Techniques and Methods 6-A19

Techniques and Methods 6-A19

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Implementation of UZF1 With MODFLOW-2005

The UZF1 Package comprises 13 subroutines and one Fortran module that are used with the Ground-Water Flow (GWF) Process of MODFLOW-2005 (Harbaugh, 2005). Each subroutine or module incorporated in the main listing of MODFLOW-2005 is identified first by the Ground-Water Flow (GWF2) Process, followed by the UZF1 Package, and then followed by the division of procedures used in MODFLOW-2005. The Fortran module is used to share and store data. Memory allocation, reading input data and initialization of variables, formulating characteristic and finite-difference equations, and computing mass balances occur within GWF2UZF1AR, GWF2UZF1RP, GWF2UZF1FM, and GWF2UZF1BD subroutines, respectively. The subroutine GWF2UZF1VKS was added to define the saturated vertical hydraulic conductivity in the unsaturated zone, either as a variable read into the UZF1 Package in the GWF2UZF1RP subroutine or obtained from the LPF or Block-Centered Flow (BCF) Packages (Harbaugh, 2005). In addition to the subroutines called from the main program, several new subroutines were included in the UZF1 Package for routing kinematic waves through the unsaturated zone. These subroutines are UZF1FLOW2, LEADWAVE2, and TRAILWAVE2. The subroutine GWF2UZF1OLF was included to add ground-water discharge to land surface and overland flow to streams and lakes. The subroutine TRANSPIRATION was included to account for ET loss from the unsaturated zone. Finally, memory deallocation, setting pointer for all data for the package, and saving pointer locations occurs within GWF2UZF1DA, GWF2UZF1PNT, and GWF2UZF1PSV, respectively.

Additionally, the MAIN program of MODFLOW-2005 has been modified to designate “UZF” in position 56 in CUNIT. Whenever “UZF” is used at the beginning of a line in the NAME file, the program will use the specified unit number and file name to obtain information for the UZF1 Package. Output from the UZF1 Package can be printed to any or all of (1) the main listing file; (2) a separate listing file; (3) separate files for each specified cell that provides a time series of selected information; and (4) separate files for budget items summed for all model cells.

The UZF1 Package has been programmed to apply infiltration to a designated underlying cell. If the water level in the underlying cell becomes less than the bottom elevation of the cell, the cell becomes inactive. When this happens, unsaturated flow will be routed vertically to the uppermost active cell. However, the unsaturated zone hydraulic properties remain unchanged even if the properties change across model layers. If all cells in the vertical column are inactive, no interaction with ground water is allowed and water is not routed through the unsaturated zone and a warning is printed in the main output file.

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