User's guide of Proceessing Modflow 5.0

248 Processing Modflow
cells, which is indicated by a negative value of THRESH. The wetting-iteration interval is 1 and
THRESH is -0.5 foot, which means that the wetting threshold is 10 percent of the thickness of
a cell. In the second simulation, wetting of cells is based on comparison to heads both in
horizontally adjacent and underlying cells (THRESH is positive). A wetting-iteration interval of
2 and a THRESH of 1.5 feet are used in order to prevent continued oscillation between wet and
dry for some cells. Due to the steepness of the head gradient and the grid discretization, the head
difference between adjacent horizontal cells is generally much larger than the head difference
between adjacent vertical cells along the mound. For example, the cell at layer 4, row 3, column
4 is supposed to be dry even though the head in the horizontally adjacent cell in column 3 is 1.4
feet above the bottom of the layer. The vertical head difference between cells in this part of the
model is much less; the difference between the head at the cell in layer 4, row 3, column 3 and
the cell below is only 0.05 foot. Thus, the neighboring cell to the right is repeatedly and
incorrectly converted to wet during the solution process if horizontal wetting is used with a
wetting threshold of 0.5 foot. The larger wetting threshold and wetting-iteration interval used in
the second simulation allow convergence to occur, but only after many iterations. In this
simulation, head in adjacent vertical cells is the best indicator of when a dry cell should become
wet.
The formation of the groundwater mound over time can be obtained with a transient
simulation. The transient simulation is run for one stress period with a length of 500,000 days.
The stress period is divided into 50 time steps with a time-step multiplier of 1.3. The first time
step is 0.3 days, and the last time step is 115,385 days. The specific yield is 20 percent and the
confined storage coefficient is 0.001. The PCG2 solver is used and cells are activated by
comparison of the wetting threshold to heads in underlying cells. The head change criterion for
closure is 0.001 foot and the residual-change criterion is 10,000 cubic feet, the wetting threshold
is 0.5 foot, the wetting factor is 0.5, and the wetting-iteration interval is 1. Fig. 6.23 shows
simulated water-table heads along row 1 at several times during the transient simulation.
Steady-state conditions were reached at the 44th time step of the transient simulation as
indicated by storage flow terms being zero (see the simulation listing file OUTPUT.DAT).
6.2.4 Simulation of a Water-Table Mound resulting from Local Recharge