DrainSedimentThickness

Conductance Interpretation has no effect for point objects. For point objects, the formulas will be applied without any modification regardless of Conductance Interpretation.

if Conductance Interpretation is set to Calculated for a Polygon Object, Conductance should be treated as if it were K/M instead of KLW/M, its units would be 1/t, and an appropriate formula would be

Kz / DrainSedimentThickness

If Conductance Interpretation is set to Direct, the formulas will be applied without any modification.

If Conductance Interpretation is set to Total the formulas will be modified by multiplying the conductance by ObjectSectionIntersectLength / ObjectLength or ObjectIntersectArea / ObjectArea.

Setting the Conductance Interpretation to Calculated or Total allows ModelMuse to calculate an appropriate conductance for a cell based on the length or area of intersection between the object and cell. However, if you calibrate your model and then change the grid size, the conductances assigned by ModelMuse may no longer be appropriate and you may have to recalibrate your model. The conductance is a coefficient of proportionality between the head difference in the cell and the flow to the boundary. When the grid size changes, the head in the cell will be averaged over a different area than before. This conductances will need to be adjusted because of this change in head.