User's guide of Proceessing Modflow 5.0

270 Processing Modflow
Modeling Approach and Simulation Results
To meet the requirement of an infinite areal extent, the modelled domain is fairly large. The
aquifer is simulated by a single model layer. An increasing grid spacing expansion is used to
extend the model boundaries (Fig. 6.39). The layer type is 0:confined. In the Layer Options
dialog box, the flags of Transmissvity and Storage Coefficient are set to User-specified. The
top and bottom elevations of the model layer are not required.
The analytical drawdown values at the borehole are specified in the Boreholes and
Observations dialog box. Both the analytical and calculated drawdown curves are shown in
Fig. 6.40. An exact comparison is not attained because of the approximations made in the
numerical model. These include: (1) use of a discrete rather than continuous spatial domain, (2)
use of a discrete rather than continuous time domain, (3) use of an iterative solution with a
convergence tolerance, and (4) artificial placement of boundaries.
In practice, you can use this model to estimate transmissivity and confined storage
coefficient by specifying the real observation time and data in the Boreholes and Observations
dialog box. By defining transmissivity and storage coefficient as estimated parameters, the
inverse models PEST or UCODE can estimate the parameters automatically. Click Models <
PEST < Run... or Models < UCODE < Run... to see how the inverse models work. Because the
analytical drawdown values were used as the observations, the results from the inverse models
2
must be transmissivity = 0.0023 m /s and storage coefficient = 0.00075.
Fig. 6.39 Configuration of the groundwater model
6.4.3 The Theis Solution - Transient Flow to a Well in a Confined Aquifer