User's guide of Proceessing Modflow 5.0

Processing Modflow 273
Fig. 6.41 Configuration of the leaky aquifer system and the aquifer parameters
Modeling Approach and Simulation Results
The modelled domain is the same as the previous example. Three model layers are used to
simulate the system. The layer type is 3:confined/unconfined (transmissvity varies). In the
Layer Options dialog box, the Storage Coefficient flag is set to user-specified and the
Transmissvity flag is calculated. All model cells in the first model layer are fixed-head cells
and all other cells are specified as active cells. A transient flow simulation is performed for a
stress period with the lenght of 49320 seconds, 20 time steps and a time-step multiplier of 1.3.
For comparison, the analytical solution is enterred in the Boreholes and Observations dialog
box. Fig. 6.42 shows the numerical and analytical drawdown-time curves at the observation
borehole, which is at a distance of 55 m from the pumping well. The match of these two curves
is very good.
While the use of the analytical solution is limited to the primary assumptions, the numerical
model can be used to evaluate pumping tests, even if the confining aquitard (Fig. 6.41) has a
higher value of the vertical hydraulic conductivity and the hydraulic head in the overlying
aquifer is not constant during the pumping. To do this, simply specify all model cells as active
cells. This is allowed because the simulation time is normally very short and the extent of the
model domain is relative large, so that at end of a transient flow simulation the drawdown
values at the model boundaries are acceptable low.
If the vertical hydraulic conductivity of the aquitard is known, we can use PEST or UCODE
6.4.4 Transient Flow to a Well in a Leaky Confined Aquifer