User's guide of Proceessing Modflow 5.0

Processing Modflow 97
MODFLOW < Wetting Capability...
The wetting capability of the Block-Centered Flow 2 package (BCF2; McDonald et al. 1991)
allows the simulation of a rising water table into unsaturated (dry) model layers.
The BCF2 package is identical to the BCF1 package of the "original" MODFLOW
(McDonald and Harbaugh, 1988) except for the wetting and drying of cells. A cell falls dry when
the head is below the bottom elevation of the cell. When a cell falls dry, IBOUND is set to 0
(which indicates a no flow or an inactive cell), all conductances to the dry cell are set to zero. No
water can flow into the cell as the simulation proceeds and the cell remains inactive.
To overcome this problem, a value THRESH, called wetting threshold, is introduced to the
BCF2 package (or later versions of this package). The computer code uses this value to decide,
whether a dry or an inactive cell can be turned into a wet (active) cell.
1. If THRESH=0, the dry cell or the inactive cell cannot be wetted.
2. If THRESH0, the cell below the dry cell (or inactive cell) and the four horizontally
adjacent cells can cause the cell to become wet.
A dry cell or an inactive cell can be turned into an active cell if the head from the previous
iteration in a neighboring cell is greater than or equal to the turn-on threshold TURNON.
TURNON ' BOT % |THRESH|
where BOT is the elevation of the bottom of the cell.
h ' BOT % WETFCT(hn & BOT)
h ' BOT % WETFCT@|THRESH|
(3.27)
To keep the stability of the numerical solution, a neighboring cell cannot become wet as a result
of a cell that has become wet in the same iteration; only variable-head cells either immediately
below or horizontally adjacent to the dry cell can cause the cell to become wet. When a cell is
wetted, its IBOUND value is set to 1 (which indicates a variable-head cell), vertical conductances
are set to the original values, and the hydraulic head h at the cell is set by using one of the
following equation.
(3.28)
(3.29)
3.6.1 MODFLOW