General-Head Boundary Package (GHB)

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Last modified on Wednesday, January 31, 1996 - 5:51:32 PM

Skip forward to Input instructions for the General-Head Boundary Package.

The original instructions for the General-Head Boundary Package are on pages 11-5 to 11-6 of McDonald and Harbaugh (1988).

Introduction: The General Head Boundary Package is used to specify water flux into or out of particular cells in a model based on the head in those cells. In this respect it resembles, the River, Drain, Evapotranspiration and Streamflow-Routing packages. Unlike those packages, however, there is neither a lower nor an upper limit to the rate of water flux. The General-Head Boundary Package calculates a flux to or from a particular cell based on the head in that cell, some user-specified boundary head, and a hydraulic conductance.

If the conductance is high enough, the flux will be large enough to keep the aquifer cell at virtually the same head as the boundary head. Thus one use of the General-Head Boundary Package is to designate cells which have a constant head during any particular stress period but which have different heads in different stress periods. However, it is usually better to use the Time-Variant Specified-Head Package for this purpose. As explained by Anderson and Woessner (1992, p. 259), use of the General-Head Boundary Package in this way can cause large errors in the water balance. If this is a problem, reduce the conductance and see if that clears up the problem while still maintaining the boundary conditions you desire. You shouldn't ignore the possibility that water balance errors are unrelated to the General-Head Boundary Package. They may indicate that the convergence criterion in whichever solver you used was not stringent enough.

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Input Instructions

• Line 1
  • Spaces 1-10, MXBND, Integer, Maximum number of general-head boundary cells active at one time
  • Spaces 11-20, IGHBCB, Integer, (You must also set ICBCFL in the Output Control option to print or save cell-by-cell flow terms.)
    • if IGHBCB > 0, IGHBCB is the unit number for cell-by-cell flow terms;
    • if IGHBCB = 0, cell-by-cell flow terms will not be printed or saved;
    • if IGHBCB < 0, General-head boundary leakage will be printed on the main output file.
• Lines 2+, One copy of Lines 2+ followed by 0 or ITMP copies of Lines 3+ are read for each stress period.
  • Spaces 1-10, ITMP, Integer
    • If < 0, General-head boundary data from the last stress period will be reused
    • If >= 0, ITMP is the number of active general-head boundary cells during the current stress period.
• Lines 3+, The number of copies of Lines 3+ depends on ITMP in Lines 2+
  • Spaces 1-10, Layer, Integer, The layer number of the cell affected by the head-dependent boundary
  • Spaces 11-20, Row, Integer, The row number of the cell affected by the head-dependent boundary
  • Spaces 21-30, Column, Integer, The column number of the cell affected by the head-dependent boundary
  • Spaces 31-40, Boundary head, the reference head used in calculating the flux to the cell affected by the head-dependent boundary
  • Spaces 41-50, Cond, hydraulic conductance of the drain (length^2/time)

Line 1, Spaces 1-10, Variable = MXBND, Integer

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Line 1, Spaces 11-20, Variable = IGHBCB, Integer

• if IGHBCB > 0, unit number for cell-by-cell flow terms;
• if IGHBCB = 0, cell-by-cell flow terms will not be printed or saved;
• if IGHBCB < 0, general-head boundary leakage will be printed in the main output file.

Lines 2+, Spaces 1-10, Variable = ITMP, Integer

• If ITMP < 0, General-head boundary data from the last stress period will be used. There will be 0 copies of Lines 3+ immediately following this copy of Line 2.
• If ITMP >= 0, ITMP is the number of active general-head boundary cells during the current stress period. There will be ITMP copies of Lines 3+ immediately following this copy of Line 2.

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Lines 3+, Spaces 1-10, Variable = Layer, Integer

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Lines 3+, Spaces 11-20, Variable = Row, Integer

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Lines 3+, Spaces 21-30, Variable = Column, Integer

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Lines 3+, Spaces 31-40, Variable = Boundary head

Q=C(H-h)

where

• Q = the flux
• C = the hydraulic conductance
• H = the Boundary head
• h = the head in the cell affected by the General-Head Boundary.

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Lines 3+, Spaces 41-50, Variable = Cond

Q=C(H-h)

where

• Q = the flux
• C = the hydraulic conductance
• H = the Boundary head
• h = the head in the cell affected by the General-Head Boundary.

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