Introduction to MODFLOW. i-1 i i+1. x i-1/2. x i+1/2. General 3D equation used in MODFLOW. Block centered grid. x i. K values in the space between nodes is calculated using the harmonic mean. The default boundary condition is no flow.
General 3D equation used in MODFLOW
Block centered grid
is calculated using the harmonic mean
leakance = Kv/thickness
resistance = thickness/Kv
(resistance is an analytic element term)
MODFLOW uses special terms (VCONT) for
vertical conductance, which is required in
3D problems. Modflow.exe requires the user
to supply values for the VCONT arrays.
GW Vistas computes the values in the VCONT
arrays for you.
VCONT = Kv/z
VCONT in MODFLOW language.
We also have horizontal conductance along rows
and along columns.
or 2nd ed. of textbook
by Domenico & Schwartz (Eqn. 7.15)
for MODFLOW’s FD equation
(Useful to think in terms of a layer transmissivity.)
Confined layer transmissivity = (KRi,j-1/2,k)(vk)
Unconfined layer transmissivity = (KRi,j-1/2,k)(hi,j,k-BOTi,j,k)
Input of Grid Spacing
The head value in the top layer could potentially
rise to infinity.
Land surface elevation is not used in MODFLOW,
except in the ET package.
matrix solution with iteration:
IADI (see chapter 5 of W&A)
*Used in MODFLOW
Note: m is time level and
n is iteration level.
an implicit approximation
with 7 diagonals
Matrix is sparse and symmetric.
The coefficient matrix has 5 diagonals
Coefficient matrix is
Symmetric and banded
1 is required
for the BAS Package