The Lake package simulates lakes whose elevations can rise or fall due to interaction with groundwater or with streams simulated using the Stream Flow Routing package (SFR). The pane for this package is on the MODFLOW Packages and Programs dialog box under Head Dependant Flux.
On the LAK: Lake package pane, the user sets three inputs for the Lake package.
Theta determines whether the solution for the lake package is solved implicitly (Theta = 1) or semi-implicitly (0.5 ≤ Theta < 1). Theta is automatically set to a value of 1.0 for all steady-state stress periods. Theta for transient simulations is limited to range from 0.5 to 1.0. A value of 0.5 represents the stage midway between the previous time step and the end of the current time step. A value of 1.0 (fully implicit) represents the lake stage at the end of the current time step. A Theta of less than 0.5 does not perform well and a zero value is undefined in the Newton iteration method. Slight errors in the solution of lake stage and seepage may result when Theta is greater than 0.5. Values greater than 0.5 have been recommended for damping oscillations in streamflow-routing equations. A value of 0.5 represents a semi-implicit method that is often called Crank-Nicolson. A value of 0.5 is generally recommended. See Theta.
Maximum number of iterations (NSSITR) is the maximum number of iterations for Newton’s method solution for equilibrium lake stages in each MODFLOW iteration for steady-state aquifer head solution.
Convergence criterion (SSCNCR) is the convergence criterion for equilibrium lake stage solution by Newton’s method. If SSCNCR = 0, a value of 0.0001 will be used instead.
Height of lake bottom undulations (SURFDEPTH) SURFDEPTH is used to reduce numerical oscillations. Values from 0.01 to 0.5 have been used successfully. See SURFDEPTH.
Print lake output (LWRT) Checking this check box will cause LWRT to be set to 0 which allows Lake package data to be printed. See LWRT.
Lake Bathymetry (TABLEINPUT) If the optional key word “TABLEINPUT” is specified on the first line of the Lake package input file then the program reads values of lake stage, volume, and area from external lake bathymetry text files. Each lake bathymetry input file must contain exactly 151 lines with values of lake stage and the corresponding lake volume, and area in each line. Seepage between lakes and groundwater is calculated on cell-by-cell basis using the area of cells inundated by the lake, and not the lake surface area specified with the external lake bathymetry file.
