1. GHB61 GENERAL HEAD BOUNDARY PACKAGE The General Head Boundary Package is used to, –Simulate head-dependent recharge or discharge across an aquifer boundary, –Simulate time-varying constant-head boundaries (no longer necessary with the now Constant Head Boundary Package)

2. GHB62 GENERAL HEAD BOUNDARY PACKAGE Flow into or out of cell i,j,k from an external source is provided in proportion to the difference between the head in the cell, h ijk and the head assigned to the external source, h Bijk. The constant head source is represented by the apparatus on the right, which holds the source head at level h Bijk. The link between the source and the cell i,j,k is represented by the block of porous material with conductance, C Bijk.

3. GHB63 GENERAL HEAD BOUNDARY PACKAGE The relation between the flow into the cell and the head in the cell is given by, Q Bijk = C Bijk (h Bijk – h ijk ) In constrast to the River, Drain and ET Packages, the GHB Package provides no limit value of flow to bound the linear function in either direction. Flow into or out of the cell continues to increase without limit. Caution must be used in using the GHB to insure unrealistic flows do not develop.

4. GHB64 GENERAL HEAD BOUNDARY PACKAGE Head-dependent recharge or discharge across an aquifer boundary Let be the recharge (+) or discharge (-) into the boundary cell i,j,k in the steady- state (usually assigned by recharge-discharge wells). Let be the head at cell i,j,k calculated by the steady-state solution. Let be the head outside the boundary that is not effected by the pumping inside the boundary. Calculate the conductance as, Use the GHB Package equation, to calculate the recharge or discharge across the boundary.

5. GHB65 GENERAL HEAD BOUNDARY PACKAGE Time-varying constant-head boundaries The GHB package allow h Bijk to vary each stress period. If the equation Q Bijk = C Bijk (h Bijk – h ijk ) is divided on both sides by C Bijk, we get, Set the conductance, C Bijk, to a very high value (e.g. C Bijk = 10 35 ), The left side of the above equation is ≈0, thus forcing, h ijk = h Bijk

6. GHB66 GENERAL HEAD BOUNDARY PACKAGE NPGHB —is the number of general-head boundary parameters. MXL —is the maximum number of general-head boundary cells that will be defined using parameters. MXACTR —is the maximum number of general-head boundary cells in use during any stress period, including those that are defined with parameters.

7. GHB67 GENERAL HEAD BOUNDARY PACKAGE IGHBCB —is a flag and unit number. IGHBCB > 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. IGHBCB = 0, cell-by-cell terms will not be written. IGHBCB < 0, boundary leakage for each GHB cell will be written to the List File when SAVE BUDGET or a non-zero value for ICBCFL is specified in output control.

8. GHB68 GENERAL HEAD BOUNDARY PACKAGE [Option] —is an optional list of character values. AUXILIARY abc or AUX abc—defines an auxiliary variable named abc, which is read for each well as part of Items 4 and 6. Up to 5 variable can be specified, each preceded by AUXLIARY or AUX. CBCALLOCATE or CBC—indicates that memory should be allocated to store cell-by-cell flows for each well in order to make these flows available for use in other packages.

9. GHB69 GENERAL HEAD BOUNDARY PACKAGE PARNAM —is the name of a parameter. This name can consist of 1 to 10 characters, and is not case specific. PARTYP —is parameter type. There is only one parameter type for the GHB Package—the general-head boundary hydraulic conductance, GHB. Parval —is the parameter value. NLST —is the number of head- dependent boundary cells that are included in the parameter.

10. GHB610 GENERAL HEAD BOUNDARY PACKAGE ITMP —is a flag and a counter. ITMP<0, non-parameter GHB data from last stress period will be reused. ITMP≥0, ITMP will be the number of non-parameter general-head boundaries read for current stress period. NP —is the number of parameters in use in current stress period. Layer —is the layer number of the cell affected by the head dependent boundary. Row —is the row number of the cell affected by the head dependent boundary.

11. GHB611 GENERAL HEAD BOUNDARY PACKAGE Column —is the column number of the cell affected by the head dependent boundary. Bhead —is the head on the boundary. Condfact —is the factor used to calculate the hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value. Cond —is the hydraulic conductance of the interface between the aquifer cell and the boundary.

12. GHB612 GENERAL HEAD BOUNDARY PACKAGE [xyz] —represent the value of any auxiliary variable for a boundary that has been defined in Item 2. The auxiliary variable must be present in each repetition of of Item 4 and 6 if they are defined in Item 2. Pname —is the name of the parameter being used in the current stress period. NP parameter names are read.

13. GHB613 GENERAL HEAD BOUNDARY PACKAGE No Parameters

14. GHB614 GENERAL HEAD BOUNDARY PACKAGE