SUB: Subsidence and Aquifer-System Compaction Package Pane

The Subsidence and Aquifer-System Compaction package is used for simulating the drainage, changes in groundwater storage, and compaction of aquifers, interbeds and confining units that constitute an aquifer system. Delays in the release of groundwater from interbed storage, and thus delays in aquifer-system compaction, can be simulated. Delayed drainage and compaction in confining units can also be simulated.

The pane for the SUB package is on the MODFLOW Packages and Programs dialog box under Subsidence.

The user defines the layers with which the interbeds are associated in the MODFLOW Layer Groups dialog box.

NN is the number of nodes used to discretize the half space to approximate the head distributions in systems of delay interbeds.

AC1 is an acceleration parameter. This parameter (ω1 in equation 25) is used to predict the aquifer head at the interbed boundaries on the basis of the head change computed for the previous iteration. A value of 0.0 results in the use of the aquifer head at the previous iteration. Limited experience indicates that optimum values may range from 0.0 to 0.6.

AC2 is an acceleration parameter. This acceleration parameter is a multiplier for the head changes to compute the head at the new iteration (ω2 in equation 27). Values are normally between 1.0 and 2.0, but the optimum is probably closer to 1.0 than to 2.0. However, as discussed following equation 27, this parameter also can be used to help convergence of the iterative solution by using values between 0 and 1.

ITMIN is the minimum number of iterations for which one-dimensional equations will be solved for flow in interbeds when the Strongly Implicit Procedure (SIP) is used to solve the ground-water flow equations. If the current iteration level is greater than ITMIN and the SIP convergence criterion for head closure (HCLOSE) is met at a particular cell, the one-dimensional equations for that cell will not be solved. The previous solution will be used. The value of ITMIN is not used if a solver other than SIP is used to solve the ground-water flow equations.

If the IDSAVE checkbox is checked, restart records for delay interbeds will be saved at the end of the simulation. The restart record file will have the extension .rst.

If the IDREST file name is specified, restart records for delay interbeds will be read in. The restart records contain the starting head and starting preconsolidation head for the delay interbeds. If you are saving restart records as well as reading restart records, you should make sure that the file that is read has a different name from the one that is saved.

The Binary output choice determines whether all the binary output selected on the Print/Save tab is saved to a single file or to multiple files.

The table on the Print/Save tab is used to choose what to print or save during the specified time periods. However those time periods are not used to specify stress periods if the chosen times do not correspond to the boundaries of the stress periods. Instead, a pair of stress periods and times steps are chosen that contain the specified starting and ending times. The first row of the table is used to choose the format with which various types of data are printed. The extensions for the binary output files that can be generated by the SUB package are listed in the table below.