Wetting Tab

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The Wetting tab on the MODFLOW Options dialog box is used to activate or deactivate the wetting option in MODFLOW and to specify options related to the wetting capability. The wetting tab is not visible if the Upstream Weighting package in MODFLOW-NWT is being used. The wetting option is not used with the Upstream Weighting package.

In MODFLOW, if the head in a cell in a convertible layer goes below the bottom of the cell, the cell becomes inactive and is removed from the model. If the Rewetting option is active, dry cells that can become active again if neighboring cells have heads higher than the base of the dry cell. Such rewetting can help make a model more realistic but it can also makes a model less stable.

•Wetting active turns on or off the rewetting option.

•WETFCT is a number used in the equation for rewetting cells (IHDWET).

•IWETIT is the iteration interval for attempting to wet cells. Wetting is attempted every IWETIT iteration. If using the PCG solver, (Hill, 1990), this applies to outer iterations, not inner iterations

•IHDWET is used to pick the equation used to rewet cells. The first equation

h = BOT + WETFCT (hn - BOT) .

is considered more realistic but less stable than the alternative equation

h = BOT + WETFCT(THRESH) .

•h = the head in the newly wetted cell.

•hn = the head in the neighboring cell that causes wetting to occur.

•BOT is the elevation of the bottom of the newly wetted cell.

•WETFCT is a user specified wetting factor.

•THRESH is the wetting threshold specified in WETDRY.

 

ModelMuse defines three data sets related to the rewetting option: Wet_Dry_Flag, Wet_Dry_Threshold, and WetDry.

•If Wet_Dry_Flag > 0, any cell adjacent to the dry cell, except the cell above the dry cell, can cause the dry cell to convert to an active cell.

•If Wet_Dry_Flag = 0, the dry cell can not convert to an active cell.

•If Wet_Dry_Flag < 0, only the cell below the dry cell can cause the dry cell to convert to an active cell.

Setting Wet_Dry_Flag to a value less than zero in areas where the slope in the water table is relatively high can help promote stability.

Wet_Dry_Threshold is the wetting threshold. The head in neighboring cells must exceed the elevation of the bottom of the dry cell plus the wetting threshold before the dry cell can convert to an active cell.

WetDry is the product of Wet_Dry_Flag and Wet_Dry_Threshold. It corresponds directly to the WETDRY data set in MODFLOW. Normally, you wouldn't set WetDry directly but you can if you want to.

In MODFLOW 6, there is an option to use the Newton Formulation in the model Name file.

NEWTON—keyword that activates the Newton-Raphson formulation for groundwater flow between connected, convertible groundwater cells and stress packages that support calculation of Newton-Raphson terms for groundwater exchanges. Cells will not dry when this option is used. By default, the Newton-Raphson formulation is not applied.

UNDER RELAXATION—keyword that indicates whether the groundwater head in a cell will be underrelaxed when water levels fall below the bottom of the model below any given cell. By default, Newton-Raphson UNDER RELAXATION is not applied.