User's guide of Proceessing Modflow 5.0

Processing Modflow 77
Vertical Hydraulic Conductivity and Vertical Leakance
As discussed in Layer Type above, there are two options to input the required vertical leakance
(VCONT) between two model layers.
You can specify the vertical leakance directly by choosing Vertical Leakance from the Parameters
menu. The specified Vertical Leakance values of a layer will be used for simulation if the Leakance
flag in the Layer Options dialog box is User-specified. In the Data Editor, the vertical leakance
between the layers i and i+1 is given as the data of the layer i. The vertical leakance is not required
for the layer at the very bottom of the model because MODFLOW assumes that the botton of the
model is underlain by impermeable material.
Note that setting the Leakance flag (in the Layer Options dialog box) of a layer to Calculated
causes PMWIN to calculate the vertical leakance by using eq. 3.1.
Effective Porosity
If the total unit volumn V of a soil matrix is divided into the volume of the solid portion V and
s
the volume of viods V , the porosity n is defined as n=V /V. Effective porosity (with the respect
v v
to flow through the medium) is normally smaller than porosity, because part of the fluid in the
pore space is immobile or partially immobile. This may occur when the flow takes place in a fine-
textured medium where adhesion (i.e., the attraction to the solid surface of the porous matrix by
the fluid molecules adjacent to it) is important.
Effective porosity is used by transport models, for example PMPATH, MOC3D or MT3D,
to calculate the average velocity of the flow through the porous medium. If a dual-porosity system
is simulated by MT3DMS, effective porosity should be specified as the portion of total porosity
filled with mobile water and the “immobile” porosity is defined through MT3DMS < Chemical
Reaction of the Models menu. A summary of representive porosity values for different soil types
can be found in Zheng and Bennett (1995) or Domenico and Schwartz (1990).
Specific Storage, Storage Coefficient and Specific Yield
For transient flow simulations, MODFLOW requires dimensionless storage terms specified for
each layer of the model. For a steady state simulation, these menu items are not used and are therefore
dimmed.
In a confined layer, the storage term is given by storativity or confined storage coefficient (=specific
-1
storage [L ] × layer thickness [L]). The storativiy is a function of the compressibility of the water
3.5 The Parameters Menu