GMS:GHB Package: Difference between revisions
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{{MODFLOW Links}} | {{MODFLOW Links}} | ||
The General Head package is similar to the [[GMS:DRN Package|Drain]] and [[GMS:RIV Package|River]] packages in that flow in or out of a cell is proportional to a difference in head. General head cells are often used to simulate lakes. General head conditions are specified by assigning a head and a conductance to a selected set of cells. If the water table elevation rises above the specified head, water flows out of the aquifer. If the water table elevation falls below the specified head, water flows into the aquifer. In both cases, the flow rate is proportional to the head difference and the constant of proportionality is the conductance. | ==General Head Boundary== | ||
The General Head Boundary (GHB) conceptually is a fixed head far from the model where it is assume as a fixed head with time (i.e. river, head will not be affected by the model stresses with time). The purpose of using this boundary condition is to avoid unnecessarily extending the model domain outward to meet the element influencing the head in the model. As a result, the general head condition is usually assigned along the outside edged of the model domain. | |||
[[File:General Head Boundary.jpg|thumb|none|400 px|Schematic of General Head Boundary.]] | |||
Where: | |||
:– K is the average hydraulic conductivity. | |||
:– W is the thickness of the saturated aquifer perpendicular to the flow direction. | |||
:– L is the boundary length perpendicular to the flow direction. | |||
:– D is the distance from the general head boundary to the model boundary. | |||
:– The GHB is the head at far distance where it is fixed (doesn’t change with time) | |||
The conductance for the arc = <math> K \times W \times \frac{L}{D} = 5 \times 100 \times \frac{1}{5000} = 0.1 m^2/day</math> | |||
==General Head Package Dialog== | |||
[[File:MODFLOW-GHB.jpg|thumb|375 px|Example of the ''MODFLOW General Head Package'' dialog]] | |||
The General Head package is similar to the [[GMS:DRN Package|Drain]] (DRN) and [[GMS:RIV Package|River]] (RIV) packages in that flow in or out of a cell is proportional to a difference in head. General head cells are often used to simulate lakes. General head conditions are specified by assigning a head and a conductance to a selected set of cells. If the water table elevation rises above the specified head, water flows out of the aquifer. If the water table elevation falls below the specified head, water flows into the aquifer. In both cases, the flow rate is proportional to the head difference and the constant of proportionality is the conductance. | |||
A set of selected cells can be specified as general head cells using the '''Point Sources/Sinks''' command in ''MODFLOW'' menu or general heads can be assigned using the [[GMS:MODFLOW Conceptual Model Approach|conceptual model]] in the [[GMS:Map Module|Map module]]. When the general head attribute is assigned to a polygons, the head and conductance are applied uniformly over the entire polygon. When assigned to an arc, the conductance is applied uniformly over the arc, but separate head values are applied to each of the nodes on the ends of the arc and the head is assumed to vary linearly between the nodes. When the attribute is assigned to a point, the head and conductance values are assigned directly to the cell containing the point. Once a set of cells have been specified, the ''General Head Package'' dialog can be used to view and edit the values assigned to the cells. | A set of selected cells can be specified as general head cells using the '''Point Sources/Sinks''' command in ''MODFLOW'' menu or general heads can be assigned using the [[GMS:MODFLOW Conceptual Model Approach|conceptual model]] in the [[GMS:Map Module|Map module]]. When the general head attribute is assigned to a polygons, the head and conductance are applied uniformly over the entire polygon. When assigned to an arc, the conductance is applied uniformly over the arc, but separate head values are applied to each of the nodes on the ends of the arc and the head is assumed to vary linearly between the nodes. When the attribute is assigned to a point, the head and conductance values are assigned directly to the cell containing the point. Once a set of cells have been specified, the ''General Head Package'' dialog can be used to view and edit the values assigned to the cells. | ||
===Stress Period=== | ===Stress Period=== | ||
The stress period field shows the current stress period and allows | The stress period field shows the current stress period and allows users to cycle through the stress periods and view the different general head properties for those stress periods in the spreadsheet below. | ||
===Use Previous=== | ===Use Previous=== | ||
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===Edit All Use Previous === | ===Edit All Use Previous === | ||
This button brings up a spreadsheet allowing the user to quickly edit the ''Use Previous'' flag for all stress periods. | |||
This button brings up a spreadsheet allowing the user to quickly edit the Use Previous flag for all stress periods. | |||
===Spreadsheet=== | ===Spreadsheet=== | ||
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===Display Cell IDs/IJK=== | ===Display Cell IDs/IJK=== | ||
The radio group at the bottom of the dialog allows the user to adjuster whether the cell ID or the cell i, j, k values are displayed for the cells containing the boundary condition/source/sink. | The radio group at the bottom of the dialog allows the user to adjuster whether the cell ID or the cell ''i'', ''j'', ''k'' values are displayed for the cells containing the boundary condition/source/sink. | ||
===Delete All=== | ===Delete All=== | ||
The '''Delete All''' button deletes all of the data currently defined in the River package and restores the River package parameters to the default values. | The '''Delete All''' button deletes all of the data currently defined in the River package and restores the River package parameters to the default values. | ||
==Related Topics== | |||
* [[GMS:Conductance|Conductance]] | |||
{{Navbox GMS}} | {{Navbox GMS}} | ||
[[Category:MODFLOW]] | [[Category:MODFLOW]] | ||
[[Category:MODFLOW-USG]] | |||
[[Category:MODFLOW Packages]] | [[Category:MODFLOW Packages]] | ||
[[Category:MODFLOW Dialogs]] | |||
[[Category:Equations]] | |||
